GB/T 11299.9-1989 Satellite communication earth station radio equipment measurement methods Part 2: Subsystem measurement Section 7: Frequency modulator Section 8: Frequency demodulator
Some standard content:
National Standard of the People's Republic of China
GB/T 11299.9—1989
Methods of measurement for radio equipment used in satellite communication earth stationsPart 2:Measurements for sub-systemsSection Seven-Frequency modulatorsSection Eight-Frequency demodulatorsPromulgated on March 1, 1989
Implementation on January 1, 1990
Ministry of Electronics Industry of the People's Republic of China
National Standard of the People's Republic of China
Methods of measurement for radio equipment used in satellite communication earth stationsPart 2:Measurements for sub-systemsSection Seven-Frequency modulatorsSection Eight-Frequency demodulators 2:Measurements for sub-systems Section Seven-Frequency modulators Section Eight-Frequency demodulators This standard is the seventh section of the series of standards for "Measurement methods for radio equipment of satellite communication earth stations"
1 Subject content and scope of application
Frequency modulator
GB/T11299.9—1989
This section specifies the measurement methods for the electrical performance of frequency modulators, and as far as possible only involves the measurement of basic modulators, and does not include the baseband part composed of the pre-emphasis network and the combination network of sound subcarrier signals, pilot signals and auxiliary signals. Section 8 gives the measurement methods for frequency demodulators. The measurements between the baseband terminals of the modulator/demodulator combination are described in the sections of Part 3 of this series of standards. 2 Definitions
For the purpose of this standard, frequency modulator refers to a subsystem that modulates the intermediate frequency carrier with the baseband signal by analog method. Such baseband signals can be frequency-division multiplexed telephone signals or television signals with a sound subcarrier, as well as pilot signals and auxiliary signals. These baseband signals are usually analog signals, but digital signals are not excluded. However, the measurement methods described in this section are only used to evaluate the performance of the modulator when transmitting analog signals.
The modulator subsystem usually consists of the following three main parts: a baseband part,
the part from baseband to intermediate frequency (modulator);
an intermediate frequency part.
3 Overview
Figure 1 is a block diagram of a typical modulator subsystem. The characteristics that need to be measured can be divided into three categories: a non-transfer characteristics:
a baseband to intermediate frequency characteristics,
some baseband to baseband transfer characteristics when connected to the measurement demodulator. The first category only involves measurements at the baseband port and at the intermediate frequency port, including the measurement of the frequency and spurious/harmonic signals at the intermediate frequency output. Approved by the Ministry of Electronics Industry of the People's Republic of China on March 1, 1989 and implemented on January 1, 1990
GB/T11299.9—1989
These measurements are described in other chapters of this series of standards. The second type of measurement forms the basic part of this section, because the nature of the component under test is the conversion from baseband to intermediate frequency. The third type of measurement is performed on a complete modulator/demodulator set, but the measurement demodulator should be replaced by the actual or system demodulator.
Because in actual applications, modulators of one design or manufacturer may work with demodulators of other designs or manufacturers, it is very necessary to know the impact of the modulator on the total tolerance of performance. Compensation effects between modulators and demodulators are not desired. Each modulator connected to the measurement demodulator should meet the specified technical conditions, which requires that the characteristics of the measurement demodulator are better than those of the modulator under test.
4 IF output characteristics
4.1 Return loss
According to the provisions of "Measurement within the intermediate frequency range" of this series of standards GB/T11299.3. This measurement should be performed when the modulator has no signal output. For example, this requirement can be achieved by making the oscillator in the modulator inoperative.
4.2 Level
According to the provisions of "Measurement within the intermediate frequency range" of this series of standards GB/T11299.3. 4.3 Carrier frequency
According to the provisions of "Measurement within the intermediate frequency range" of this series of standards GB/T11299.3. 4.4 IF spurious and/or harmonic signals
4.4.1 Measurement method
Use a suitable spectrum analyzer or frequency-selective level meter to check the IF output of the modulator to confirm that the level of any IF spurious and/or harmonic signals is within the specified range. It should be noted that the measurement of intermediate frequency spurious and harmonic signals should be carried out without modulation and with the energy dispersal generator turned off.
4.4.2 Result presentation
The measurement results are preferably presented by a photograph, a chart displayed by a calibrated spectrum analyzer or a curve recorded by an XY recorder. Alternatively, the level of the more important spurious and/or harmonic signals relative to the useful signal is expressed in decibels. 4.4.3 Details to be specified
When this measurement is required, the equipment technical specifications should include the following: a. The maximum allowable level of spurious signals relative to the useful signal, expressed in decibels; b. The maximum allowable level of harmonic signals relative to the useful signal, expressed in decibels; c. The frequency range required for measurement;
d. The level of the useful signal.
5 Baseband input impedance and return loss
According to the provisions of this series of standards GB/T11299.4 "Baseband measurement". 6 Frequency deviation sensitivity
6.1 Definitions and general considerations
For a sinusoidal signal of a given frequency, the frequency deviation sensitivity Sm of the modulator can be expressed as the ratio of the frequency deviation ∆f to the baseband input voltage V,:
Both V and ∆f are expressed as peak or RMS values. 2
(MHz/V)
GB/T 11299.9—1989
Due to the influence of the pre-emphasis network, the frequency deviation sensitivity of the modulator is usually a function of the baseband frequency. When it is possible to obtain the baseband input point after the pre-emphasis network (Figure 1), the measured modulator frequency deviation sensitivity is independent of the baseband frequency used. Note: 1) References 1 and 2.
6.2 Measurement method
The measurement method is the "Bessel null" method. It is based on the first disappearance of the carrier spectrum line at the modulation index m: given below in the case of sinusoidal modulation.
Af=2.40483
Where: Af——peak frequency;
at——modulation frequency.
This "zero point" seen on the spectrum analyzer, that is, the point where the intermediate frequency carrier disappears for the first time, cannot be an ideal zero value due to the influence of the residual harmonic distortion of the baseband signal generator. However, a carrier level drop of 30dB or more is sufficient. Because there are multiple modulation index values that can obtain the carrier zero point, the best way to determine the first zero point is to increase the modulation voltage steadily from zero until the carrier disappears for the first time. The measurement steps are as follows:
a. Adjust the baseband signal generator to the frequency required to measure the frequency deviation sensitivity b. Set the output voltage of the baseband signal generator to zero, and then steadily increase the voltage until the intermediate frequency carrier disappears for the first time on the spectrum analyzer,
c. Measure the root mean square voltage V at the baseband input of the modulator. d. Then calculate the modulator frequency deviation sensitivity Sm2 at the modulation frequency according to formula (3). 404 83f (MHz/V)
Note: Because the intermediate frequency bandwidth corresponding to the modulation index of 2.40483 increases linearly with the increase of the modulation frequency, the modulation frequency must be limited when using this method to measure the frequency deviation sensitivity so that the spectrum of the modulated signal does not exceed the system bandwidth. Another method is to use a calibrated measurement demodulator to measure the frequency deviation sensitivity.
6.3 Result expression
The measurement result should be expressed as follows:
"Frequency deviation sensitivity S...MHz" or "When the baseband input level is .….dBm, the root mean square frequency deviation is...kHz". 6.4 Details to be specified
When this measurement is required, the equipment technical specifications should include the following: a. Measurement method (6.2 or its note)
b. Frequency of the baseband input signal;
c. Frequency deviation of the intermediate frequency output signal;
d. Required frequency deviation sensitivity or output frequency deviation at a specified input level, e. Baseband connection point (i.e. before or after the pre-emphasis network, see Figure 1); f. Pre-emphasis characteristics adopted (if used). 7 Modulation polarity
7.1 Definition and general considerations
If a positive change in the input voltage causes the intermediate frequency to increase, the polarity of the frequency modulator is positive. Modulation polarity is very important in television transmission.
7.2 Measurement method
A simple method to check the modulation polarity is to apply an asymmetrical waveform to the modulator under test and apply the intermediate frequency signal output by the modulator to a measurement demodulator of known polarity. If the demodulator output signal polarity is the same as the modulated input signal polarity, then the modulator polarity is the same as this known demodulated polarity. Another method is to apply a signal consisting of a horizontal sync pulse and a positive peak brightness signal to the baseband input and observe the modulator IF spectrum displayed by the spectrum analyzer. For positive modulation polarity, the frequency of the highest level spectral line will be higher than the frequency of the baseband modulation. 8 Differential Gain/Nonlinearity and Differential Phase/Group Delay 8.1 Definitions and General Considerations
The baseband signal that excites the modulator under test consists of a small amplitude, high frequency sinusoidal signal (test signal) with constant amplitude and phase superimposed on a lower frequency, large amplitude signal (sweep signal). At the modulator IF output, the frequency deviation caused by the test signal corresponds to a sinusoidal frequency shift whose magnitude and phase depend on the instantaneous value of the sweep signal voltage. The differential gain (DG) and differential phase (DP) of the modulator under test are defined as functions of this instantaneous value. Given by the following formula: DG(X)=A(X)
DP(X)=(X)-
Where: X is the instantaneous value of the input sweep signal; DG(X) is the modulator differential gain, which is a function of X; A(X) is the magnitude of the output frequency deviation generated by the test signal, which is a function of X. A is the magnitude of the output frequency deviation generated by the test signal when the sweep signal is zero; DP(X) is the modulator differential phase, which is a function of X; (X) is the phase of the output frequency deviation generated by the test signal, which is a function of X; $ is the phase of the output frequency deviation generated by the test signal when the sweep signal is zero. (4)
（5）
For an ideal modulator without distortion, the differential gain and differential phase are both zero, while for an actual modulator, the above functions will present variables. The actual modulator characteristics can be represented by these functions themselves or by differential gain distortion and differential phase distortion. Differential gain distortion and differential phase distortion are defined as the difference between the two extreme values of the above functions, usually expressed in percentage and degree, respectively: lAmax-Amn
DG distortion (percentage) = 100
DP distortion (degrees) = dmax-minWww.bzxZ.net
+·. (7)
The choice of test signal frequency depends on which part of the modulator is to be evaluated and which parameters need to be measured (i.e. differential gain or nonlinearity, differential phase or group delay). The definition of nonlinearity and group delay and the factors for selecting the test frequency can be referred to GB/T11299.4\Baseband measurement in this series of standards.
The same method is used to measure differential gain and nonlinearity, but the test frequencies used are different. Nonlinearity is an important performance parameter of the modulator because it indicates the deviation of the output frequency/input voltage characteristic from the ideal linear response. The test signal frequency used to measure nonlinearity is relatively low, and its typical range is 50 to 500kHz. 8.2 Measurement method
The measurement of differential gain/nonlinearity and differential phase/group delay of the modulator requires a measurement demodulator. This demodulator should have smaller differential gain/nonlinearity and differential phase/group delay than the modulator under test. In this measurement In the measurement demodulator, a heterodyne type demodulator can be formed. Its heterodyne oscillator is controlled by a sweep signal, which suppresses the frequency deviation caused by the sweep signal, thereby virtually eliminating the distortion of the demodulator. Figure 2 shows a simplified configuration for measuring the differential gain and differential phase of the modulator. The complete measurement device including the measurement demodulator shown in the figure is a commercial line analyzer, radio line tester or system analyzer. For the measurement of the modulator, the "line analyzer" usually consists of the following two parts: a. The transmitting part connected to the baseband input of the device under test. This part consists of a sweep signal source and a test source. b. The receiving part connected to the intermediate frequency output of the device under test. This part consists of the measurement demodulator and the bandpass filter, envelope 4 behind it.
GB/T11299.9—1989
It consists of a detector, a phase detector, and an oscilloscope with calibrated horizontal and vertical axes. Its bandpass filter is used to extract the test signal, and the detector is used to provide differential gain/non-linearity and differential phase/group delay signals. The horizontal deflection of the oscilloscope is generated by the demodulated sweep signal, which is taken out from the measurement demodulator after being fed to the low-pass filter. Note: ① When using a high test frequency, the intermediate frequency measured range will not be approximate to the sweep width, but to the sweep width plus twice the test frequency. Modern line analyzers provide a working mode that keeps the measured range unchanged when the test frequency is changed. ② It must be ensured that the baseband amplifier before the modulator is not overdriven by the large amplitude sweep signal. For To meet this requirement, the applied sweep amplitude is usually limited or the baseband portion of the modulator is excluded from the measurement, thus allowing the entire modulator characteristic to be measured with a large amplitude sweep signal. The baseband portion before the modulator must also be removed from the measurement when the low-end cutoff frequency of the baseband amplifier prevents the sweep signal from being transmitted.
8.3 Presentation of Results
The results of differential gain and differential phase measurements are best presented as a photograph, graph, or XY recorder of the functions displayed on an oscilloscope with both axes properly calibrated. A single photograph, graph, or XY recorder showing both functions simultaneously may be presented. Alternatively, a textual statement of differential gain distortion/nonlinearity, differential phase distortion/group delay, and sweep range may be provided. 8.4 Details to be specified
When this measurement is required, the equipment specifications shall include the following: a. The IF sweep range (e.g. ±10 MHz); b. The differential gain distortion allowed within the above range (e.g. 3%) c. The differential phase distortion allowed within the above range (e.g. 0.8°); d. The test frequency to be used,
e. The baseband connection point (i.e. before or after the baseband amplifier). 9 Unwanted amplitude modulation
9.1 General considerations
Frequency modulators usually have slight amplitude modulation, which is either caused by the modulator itself or by the amplitude/frequency characteristics of the IF circuitry following the modulator. This amplitude modulation is undesirable because the subsequent AM/PM conversion or the demodulator's sensitivity to AM may cause additional baseband distortion. 9.2 Measurement Method
Figure 2 shows a simplified configuration for measuring this unwanted amplitude modulation. This is the same configuration as that used to measure differential gain and differential phase, but an IF envelope detector is used in the receiving section instead of the measuring demodulator and its subsequent circuits. The modulator under test is excited by a swept signal source (without the test signal in the transmitting section), and the IF output level is detected by the IF envelope detector. The output of the detector is used for the vertical deflection of the display. The shape of this characteristic curve within the specified frequency range is a measure of the amplitude modulation. The sweep width corresponding to the highest frequency deviation under the operating conditions is selected for this measurement. The receiving section of commercial line analyzers has an operating type switch that allows the measurement of differential gain or differential phase through the measuring demodulator as detailed in 8.2, and also allows the measurement of the IF amplitude/frequency characteristic through the IF envelope detector. 9.3 Presentation of results
The measurement results of unwanted amplitude modulation of frequency modulators are preferably presented by a photograph of the intermediate frequency amplitude/frequency characteristic displayed on an oscilloscope with both axes properly calibrated, a diagram, or a curve recorded by an XY recorder. The difference between the extreme values of the characteristic and the sweep range used may also be described in words. 9.4 Details to be specified
When this measurement is required, the equipment specifications shall include the following: a. Sweep width, MHz;
b. Tolerance of the intermediate frequency amplitude/frequency characteristic.
10 Baseband amplitude/frequency characteristic
10.1 Definition
GB/T11299.9—1989
The baseband amplitude/frequency characteristic of a modulator is a curve showing the ratio of the intermediate frequency deviation to the reference frequency deviation (expressed in decibels) as a function of the baseband modulation frequency under the condition that the baseband input signal amplitude remains unchanged. The reference frequency deviation is the frequency deviation at the specified baseband frequency. 10.2 General Considerations
Measurement of the baseband amplitude/frequency characteristic of a modulator requires a measurement demodulator. By definition, a measurement demodulator measuring this characteristic should provide a nominal constant amplitude baseband output signal as a function of the modulation frequency when the frequency deviation of its input signal is constant. Small amplitude baseband signals should be used for the measurement to avoid the presence of higher order sideband signals of significant amplitude at higher modulation frequencies. If the modulator under test cannot be separated from its pre-emphasis network, the measurement demodulator must be equipped with a calibrated corresponding pre-emphasis network. However, in some cases, the pre-emphasis network can be separated from the modulator, so that the amplitude/frequency characteristic of the basic modulator can be measured. In this case, the baseband amplitude/frequency characteristic of the pre-emphasis network should be measured separately. NOTE: It is currently not possible to separate the contribution of the measured modulator/measurement demodulator to the overall baseband frequency characteristic, since the contributions of the measurement demodulator/modulator under test are of the same order of magnitude. Therefore, this measurement is usually performed using the modulator and demodulator of the system under test, and only the overall characteristic is specified. 10.3 Measurement method
Figure 3 of this series of standards GB/T11299.4 shows the measurement configuration. The "device under test" between the baseband terminals mentioned is composed of a measurement demodulator and a modulator under test connected to each other at an intermediate frequency. 10.4 Result presentation
When the frequency sweep method is used for measurement, the results should be presented using a photograph of the oscilloscope display, a graph, or a curve recorded by an XY recorder. When the measurement results are not presented graphically, they should be presented as follows: "The baseband amplitude/frequency characteristics of the modulator (or modulator and demodulator connected back to back) are between +0.2 and -0.1 dB from 300 kHz to 8 MHz relative to the value at 1 MHz." When measuring point by point, it can be presented in a table or as shown in the above example. 10.5 Details to be specified
When this measurement is required, the equipment technical specifications should include the following: a. Baseband reference frequency,
b. Baseband frequency range;
c. Tolerance of baseband amplitude/frequency characteristics; d. Intermediate frequency deviation at the reference frequency point
e. Pre-emphasis/de-emphasis characteristics (when necessary). 11 Measurement of frequency division multiplexing telephone
It is currently impossible to measure the contribution of the measured modulator to the intermodulation noise separately, because the measurement demodulator also contributes noise of the same order of magnitude. Therefore, it is usually necessary to use the system demodulator to perform noise measurements according to the measurement method given in Section 4 of Part 3 of this series of standards "Measurement of frequency division multiplexing transmission", and only specify the noise value of the entire modulator/demodulator. For the measurement of the basic noise of the modulator that is independent of the load, the baseband measurement can be performed in the absence of modulation and connected to a measurement demodulator with known basic noise characteristics (see Chapter 10 of GB/T11299.9).
12 Television measurement
It is currently impossible to measure the contribution of the measured modulator to the waveform distortion separately, because the measurement demodulator also contributes distortion of the same order of magnitude. Therefore, it is usually necessary to use a system demodulator to perform distortion measurements according to the method given in the system standard GB/T11299.14 "Measurement of television transmission", and only specify the distortion value of the entire modulator/demodulator. Note: Most nonlinear waveform distortions are not affected by the basic modulator/demodulator itself, but by the baseband part (including band-limiting filters, pre-emphasis and de-emphasis networks, etc.). If these parts can be separated, their performance can be measured directly at the baseband. When measuring the basic noise of the modulator (that is, when not modulated), the modulator can be connected to a measurement demodulator with known basic noise performance for baseband measurement.
13 Carrier Energy Dispersion Subsystem
13.1 Measurement Method
GB/T112 99.9—1989
The following measurements can be performed on the carrier energy dispersal subsystem (see reference 3). a. Connect the oscilloscope to the output of the energy dispersal signal generator (see Figure 1) and check the waveform amplitude and synchronization of the signal (if necessary).
For television transmission, verify whether the energy dispersal signal is synchronized with the field synchronization waveform of the television signal. b. To measure the frequency stability of the energy dispersal signal over time, a frequency meter connected to a recorder should be used. c. In order to measure whether the amplitude of the energy dispersal signal changes with the baseband load, a measurement demodulator with a baseband low-pass filter (such as 2kHz) should be connected to the intermediate frequency output of the modulator of the system under test, and the amplitude of the energy dispersal signal should be measured at the baseband output. The amplitude is the baseband function of the input signal level. The baseband input signal is the white noise test signal for frequency division multiplexed telephones (see Chapter 11 of this section) and the television test signal for television (see Chapter 12 of this section). 13.2 Presentation of results
The measurement results shall be presented as follows:
a. A photograph of the energy dispersal signal waveform and an indication of its amplitude; b. A record of the frequency stability of the energy dispersal signal generator over a specified time; c. The variation of the energy dispersal signal level with baseband load. 13.3 Details to be specified
When this measurement is required, the equipment specifications shall include the following: a. Amplitude of the signal,
b. Frequency and tolerance,
c. The permitted level variation with baseband load. Section 8
Frequency demodulator
14 Subject matter and scope
This section gives methods for measuring the electrical performance of frequency demodulators. This includes the measurement of thresholds and carrier-to-noise ratios, since these are essential parameters in satellite communication systems. As far as possible, only basic demodulator measurements are covered, excluding devices consisting of a de-emphasis network and a network combining sound subcarrier signals, pilot signals and auxiliary signals. Section 7 gives methods for measuring frequency modulators. Measurements between baseband terminals of modulator/demodulator combinations are described in the sections of Part 3 of this series of standards.
15 Definitions
For the purposes of this standard, a frequency demodulator is a subsystem that demodulates an intermediate frequency carrier modulated by a baseband signal in an analog manner. The baseband signal may be a frequency division multiplexed telephone or television signal with a sound subcarrier signal, pilot signal and auxiliary signal. Such baseband signals are usually analog signals, but digital signals are not excluded. However, the measurement methods described in this section are only used to evaluate the performance of the demodulator when transmitting analog signals.
The demodulator subsystem usually consists of the following three main parts: an intermediate frequency part
an intermediate frequency to baseband part (such as a frequency discriminator); and a baseband part.
16 Overview
Figure 3 is a block diagram of a typical satellite communication earth station demodulator. 7
GB/T11299.9—1989
There are two different types of demodulators in common use, namely ordinary demodulators and threshold extension demodulators. The characteristics that need to be measured can be divided into three categories: non-transfer characteristics;
intermediate frequency to baseband characteristics,
some baseband to baseband transfer characteristics when connected to the measurement modulator. The first type of measurement applies to measurements at the intermediate frequency input end (Chapter 4) and measurements at the baseband output end (Chapter 5). The second type of measurement forms the basic part of this section. This is due to the nature of the component under test that transforms from intermediate frequency to baseband. In order to evaluate the effect of the IF input level, some specified measurements shall be made at the specified nominal, minimum and maximum values of the IF input level. Note: Measurements of the effect of amplitude modulation are not included in this standard. Since the input level is assumed to be well within the operating range of the limiter, the subsequent AM/PM conversion can be ignored.
The third type of measurement is made on a complete modulator/demodulator set, but the actual or system modulator shall be replaced by the measurement modulator.
Because in actual applications a demodulator of one design or manufacturer may work with modulators of other designs or manufacturers, it is necessary to know the effect of the demodulator on the overall performance tolerance. Compensation effects between demodulator and modulator are undesirable. Each demodulator connected to the measurement modulator shall meet the specified technical conditions, which requires that the characteristics of the measurement modulator be better than those of the demodulator under test.
17 IF input return loss
According to this series of standards GB/T11299.3 "Measurements within the IF range", it may also be necessary to measure at the IF harmonics. 18Baseband output impedance and return loss
Measured according to this series standard GB/T11299.4\Baseband". 19Frequency deviation sensitivity
19.1 Definitions and General Considerations
For a sinusoidal signal of a given frequency, the frequency offset sensitivity S of a demodulator can be expressed as the ratio of the baseband output voltage V to the frequency offset Af:
S—(V/MHz)
V. Both ∆f and ∆f are expressed as peak or RMS values. (8)
Due to the effects of the de-emphasis network, the frequency offset sensitivity of a demodulator is usually a function of the baseband frequency. However, in some cases, it is possible to obtain the baseband output point before the de-emphasis network (Figure 3), in which case the measured demodulator frequency offset sensitivity is independent of the baseband frequency used. 19.2 Measurement Methods
Using a test signal with a known frequency offset, there are two methods for obtaining the frequency offset sensitivity, namely the Bessel null method and the dual signal method discussed below.
In the first method, a relatively low modulation frequency (e.g., less than 2 MHz) is used and the measurement is made at a modulation index of exactly 2.40483. The second method requires a higher modulation frequency (e.g. greater than 2 MHz) and a low modulation index (e.g. not exceeding 0.2). Therefore, the latter method is particularly suitable for measurements at pilot and sound subcarrier frequencies. 19.2.1 Bessel null method
Figure 4 shows a configuration suitable for measuring and calibrating the frequency deviation sensitivity of the demodulator. The measurement method is the Bessel null method. The calibration of the modulator frequency deviation is based on the fact that in the case of sinusoidal modulation, the carrier spectrum disappears for the first time at the following modulation index.
Where: Af - peak frequency deviation;
at - modulation frequency.
GB/T 11299.9—1989
This "zero point" seen on the spectrum analyzer, that is, the point where the intermediate frequency carrier disappears for the first time, cannot be an ideal zero value due to the influence of the residual harmonic distortion of the baseband signal generator. However, it is sufficient when the carrier level drops by 30 dB or more. Because there are multiple modulation index values that can obtain the carrier zero point, the best way to determine the first zero point is to increase the modulation voltage steadily from zero until the carrier disappears for the first time. The measurement steps are as follows:
a. Adjust the baseband generator to the frequency required to measure the frequency deviation sensitivity, b. Set the output voltage of the generator to zero, and then steadily increase the voltage until the intermediate frequency carrier disappears for the first time on the spectrum analyzer, c. Measure the root mean square voltage Vbd of the demodulator baseband output. Then calculate the demodulator frequency deviation sensitivity Sa at the modulation frequency f according to formula (10): 2V
Sa-2.40483f
(V/MHz)
Note: Because the intermediate frequency bandwidth corresponding to the modulation index of 2.40483 increases linearly with the increase of the modulation frequency, when using this method to measure the frequency deviation sensitivity, the modulation frequency must be limited so that the spectrum of the modulated signal does not exceed the system bandwidth. Another method is to use a calibrated measurement demodulator to measure the frequency deviation sensitivity.
19.2.2 Dual Signal Method
Figure 5 shows a configuration suitable for measuring frequency deviation sensitivity using the dual signal method. This method is often used to calibrate the frequency deviation sensitivity of a demodulator at low modulation indices (not exceeding 0.2) and high modulation frequencies (between 2 and 10 MHz). This method is therefore particularly suitable for measurements at pilot signals and sound subcarrier frequencies. Two intermediate frequency crystal oscillators are used to generate the exact frequency deviation at a specified frequency point. The two oscillators have equal output levels but different frequencies. The first oscillator operates at the nominal carrier frequency (for example, 70 MHz) and the second oscillator operates at a frequency that differs from the carrier frequency by a known value fx.
As shown in Figure 5, the output signal of crystal oscillator 2 is appropriately attenuated as specified below and added to the signal of crystal oscillator 1. Attenuator 2 is then adjusted to make the level of the composite signal suitable for the input level of the demodulator under test. Due to the limiting action of the demodulator, a pure angle modulated signal is actually generated. In order to reduce the unwanted amplitude modulation, an external limiter must be inserted before the demodulator under test. This limiter should have a low AM/PM conversion to reduce the measurement error to an acceptable level. The RMS frequency deviation is given by the following formula:
Where: al-
-the voltage attenuation of attenuator 1.
From this formula, the required attenuation α can be calculated. For example, to produce a 140kHz RMS frequency deviation at f of 8500kHz, the required attenuation is 201og10gl. αl is: al
This value corresponds to 32.7dB.
In practice, a sufficiently high modulation frequency should be used so that fx>Af (for example: 201oga'>14dB). When a known frequency deviation is generated using the above method, the frequency deviation sensitivity of the demodulator can be calculated using formula (13): 2Vng
where: V is the root mean square voltage of the demodulator output frequency fx. 19.3 Expression of results
(V/MHz)
The measurement results shall be expressed as follows:
GB/T11299.9—1989
“The frequency deviation sensitivity S.) is ….V/MHz”, or when the RMS frequency deviation is ….kHz, the baseband output level is …dBm”. 19.4 Details to be specified
When this measurement is required, the equipment specifications shall not include the following: a. Measurement method (6.2.1 or 6.2.2); b. Modulation frequency of the intermediate frequency input signal when the Bessel null method is used. Or the difference between the two input carrier frequencies fx when the dual signal method is used
c. Frequency deviation of the intermediate frequency input signal;
d. Required frequency deviation sensitivity, or the output at the specified frequency deviation. Output level; e. Baseband connection point (i.e. before or after the de-emphasis network, see Figure 3), f. De-emphasis characteristics used (if used); name. Intermediate frequency input level (maximum, nominal and minimum values). 20 Demodulation polarity
20.1 Definition and general considerations
If the increase in intermediate frequency causes a positive change in the output voltage, the demodulation polarity of the frequency demodulator is positive. It can be seen from this series of standards GB/T11299.14 that modulation polarity is very important in television transmission. 20.2 Measurement method
A simple way to check the demodulation polarity is to add an asymmetrical waveform to the measurement modulator of known modulation polarity, and apply this intermediate frequency signal to the demodulator under test. If the polarity of the demodulator output signal is the same as the polarity of the modulator input signal, the demodulation polarity is the same. The known modulation polarity is the same.
Another method is to use a low-frequency modulation signal to cause a large frequency deviation in the intermediate frequency carrier. And add this modulated carrier and a small-amplitude continuous wave intermediate frequency carrier of known frequency to the input of the demodulator under test. At the output of the demodulator, the beat frequency between the interfering carrier and the modulated carrier can be seen on the oscilloscope. If the intermediate frequency of the interfering carrier is increased, the voltage at the beat frequency point also increases, then the demodulation polarity is positive. The measurement configuration and oscilloscope display are shown in Figure 6. 21 Differential Gain/Nonlinearity and Differential Phase/Group Delay 21.1 Definitions and General Considerations
The demodulator under test is excited by an intermediate frequency carrier. The intermediate frequency carrier is modulated by a sinusoidal test signal superimposed on a low-frequency sweep signal. The frequency deviation generated by the sinusoidal test signal is constant in size and phase. At the demodulator base At the output end of the band, the amplitude and phase of the demodulated test signal depend on the instantaneous value of the scanned carrier frequency. The differential gain (DG) and differential phase (DP) of the demodulator under test are defined as functions of this instantaneous value. Given by the following formula: DG(x)=4()-1
DP(X)=(X)—虫
Where: X——the instantaneous value of the input carrier frequency, DG(X)——demodulator differential gain, which is a function of X; - the output test signal amplitude, which is a function of X; A(X)-
A. The amplitude of the output test signal at the carrier frequency point in the middle of the band; DP(X)-
-demodulator differential phase, which is a function of X; d(x)—
output test signal phase, which is a function of X: - the output test signal phase at the carrier frequency in the middle of the band. (14)
For an ideal demodulator without distortion, the differential gain and differential phase are both zero, while for a practical demodulator, the above function will be 10
GB/T11299.9—1989
presents variables. The characteristics of actual demodulators can be expressed either by these functions themselves or by differential gain distortion and differential phase distortion. Differential gain distortion and differential phase distortion are both defined as the difference between the two extreme values of the above functions, usually expressed in percentage and degree respectively: AmaxAmin
DG distortion (percentage) = 100
DP distortion (degrees) = dmax—nin
The choice of test signal frequency depends on which part of the demodulator is to be evaluated and which parameters need to be measured (i.e. differential gain or nonlinearity, differential phase or group delay). The definitions of nonlinearity and group delay and the factors to be considered in choosing the test signal frequency are given in this series of standards GB/T11299.4 "Baseband Measurements".
The same method is used to measure differential gain and nonlinearity, but the test frequencies used are different. Nonlinearity is an important performance parameter of a demodulator because it represents the deviation of the output voltage/input frequency characteristic from an ideal linear response. The test signal frequency used to measure nonlinearity is relatively low, typically in the range of 50 to 500 kHz. 21.2 Measurement Methods
The measurement of differential gain/nonlinearity and differential phase/group delay of a demodulator requires an ideal modulator. By definition, an ideal modulator, when stimulated by a test and swept composite signal, has a constant magnitude and phase of the frequency deviation produced by the test signal and is independent of the instantaneous value of the swept carrier frequency.
When testing in this way, an approximately ideal modulator can be constructed by the following method. Two modulators are used at frequencies much higher than the intermediate frequency, and the difference between the two is the intermediate frequency. One modulator is frequency modulated with the swept signal and the other modulator is frequency modulated with the test signal. The swept intermediate frequency signal with constant test signal frequency deviation magnitude and phase is heterodyned from these two signals to the intermediate frequency signal. Figure 7 shows a simplified measurement equipment configuration for the differential gain and differential phase of a demodulator. The configuration of the ideal modulator is shown in the dashed line marked "transmit part". In the dashed line marked "receive part", the test signal component is taken out by a bandpass filter tuned to the test frequency, and the amplitude and phase modulation of the output test signal are detected by an envelope detector and a phase detector. The differential gain and differential phase signals are then supplied to the vertical axis of the display. In some cases, a low-pass filter is configured at the output of the demodulator to separate the sweep voltage applied to the oscilloscope. In other cases, this voltage is provided by a sweep signal generator. A suitable phase shifter is also required for the measurement. Note: ① Commercial test equipment, usually called "line analyzer", has the test configuration function within the dashed line of Figure 7. Although not shown in Figure 7, this test equipment usually has additional equipment to calibrate the vertical and horizontal axes of the display. ② When a high test frequency is used, the measured frequency range will not be close to the sweep width, but approximately the sweep width plus twice the test signal frequency.
③ It must be ensured that the baseband amplifier after the demodulator is not overdriven by the amplitude sweep signal. To meet this requirement, the applied sweep width should normally be limited. Alternatively, the baseband portion of the demodulator may be excluded from the measurement, thus allowing sufficient sweep width to measure the characteristics of the entire demodulator. The baseband amplifier must also be removed from the measurement when the low-end cutoff frequency of the baseband amplifier prevents the transmission of the sweep signal. 21.3 Presentation of results
The results of differential gain and differential phase measurements are best presented as a photograph of the function displayed on an oscilloscope with both axes properly calibrated. Usually a single photograph is used to present both functions. Alternatively, a textual description of differential gain distortion, differential phase distortion and sweep range may be provided. 21.4 Details to be specified
When this measurement is required, the following shall be included in the equipment specifications: a. Intermediate frequency sweep range (e.g. ±10 MHz); b. The differential gain distortion allowed within the above range (e.g. 3%); c. The differential phase distortion allowed within the above range (e.g. 0.8); d. The test frequency used;
e. The baseband connection point (i.e. before or after the baseband amplifier); f. The intermediate frequency input level (maximum, nominal and minimum). 22 Baseband Amplitude/Frequency Characteristics
22.1 Definition
Tip: This standard content only shows part of the intercepted content of the complete standard. If you need the complete standard, please go to the top to download the complete standard document for free.