GB/T 4958.6-1988 Measurement methods for equipment used in terrestrial radio-relay systems Part 2: Subsystem measurements Section 5: Frequency demodulator
Some standard content:
National Standard of the People's Republic of China
GB/T4958.6—1988
idtIEC487——2—5
Methods of measurement for equipment used in terrestrial Radio-relay systems
Part 2.Measurementsforsub-systemsSectionFive-FreguencydemodulatorsPromulgated on March 28, 1988
Implementation on February 1, 1989
Ministry of Posts and Telecommunications of the People's Republic of China
National Standard of the People's Republic of China
Methods of measurement for equipment used in terrestrial Radio-relay systemsPart 2: Measurements for sub-systems
SectionV-Frequencydemodulators
Methods of measurement for equipment used in terrestrial Radio-relaysystems
Part 2:Measurements for sub-systems Section Five-Frequency demodulators 621.396:
621.317.08
GB/T4958.6—1988
IEC487—2—5
This standard is one of the national standards "Measurement methods for equipment used in ground radio-relay systems" series. This standard is equivalent to the international standard IEC487-2—5 "Measurement methods for equipment used in ground radio-relay systems Part 2: Measurement of subsystems Section 5-Frequency demodulators". 1 Scope
This section gives the measurement methods for the electromechanical characteristics of frequency demodulators. Because line-of-sight radio-relay systems usually do not require threshold characteristics, they are not included here. Moreover, if possible, only the measurement of the basic demodulator is considered, and the baseband part of the demodulator is not included, including the de-emphasis network and the network related to the accompanying audio subcarrier signal, pilot signal and auxiliary signal. The measurement methods for frequency modulators have been given in Section 4. The end-to-end measurements of the modulator and demodulator system baseband are given in the sections of Part 3 of this series of standards.
2 Definitions
For the purpose of this standard, a frequency demodulator is a subsystem that demodulates, in an analog manner, an intermediate frequency signal modulated with a baseband signal, which may be a multi-channel telephone signal or a television signal including audio subcarrier signals, pilot signals and auxiliary signals. This baseband signal is usually analog, but digital signals are not excluded. However, the measurement methods described in this section are intended to evaluate the performance of the demodulator when transmitting analog signals.
The demodulator subsystem usually consists of the following three main parts: the intermediate frequency section;
-IF to baseband section (discriminator);
baseband section.
3 OverviewwwW.bzxz.Net
The configuration of a typical demodulator used in a terrestrial radio-relay system is shown in Figure 1. The measured characteristics can be divided into the following three categories: non-transmission characteristics;
-IF to baseband characteristics;
-When measuring modulators, certain baseband to baseband transmission characteristics. The first type of measurement applies to the measurement of the IF input end (Article 4) and the measurement of the baseband output end (Article 5). Approved by the Ministry of Posts and Telecommunications of the People's Republic of China on March 28, 1988, and implemented on February 1, 1989
GB/T4958.6—1988
The second type of measurement is the main part of this standard, because the characteristic of the equipment under test is to convert the IF to baseband. In order to evaluate the influence of the IF input level, certain specified tests should be carried out under the conditions of rated, minimum and maximum specific IF input levels. Note: In this standard, the measurement of the influence caused by amplitude modulation is not included. Because the input levels are all within the working range of the limiter, it is assumed that the amplitude modulation/phase modulation conversion of the limiter can be ignored.
The third type of measurement includes the entire modulator and demodulator system, that is, it only includes the case where the measured modulator replaces the actual modulator or system modulator.
It is necessary to understand the individual impact of the demodulator itself on the total characteristic tolerance. Because a demodulator of one design or one manufacturer may work with another modulator. Compensation between modulator and demodulator is undesirable. Each demodulator and the measured modulator should meet the specified technical conditions when connected. This method requires that the performance of the measured modulator is better than the specified performance of the measured demodulator. 4 Intermediate frequency input return loss
See Section 3 of Part 1 of this series of standards - Measurements within the intermediate frequency range. (GB6662-86) It may also be required to measure at the intermediate frequency harmonics.
5 Baseband output impedance and return loss
See Section 4 of Part 1 of this series of standards: Baseband measurements. 6 Demodulator Frequency Offset Sensitivity
6.1 Definitions and General Considerations
For a sinusoidal signal of a given frequency, the frequency offset sensitivity S of a demodulator is expressed as the ratio of the baseband output voltage Vf to the frequency offset A:
(V/MHz)
Both parameters Vf and Af are expressed as peak values or as effective values. (1)
Due to the influence of the de-emphasis network, the frequency offset sensitivity of a demodulator is usually a function of the baseband frequency. Sometimes, the baseband output voltage Vf can be obtained before the de-emphasis network (see Figure 1). The frequency offset sensitivity of the demodulator under test is independent of the baseband frequency used. 6.2 Measurement Methods
There are two methods for measuring frequency offset sensitivity using a test signal with a known precise frequency offset, namely the Bessel null method and the dual signal method discussed below.
The first method uses a precisely given modulation index of 2.40483 and performs measurements at relatively low frequencies (e.g., below about 2 MHz). The second method uses a low modulation index (e.g., not exceeding about 0.2) and performs measurements at relatively high modulation frequencies (e.g., greater than 2 MHz). Therefore, the second method is particularly useful for measurements at the pilot and sound subcarrier frequencies. 6.2.1 Bessel Null Method
In order to measure the demodulator frequency deviation sensitivity and calibrate the frequency deviation of the measurement modulator, a reasonable configuration is shown in Figure 2. This measurement method is called the Bessel Null Method. The calibration of the frequency deviation sensitivity of the measured modulator is based on the following fact: In the case of sinusoidal modulation, the carrier frequency spectrum line disappears for the first time when the modulation index m is given by the following formula: 4f = 2.40483
Where: Af is the peak frequency
So the modulation frequency
The "zero point" or the first disappearance point of the intermediate frequency carrier is observed on the spectrum analyzer, but due to the residual harmonic distortion of the baseband signal generator, it is impossible to obtain a true zero value. However, a reduction of the carrier level by 30dB or more can be considered to have reached the zero value. Because there are many values of the modulation index that can obtain a carrier zero, the best way to ensure the first zero point is to increase the modulation voltage smoothly from zero to the point where the carrier first disappears.
The measurement procedure is as follows:
GB/T 4958.6—1988
a. Adjust the baseband generator to the frequency required to measure the frequency deviation sensitivity. b. First, set the output level of the signal generator to zero, and then steadily increase the output level of the signal generator until the IF carrier disappears for the first time on the spectrum analyzer.
c. Measure the effective value of the voltage Vb at the baseband output of the demodulator. d. When the modulation frequency is fi, the demodulator frequency deviation sensitivity is calculated by the following formula: 2V
Sa-2.40483fi
(V/MHz)）
Note: Since the modulation index 2.40483 corresponds to an IF bandwidth that increases linearly with the modulation frequency. Therefore, the application of this method is limited to modulation frequencies not exceeding one-third of the highest baseband frequency. 6.2.2 Dual Signal Method
In order to measure the demodulator frequency deviation sensitivity using the dual signal method, a reasonable configuration is shown in Figure 3. This method of calibrating the demodulator frequency deviation sensitivity is suitable for using low modulation indices (maximum about 0.2) and high modulation frequencies (2 to 10 MHz). Therefore, this method is particularly suitable for pilot and sound subcarrier frequencies.
Use two intermediate frequency crystal oscillators with equal output levels but different frequencies to produce an accurate frequency deviation at a specified frequency. The first crystal oscillator operates at the nominal carrier frequency (i.e. 70MHz), and the second frequency differs from the nominal carrier frequency by a known value fx. As shown in Figure 3, the output signal of crystal oscillator No.2 is appropriately attenuated as specified below and added to the signal of crystal oscillator No.1. Then, attenuator No.2 Adjust the mixed signal level to a level suitable for the input level of the demodulator under test. Due to the limiting effect of the demodulator, an almost pure angle modulated signal can be generated. In order to reduce the undesired amplitude modulation, an external limiter must be inserted before the demodulator under test. The amplitude modulation/phase modulation conversion of this limiter should be very small so that the measurement error can be reduced to an acceptable value. The effective frequency deviation is given by the following formula:
Where: α' is the voltage attenuation value of attenuator 1. (4)
From this formula, the required voltage attenuation value α can be calculated. For example, in order to produce an effective value of 140kHz frequency deviation at a frequency f of 8500kHz, the required attenuation is 201og10α', where a is given by the following formulaal
It corresponds to 32.7dB.
In practice, a sufficiently high modulation frequency is used to satisfy f>Af (i.e. 201og10a>14dB). When the frequency deviation is known by the above method, the frequency deviation sensitivity of the demodulator can be calculated by the following formula: 2. Va (V / MHz)
Where: V is the effective value of the voltage at the output of the demodulator when the frequency is fx. 6.3 Expression of results
The results should be given in the following way:
"Frequency deviation sensitivity (S) is .V/MHz" or "Baseband output level when the effective value of frequency deviation is ...kHz is .dBm" 6.4 Details to be specified
If necessary, the detailed equipment specification should include the following: a. Measurement method (6.2.1 or 6.2.2); b. Modulation frequency of the intermediate frequency input signal when using the Bessel null method or the difference between the two input carrier frequencies when using the dual signal method 3
c. Frequency deviation of the intermediate frequency input signal
GB/T4958.6—1988
d. Required frequency deviation sensitivity or output level at a specified frequency deviation e. Baseband connection point (i.e. before or after de-emphasis, see Figure 1); f. De-emphasis characteristics used (whether appropriate); g. IF input level (maximum, nominal and minimum). 7 Demodulation direction
7.1 Definitions and general considerations
The demodulation direction of a frequency demodulator is positive if an increase in the IF frequency causes the output voltage to change in the positive direction. In television transmission, the demodulation direction is very important.
7.2 Measurement method
A simple method to check the direction of a demodulator is to modulate a measurement modulator with a known modulation direction with an asymmetrical waveform signal and apply the IF signal output by the modulator 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 direction is the same as the known modulation direction. Another approach is to use a low frequency modulating signal to generate a large IF deviation. This modulated carrier is then applied to the input of the demodulator under test together with a small signal of equal amplitude at a known IF frequency. At the demodulator output, the beat frequency between the interfering carrier and the modulated carrier can be seen on the oscilloscope screen. If the beat frequency moves to a higher level as the interfering carrier frequency increases, the demodulation direction is positive.
The measurement configuration and oscilloscope display are shown in Figure 4. 8 Differential Gain/Nonlinearity and Differential Phase/Group Delay 8.1 Definitions and General Considerations
The demodulator under test is excited by an IF carrier which has been modulated by a sinusoidal test signal of equal amplitude and constant phase difference superimposed on a low frequency swept signal. At the baseband output of the demodulator, the demodulated test signal amplitude and phase are related to the instantaneous value of the swept carrier frequency. The differential gain and differential phase of the demodulator under test are defined as a function of this instantaneous value, given by the following equation: DG(X)=A(X)
DP(X)=(X)-0
where X is the instantaneous value of the input carrier frequency;
represents the function of the differential gain of the demodulator; A(X) is the amplitude of the output test signal, which is a function of X, Ao
is the amplitude of the test signal output at the center frequency of the carrier frequency;
represents the function of the differential phase of the demodulator; DP(X)-
g(x) is the phase of the output test signal, which is a function of X, o
is the phase of the test signal output at the center frequency of the carrier frequency. (7)
For an ideal demodulator without distortion, the differential gain and differential phase are both zero. For an actual demodulator, the above functions will be variable. The actual demodulator is characterized either by these functions themselves or by differential gain and phase distortion. The latter is defined as the difference between the extreme values of the above functions, usually expressed in percentage and degree respectively as follows: DG distortion (percentage) = 100 × (Am × two Amm) Ao
DP distortion (degrees) = Pmx one min
The choice of test signal frequency depends on the part of the demodulator to be evaluated and the parameters to be measured (i.e. differential gain or nonlinearity, differential phase or group delay). The definitions of nonlinearity and group delay and some factors that determine the choice of test signal frequency are given in Section 4 of Part 1 of this series of standards 4
"Baseband Measurements". GB/T4958.6—1988
DG and nonlinearity are measured in the same way, but using different test frequencies. Nonlinearity is an important performance parameter of the demodulator because it indicates the deviation of the output voltage/input frequency characteristic from the ideal linear response. Nonlinearity measurements use relatively low test signal frequencies, which typically range from 50kHz to 500kHz. 8.2 Measurement Methods
In order to measure the differential gain/nonlinearity and differential phase/group delay of a demodulator, an ideal modulator is required. By definition, when an ideal modulator is excited with a composite test signal and a swept signal, it will produce a test signal modulation with a constant frequency deviation and phase, which is independent of the instantaneous value of the swept carrier frequency.
For this purpose, the following configuration is a very close approximation to an ideal modulator. Two modulators with frequencies much higher than the intermediate frequency are used, which are separated in frequency by an intermediate frequency, one of which is modulated by the swept signal and the other by the test signal. By heterodyning the two signals to the intermediate frequency, a swept intermediate frequency test signal is produced, which has a constant frequency deviation in amplitude and phase. A simplified configuration for measuring demodulators DG and DP is shown in Figure 5. The above ideal modulator is configured within the dotted line marked with "transmitting part" in the figure. Within the dotted line marked with "receiving part". A bandpass filter tuned to the test frequency is used to extract the test signal component. An envelope detector and a phase detector are used to detect the amplitude modulation and phase modulation of the output test signal, and the DG and DP signals are supplied to the vertical deflection of the display. In some cases, a low-pass filter is configured at the output of the demodulator to obtain the scanning voltage supplied to the oscilloscope. In other cases, this voltage can be provided by the scanning signal generator. A suitable phase shifter is also required. Note: ① Commercial test equipment, usually called "line ② When using a high-frequency test signal, the frequency range of the probe will not be close to the sweep width but will be approximately the sweep width plus twice the test signal frequency. ③ It must be ensured that the baseband amplifier behind the demodulator is not overloaded when the amplitude sweep signal is large. To meet this requirement, the sweep width is often limited to the applicable width. In addition, the baseband part of the demodulator can be separated from the measurement part, which allows the sweep width to be wide enough to measure the characteristics of the entire demodulator. . When the low-end cutoff frequency of the baseband amplifier is high and cannot transmit the scanning signal, it is also necessary to separate the baseband amplifier part from the demodulator.
8.3 Representation of results
The differential gain and differential phase are best given by displaying photos of functions with appropriate calibration on two axes. Usually a photo that can display two functions at the same time is given. In addition, the measured differential gain distortion, differential phase distortion and scanning range can be described in words. 8.4 Details to be specified
If necessary, the detailed equipment specifications should include the following items: a. Intermediate frequency scanning range (for example ±10MHz); b. Within the above range c. The maximum differential gain distortion allowed within the above range (e.g. 3%) c. The maximum differential phase distortion allowed within the above range (e.g. 0.8°); d. The test frequency used;
e. The baseband connection point (e.g. before or after the baseband amplifier) f. The intermediate frequency input level (maximum, nominal and minimum values). 9 Baseband Amplitude/Frequency Characteristics
9.1 Definition
The baseband amplitude/frequency characteristic of the demodulator is a curve that represents the ratio of the baseband output level to the reference level (expressed in decibels). It is a function of the baseband modulation frequency when the intermediate frequency input frequency deviation is constant. The reference level is the output level at the specified baseband frequency. 9.2 General considerations
To measure the baseband amplitude/frequency characteristic of a demodulator, a measurement modulator is required. By definition, a measurement modulator for measuring this characteristic provides an IF output signal with a constant frequency deviation, which is a function of the input baseband frequency at a constant baseband input level. In order to avoid high-order sidebands of significant amplitude at the highest modulation frequency, a small frequency deviation should be used. If the demodulator under test cannot be separated from the de-emphasis network, a measurement modulator with a calibrated and corresponding pre-emphasis network must be used. However, in some cases, the de-emphasis network can be separated from the demodulator, so that the amplitude/frequency characteristic of the basic demodulator can be measured. In this case, the baseband amplitude/frequency characteristic of the de-emphasis network should be measured separately. The demodulator baseband amplitude/frequency characteristic is preferably measured under several specified IF input level conditions. NOTE: Currently, it is not possible to isolate the contribution of the baseband frequency characteristics of the modulator/demodulator under test, because the contribution of the measuring demodulator/modulator is of the same order of magnitude as that of the modulator/demodulator under test. It is therefore customary to test the demodulator/modulator system and to specify the overall modulator/demodulator characteristics. 9.3 Measurement method
Figure 3 in Section 4 of Part 1 of this series of standards gives a diagram of the measurement configuration. Note that the "equipment under test" between the baseband terminal and the terminal consists of a measured modulator and a measured demodulator interconnected at an intermediate frequency. 9.4 Presentation of results
For swept frequency measurements, a photograph or an XY record of the oscilloscope display should be given. When the measurement results are not given graphically, they should be given as follows:
"The baseband amplitude/frequency characteristic of the demodulator (or modulator and demodulator connected back to back) from 300kHz to 8MHz is within the range of +0.2dB to -0.1dB relative to the value at the 1MHz point". Point-by-point measurements can be given in a table or in the above presentation method. 9.5 Details to be specified
If necessary, the following items should be included in the detailed equipment specification: a. Baseband reference frequency,
b. Baseband frequency range;
|c. Tolerance of baseband amplitude/frequency characteristics;
d. IF frequency deviation at the reference frequency point;
e. Pre-emphasis/de-emphasis characteristics (when required); f. IF input level (maximum, nominal and minimum values) 10 Frequency-division multiplexed telephone measurements
Currently, it is not possible to separate the intermodulation noise contribution of the demodulator under test because the measured modulator has a contribution of the same order of magnitude as the demodulator under test. Therefore, for this test, the system modulator is usually used and only the total modulator/demodulator noise value is specified. In addition to the details that should be specified listed in Section 4 of Part 3 of this series of standards "Frequency-division multiplexing measurements", the IF input level range should also be specified.
In order to measure the basic noise of the demodulator (i.e., without Noise loading), a very low noise constant amplitude wave generator such as a crystal oscillator or synthesizer may be used in place of the unloaded system modulator. 11 Television measurements
Currently, it is not possible to isolate the waveform distortion contribution of the demodulator under test, because the measurement modulator has a contribution of the same order of magnitude as the demodulator under test. Therefore, for this test the system modulator is usually used and only the total modulator/demodulator distortion value is specified. The measurement methods used are given in Section 3 of Part 3 of this series of standards, "Measurements on black-and-white and color television transmissions". NOTE: Most linear and non-linear waveform distortions are not affected by the basic modulator/demodulator, but by the baseband sections (including band-limiting filters, pre-emphasis networks, de-emphasis networks, etc.). In some cases, these sections can be separated and their performance can be compared. The measurement is made directly at the baseband. In addition to the measurements specified in Section 3 of Part 3 of this series of standards, the IF input level range may also be specified. In order to measure the demodulator noise floor, it is possible to use a very low noise constant amplitude wave generator such as a crystal oscillator or synthesizer to replace the unloaded system modulator. 6
GB/T4958.6—1988
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Figure 1. Configuration of a typical demodulator subsystem
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Figure 2. Equipment configuration for measuring demodulator frequency deviation sensitivity using the Bessel zero value methoda
Figure 3. Equipment configuration for measuring demodulator frequency deviation sensitivity using the dual signal method12
Figure 4. Equipment configuration for measuring demodulation direction
Additional remarks:
GB/T4958.6—1988
Figure 5. Simplified configuration for measuring differential gain and differential phase of demodulator This standard is under the jurisdiction of the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. This standard is drafted by the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. The main drafters of this standard are Duan Zhongxian and Wu Bingmei.Equipment configuration for measuring demodulator frequency deviation sensitivity using Bessel null method a
Figure 3. Equipment configuration for measuring demodulator frequency deviation sensitivity using dual signal method 10th edition situation
Please 12
Figure 4. Equipment configuration for measuring demodulation direction
Additional instructions:
GB/T4958.6—1988
Figure 5. Simplified configuration for measuring demodulator differential gain and differential phase This standard is under the jurisdiction of the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. This standard was drafted by the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. The main drafters of this standard are: Duan Zhongxian and Wu Bingmei.Equipment configuration for measuring demodulator frequency deviation sensitivity using Bessel null method a
Figure 3. Equipment configuration for measuring demodulator frequency deviation sensitivity using dual signal method 10th edition situation
Please 12
Figure 4. Equipment configuration for measuring demodulation direction
Additional instructions:
GB/T4958.6—1988
Figure 5. Simplified configuration for measuring demodulator differential gain and differential phase This standard is under the jurisdiction of the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. This standard was drafted by the Industrial Standardization Institute of the Ministry of Posts and Telecommunications. The main drafters of this standard are: Duan Zhongxian and Wu Bingmei.
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