GB/T 4958.5-1988 Measurement methods for equipment used in terrestrial radio-relay systems Part 2: Subsystem measurements Section 4: Frequency modulator
Some standard content:
National Standard of the People's Republic of China
GB/T 4958.5—1988
idtIEC48724
Methods of measurement for equipment used In terrestrial radio-relay systemsPart 2.Measurements for subsystems SectionFour-Frequency modulators
Promulgated on March 28, 1988
Implemented 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 subsystems SectionFour-Frequency modulators systems Part 2: Measurements for sub-systems Section Four-Frequency modulators
621.396:
621.317.08
GB/T4958.5—1988
IEC487—2—4
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—4 "Measurement methods for equipment used in ground radio-relay systems Part 2: Measurement of sub-systems Section 4 - Frequency modulators" 1 Scope
This section gives the measurement methods for the mechanical and electrical characteristics of frequency modulators. Moreover, if possible, only the measurement of the basic modulator is considered, and the baseband part of the modulator is not included, namely the pre-emphasis network and the network related to the accompanying audio subcarrier signal, pilot signal and auxiliary signal. The measurement method of the frequency demodulator is given in Section 5. The measurements between the baseband end-to-end of the modulator and demodulator components are given in the sections of Part 3 of this series of standards.
2 Definitions
For the purpose of this standard, a frequency modulator is a subsystem that modulates an intermediate frequency (IF) carrier signal with a baseband signal in an analog manner. This baseband signal may be a frequency division multiplexed (FDM) multiplexed telephone signal or a television signal including an audio subcarrier signal, a pilot signal and ancillary 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 modulator when transmitting analog signals.
The modulator subsystem usually consists of the following three main parts: baseband section;
-baseband to IF section (modulator);
-IF section.
3 Overview
The configuration diagram of a typical modulator subsystem is shown in Figure 1. The measured characteristics can be divided into the following three categories: Non-transmission characteristics:
Baseband to IF characteristics;
- and measurement of demodulator joint measurement, some baseband to baseband transmission characteristics. The first category involves measurements only at the baseband port and only at the IF port, including frequency measurements of the IF output and spurious/harmonic signal measurements. These measurements are described in other chapters of this series of standards. 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/T4958.5—1988
The second category of measurements constitutes the main part of this standard because the characteristics of the equipment under test are converted from baseband to IF. The third category of measurements includes the entire modulator and demodulator system, that is, only the measurement of the demodulator replaces the actual demodulator or the system demodulator.
It is necessary to understand the individual contribution of the modulator itself to the overall characteristic tolerance, because a modulator of one design or one manufacturer may work with another demodulator, so the compensation between the modulator and demodulator is undesirable. Each modulator and the measuring demodulator should meet the specified technical conditions when connected. This method requires that the performance of the measuring demodulator is better than the specified performance of the measured modulator. 4 Intermediate frequency output characteristics
4.1 Return loss
See the third section of the first part of this series of standards-measurements within the intermediate frequency range. (GB6662-86) 4.2 Level
See the third section of the first part of this series of standards. (GB6662-86) 4.3 Carrier frequency
See the third section of the first part of this series of standards. (GB6662-86) 4.4 Parasitic and/or harmonic signals
See the third section of the first part of this series of standards. (GB6662-86) 5 Baseband input impedance and return loss
See Section 4 of Part 1 of this series of standards -
Baseband measurement.
6 Frequency deviation sensitivity
6.1 Definition and general considerations
The frequency deviation sensitivity Sm of a modulator for a sinusoidal signal of a given frequency is expressed as the ratio of the frequency deviation Af to the baseband input voltage V: f(MHz/V)
V. Both parameters ∆f and ∆f are expressed in peak values or in effective values. (1)
Due to the influence of the pre-emphasis network, the frequency deviation sensitivity of the modulator is usually a function of the baseband frequency. If the baseband voltage V' is directly input after the pre-emphasis network (see Figure 1), the frequency deviation sensitivity of the modulator under test is independent of all baseband frequencies. 6.2 Measurement method
The measurement method adopts the "Bessel zero value method", which is based on the case of sinusoidal wave modulation. At the following modulation index m, the carrier frequency spectrum line has the first zero amplitude:
Where: Af is the peak frequency deviation
So the modulation frequencywww.bzxz.net
Af=2.40483
（2）
The "zero" value, that is, the point where the intermediate frequency carrier frequency disappears for the first time, can be observed on the spectrum analyzer. However, due to the residual harmonic distortion of the baseband signal generator, it is impossible to obtain a true zero value. However, a carrier level reduction of 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 disappears for the first time.
The measurement procedure is as follows:
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, then increase the output level of the signal generator slowly and continuously until the intermediate frequency carrier disappears for the first time on the spectrum analyzer.
c. Measure the effective value of the voltage Vs at the baseband input of the modulator. 2
GB/T4958.5—1988
d. When the modulation frequency is, the modulator frequency deviation sensitivity is calculated by the following formula: Ss =2: 40483 f (MHz/V)
Note: Since the modulation index 2.40483 corresponds to an intermediate frequency bandwidth, the intermediate frequency bandwidth 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. Another method is to use a calibrated measurement demodulator. 6.3 Representation of results
The results should be given in the following way:
"The frequency deviation sensitivity (S.) is ....MHz/V" or "The effective value of the frequency deviation is ....kHz when the baseband input level is ....dBm". 6.4 Details to be specified:
If necessary, the detailed equipment specification should include the following: a. Measurement method (6.2 or 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 the specified input level, e. Baseband connection point (i.e. before or after the pre-emphasis network), (see Figure 1); f. Pre-emphasis characteristics adopted (whether appropriate). 7 Modulation direction
7.1 Definition and general considerations
If the input voltage changes in the positive direction, resulting in an increase in the intermediate frequency, the modulation direction of the frequency modulator is positive. In television transmission, the modulation direction is very important.
7.2 Measurement Methods
A simple method to check the modulation direction is to apply an asymmetrical waveform to the modulator under test and to apply the IF signal to the test demodulator with a known demodulation direction. If the demodulator output signal has the same polarity as the modulator input signal, then the modulation direction is the same as the known demodulation direction.
Another method is to apply a line sync pulse and a positive peak luminance signal to the baseband input and observe the modulator IF spectrum on a spectrum analyzer. For the positive modulation direction, the highest level line will be above the carrier frequency. 8 Differential Gain/Nonlinearity and Differential Phase/Group Delay 8.1 Definitions and General Considerations
The modulator under test is excited by a baseband signal consisting of a small amplitude, relatively high frequency sinusoidal test signal with fixed amplitude and phase superimposed on a low frequency, relatively large amplitude sweep signal. At the modulator IF output, the frequency deviation caused by the test signal corresponds to a sinusoidal frequency offset whose amplitude 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, and are given by the following formula. DG(X)=A(X)
DP(X)=X)
Where: X is the instantaneous value of the input scanning signal DG(X) represents the differential gain function of the modulator
A(X) is the output amplitude deviation caused by the test signal, which is a function of X. A is the output amplitude deviation caused by the test signal when the scanning signal is zero value. DP(X) represents the differential phase function of the modulator. A(X) is the output phase deviation caused by the test signal when the scanning signal is zero value. (4)
GB/T4958.5—1988
9 is the output phase deviation caused by the test signal when the scanning signal is zero value. For an ideal undistorted modulator, the differential gain and differential phase are both zero. For an actual modulator, the above functions will vary. A real modulator is characterized either by these functions themselves or by differential gain distortion and differential phase distortion. The latter are defined as the difference between the extreme values of the above functions and are usually expressed in percentage and degree respectively as follows: DG distortion (percent) = 100 × (Amm = Am) A.
DP distortion (degrees) = qmax – 9min
The choice of test signal frequency depends on which part of the modulator is to be evaluated and which parameters are to be measured (i.e. differential gain or nonlinearity, differential phase or group delay). Definitions of nonlinearity and group delay and some of the factors that determine the choice of test signal frequency are given in Part 1, Section 4, "Baseband Measurements" of this series of standards. DG and nonlinearity are measured in the same way, but at different test frequencies. Nonlinearity is an important performance parameter of the modulator because it indicates the deviation of the output frequency/input voltage characteristic from an ideal linear response. Nonlinearity is measured using relatively low test signal frequencies, typically in the range of 50kHz to 500kHz. 8.2 Measurement Methods
In order to measure the differential gain/nonlinearity and differential phase/group delay of a modulator, an ideal demodulator is required. Such a demodulator should have very small differential gain/nonlinearity and differential phase/group delay compared to the modulator under test. For this purpose, the measuring demodulator can consist of a heterodyne demodulator, the heterodyne oscillator of which is controlled by a swept signal, so that the frequency deviation caused by the swept signal is suppressed, thereby substantially suppressing the distortion of the demodulator. A simplified equipment configuration for measuring the differential gain and differential phase of a modulator is shown in Figure 2. The figure shows the complete test layout including the measuring demodulator, i.e. a commercial line analyzer. For modulator measurements, the line analyzer usually consists of the following two parts: a. A transmitting part consisting of a swept signal source and a test signal source connected to the baseband input of the device under test. b. A receiving part connected to the intermediate frequency output of the device under test. This section consists of a measurement demodulator followed by a bandpass filter to extract the test signal, an envelope detector and a phase detector to provide differential gain/nonlinearity and differential phase/group delay, and a display device with calibrated horizontal and vertical axes and a built-in oscilloscope. The horizontal deflection of the oscilloscope is caused by the demodulated swept signal taken from the low-pass filter and fed by the measurement demodulator.
Note: ① When a high test frequency is applied, the intermediate frequency range to be measured will not be close to the sweep width, but close to the sweep width plus twice the test frequency. ② It must be ensured that the baseband amplifier in front of the modulator is not overdriven by a large amplitude sweep signal. To meet this requirement, it is often necessary to limit the sweep amplitude that may be applied. On the other hand, the baseband part of the modulator may be removed from the measurement, thus allowing the use of a large amplitude sweep signal to determine the characteristics of the entire modulator. When the low-end cutoff frequency of the baseband amplifier is high and cannot transmit the sweep signal, it is also necessary to separate the baseband amplifier part from the modulator. 8.3 Presentation of results
Differential gain and differential phase are preferably presented as a photographic display of the functions with suitable calibration on both axes. A photograph showing both functions simultaneously is usually presented. In addition, a textual statement may be given of the measured differential gain distortion, differential phase distortion and sweep range. 8.4 Details to be specified
If necessary, the detailed equipment specification shall include the following items: a. Intermediate frequency sweep range (e.g. ±10 MHz) b. Permissible differential gain distortion within the above range (e.g. 3%) c. Permissible differential phase distortion within the above range (e.g. 0.8°) d. Test frequency used;
e. Baseband connection point (e.g. before or after the baseband amplifier). 9 Parasitic amplitude modulation
9.1 General considerations
GB/T4958.5—1988
Frequency modulators usually exhibit a small degree of amplitude modulation, which may be generated by the modulator itself or may be caused by the amplitude/frequency response of the intermediate frequency circuitry following the modulator. This amplitude modulation is undesirable because it may cause additional baseband distortion due to subsequent AM-to-PM conversion or due to the AM sensitivity of the demodulator. 9.2 Measurement Method
A simplified configuration for parasitic AM measurements is shown in Figure 2. This is the same configuration as for differential gain and differential phase measurements, but in the receive section, instead of measuring the demodulator and subsequent circuits, an IF envelope detector is used. The modulator under test is excited with a swept signal source (without using the test signal in the transmit section), and the IF output level is detected by the IF envelope detector, the detector output of which is used for the vertical deflection of the oscilloscope. The shape of the characteristic curve over the specified frequency range is a measure of the amplitude modulation. For this measurement, the sweep width corresponding to the highest frequency under the operating conditions is selected. Commercial line analyzers have an operating mode switch in the receive section that allows the measurement of DG or DP using the demodulator as detailed in 8.2, or the measurement of the IF amplitude-frequency characteristic using the IF envelope detector.
9.3 Presentation of Results
The parasitic amplitude modulation of a frequency modulator is best presented by means of a photograph of the IF amplitude-frequency characteristic displayed on a properly calibrated two-axis oscilloscope. Alternatively, a textual statement may be made of the difference between the characteristic extremes over the corresponding sweep range. 9.4 Details to be Specified
The following items shall be included in the detailed equipment specification, if necessary: a. Sweep width (MHz)
b. Tolerance of the IF amplitude-frequency characteristic.
10 Baseband Amplitude/Frequency Characteristic
10.1 Definition
The baseband amplitude/frequency characteristic of a modulator is a curve representing the ratio of the IF frequency deviation to the reference frequency deviation (expressed in decibels). It is a function of the baseband modulation frequency when the baseband input amplitude is constant. The reference frequency deviation is the deviation at a specified baseband frequency. 10.2 General Considerations
To measure the baseband amplitude/frequency characteristic of a modulator, a measurement demodulator is required. By definition, a demodulator for this purpose provides a nominally constant baseband output signal which is a function of the modulation frequency of the input signal frequency deviation. In order to avoid high-order sidebands of significant amplitude at the highest modulation frequencies, a baseband signal of small amplitude should be used. If the modulator under test cannot be separated from the pre-emphasis network, a measurement demodulator with a calibrated corresponding de-emphasis network must be used. However, in some cases, the pre-emphasis network can be separated from the modulator, so that the amplitude/frequency characteristics of the basic modulator can be measured. In this case, the baseband amplitude/frequency characteristics of the pre-emphasis network should be measured separately. NOTE: At present, it is not possible to separate the contribution of the baseband frequency characteristics of the modulator/demodulator under test because the measurement demodulator/modulator has contributions of the same order of magnitude as the modulator/demodulator under test. It is therefore customary to test the modulator/demodulator system and specify the overall modulator/demodulator characteristics. 10.3 Measurement method
Figure 3 in Section 4 of Part 1 of this series of standards shows a diagram of the measurement configuration. Note that the "device under test" between the baseband terminal and the terminal consists of the measurement demodulator and the modulator under test interconnected at the intermediate frequency. 10.4 Presentation of results
For swept frequency measurements, a photograph or XY record of the oscilloscope display shall be given. When the measurement results are not given graphically, they shall be given as follows:
"The baseband amplitude/frequency characteristic of the modulator (or modulator and demodulator connected back to back) from 300 kHz to 8 MHz is within +0.2 dB to -0.1 dB relative to the value at 1 MHz". Point-by-point measurements may be given in a tabular form or as above. 10.5 Details to be specified
If necessary, the following items shall be included in the detailed equipment specification: a. Reference frequency;
b. Baseband frequency range;
c. Baseband amplitude/frequency characteristic tolerance;
d. Intermediate frequency deviation at the reference frequency;
e. Pre-emphasis/de-emphasis characteristics (when required). 11Frequency Division Multiplex Telephone Measurements
GB/T4958.5—1988
At present, it is not possible to distinguish the intermodulation noise contribution of the modulator under test, because the measurement demodulator has a contribution of the same order of magnitude as the modulator under test. Therefore, for this test, the system demodulator is usually actually used and only the total modulator/demodulator noise value is specified. The measurement method given in Part 3, Section 4 of this series of standards "Frequency Division Multiplex Transmission Measurements" is applied. In order to measure the floor noise of the modulator (i.e. without noise loading), baseband measurements can be made in the absence of modulation in connection with a measurement demodulator of known floor noise performance (see Part 2, Section 5 of this series of standards: Frequency Demodulator). 12TV Measurements
At present, it is not possible to distinguish the waveform distortion contribution of the modulator under test, because the measurement demodulator has a contribution of the same order of magnitude as the modulator under test. Therefore, for this test, it is usually practical to use the system demodulator and only the total modulator/demodulator noise value is specified. The measurement method given in Part 3, Section 4 of this series of standards "Frequency Division Multiplex Transmission Measurements" is applied. In order to measure the floor noise of the modulator (i.e. without noise loading), baseband measurements can be made in the absence of modulation in connection with a measurement demodulator of known floor noise performance (see Part 2, Section 5 of this series of standards: Frequency Demodulator). Therefore, for this test the system demodulator is usually used and only the total modulator/demodulator distortion value is specified. The measurement method used is given in Part 3 of this series of standards, Section
Measurements on black-and-white and color television transmissions".
Note: Most linear and nonlinear waveform distortions are not affected by the basic modulator/demodulator, but are affected by the baseband (including band-limiting filters, pre-emphasis networks and de-emphasis networks, etc.). In some cases, this is done in order to measure the baseband noise of the modulator (i.e., measurements without noise). The demodulator is connected to the baseband and their performance is measured directly at baseband (see Section ·|tt|| of this series of standards). In the case of modulation, the baseband noise is measured with a known baseband. Noise performance section 5).
111-[Good 4 yuan
Typical configuration of modulator subsystem
Shangzhong teaching inspection 1
Figure 2 Simplified configuration for measuring differential gain, differential phase and intermediate frequency amplitude frequency characteristics of modulator 6
Additional notes:
GB/T4958.5—1988
This standard is under the jurisdiction of the Post and Telecommunications Industry Standardization Research Institute of the Ministry of Posts and Telecommunications. This standard was drafted by the Post and Telecommunications Industry Standardization Research Institute of the Ministry of Posts and Telecommunications. The main drafters of this standard are Duan Zhongxian and Wu Bingmei. 7
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