GB/T 11299.6-1989 Satellite communication earth station radio equipment measurement methods Part 2: Subsystem measurement Section 1: Overview Section 2: Antenna (including feed network)
Some standard content:
National Standard of the People's Republic of China
Methods of measurement for radio equipment used in satellite earth stationsPart 2 : Measurements for sub-systemsSection One-General
Section Two-Antenna(including feed network)This standard is one of the standards in the series of "Methods of Measurement for Radio Equipment in Satellite Communication Earth Stations" GB11299.6--89
This standard is equivalent to the International Electrotechnical Commission standard IEC510-2-1: "Methods of Measurement for Radio Equipment in Satellite Communication Earth Stations Part 2: Sub-system Measurements Section One-General Section Two-Antenna(including feed network)". Section 1 Overview
1 Subject and Scope of Application
The measurement methods given in Section 2 are applicable to the subsystem shown in Figure 1 of this series of standards GB11299.1-89 "Measuring Methods for Radio Equipment of Satellite Communication Earth Stations Part 1 Section 1 General Principles". 2 Purpose
The purpose of Section 2 is to describe the measurement methods for the electrical performance of subsystems in satellite communication earth station equipment. 3 Definition
A subsystem is a combination of circuits or devices that completes a certain function (such as modulation, frequency conversion, amplification), and its electrical and mechanical properties are specified.
Subsystems that can complete similar functions or can be tested by similar methods are grouped in the same section. Section 2 Antennas (including feed networks)
4 Subject and Scope of Application
This section of the standard specifies the measurement methods for the electrical performance of satellite communication earth station antennas, and also includes some definitions specifically for antennas. 5 Definitions
The definitions of general terms used in this section of the standard shall refer to GB1417--78 "Terms and Terms of Common Telecommunication Equipment". However, some terms are not included in GB1417 or have different definitions from those in this section of the standard. Therefore, the definitions given in the following clauses must be used for this section of the standard. Approved by the Ministry of Electronics Industry of the People's Republic of China on March 1, 198954
Implementation on January 1, 1990
5.1 Broadband Subsystem
GB11299.6--89
The antenna subsystem is the first part of the earth station communication equipment. As shown in Figure 1, it consists of an antenna and a feed network. The antenna consists of a main reflector, a primary radiator, and sometimes a sub-reflector. The feed network usually includes one or more duplexers, which are connected to the tracking receiver and the transceiver splitter, combiner and switch equipment through a waveguide feeder.
5.2 Gain Reference Antenna
A gain reference antenna is an antenna with a defined structure that can be accurately replicated. Its gain and directivity coefficients are better than those of a half-wave dipole antenna. When such an antenna can be determined by calculation and its sufficient consistency is confirmed by measurement, it can be used as a conversion standard for antenna gain measurements. 5.3 Boresight Direction
The boresight direction is the direction corresponding to the special performance of the antenna directivity pattern. For a tracking antenna, the boresight direction is the direction in which the tracking signal is zero1. For a non-tracking antenna, the boresight direction is the direction of maximum power transmission.
5.4 Axis Ratio (or Ellipse Ratio)
The axial ratio (or ellipse ratio) is the ratio of the major axis to the minor axis of the polarization ellipse. 5.5 Dual Polarization Antenna
A dual polarization antenna is an antenna that can simultaneously transmit or receive signals with two independent polarizations. If the two polarizations are orthogonal, they are called orthogonal polarization signals.
Note: A dual polarization antenna has two or more ports. 5.6 Antenna effective area (in a given direction) The antenna effective area in a given direction is the ratio of the effective power (P,) at the matched terminal of the receiving antenna to the power per unit area (S) of a plane wave incident on the antenna from that direction, the polarization of which is consistent with the polarization of the electromagnetic wave radiated by the antenna when used as a transmitter. 6 Measurement conditions
The measurements described in this standard can be carried out under different environmental conditions, the restrictions of which should be agreed upon by the parties concerned. For example: - wind speed
solar radiation
- temperature range
It should be recognized that mechanical deformations of the antenna geometry caused by factors such as gravity, wind and antenna pointing angle can affect the measurement results, especially the measurement results of gain and cross-polarization discrimination. The measurements should be carried out in all frequency bands given in the equipment specifications. 7 Polarization of the antenna
7.1 Polarization efficiency
7.1.1 Definition and general considerations
Polarization efficiency (n) is a coefficient less than or equal to 1 used in equation (1). P.,e)A(p,o).sn
武中：A. .0）The effective area of the receiving antenna in a given incident direction (,); Using Notes:
1) This originally refers to differential mode tracking, while for extreme value tracking the antenna boresight direction is still the direction of maximum power transmission. (1)
Polarization efficiency;
GB 11299.6—89
Power density of plane waves incident from the direction (,の); P (Φ,) is the power transmitted from the receiving antenna to the matched load when there is no resistance loss. Note: The effective area or gain of the large line (Equation 7) is measured by the method given in 8.2.3. When a randomly polarized signal source (i.e., its energy is evenly distributed between the cross polarizations) is used, the polarization efficiency is 50%. The general expression for calculating the polarization efficiency () is: (rirz ± 1)\cos*α+ (ri ± r2)sin\α(r2+(rz-+ 1)
Wu Zhong: r-
When the antenna is used for transmission, the voltage axis ratio of the far zone (Fraunhofcr zone) radiated in a given direction; r is the voltage axis ratio of the incident plane wave in the same direction; α is the angle difference between the major axes of the two polarization ellipses, rad. Note: ①When the two polarizations rotate in the same direction, take a positive sign, and when the rotation directions are opposite, take a negative sign. ②) The polarization efficiency (n) can also be expressed by formula (3): = (1+r21+)+4nt121-2c02a
2(1 +)(1 + r)
③If 71, the antenna is polarization matched to the incident wave. ③=0, then the antenna is orthogonally polarized to the incident wave, and the antenna polarization is said to be orthogonal to the incident wave polarization. 7.2 Cross-polarization discrimination
7.2.1 Definition and general considerations
The cross-polarization discrimination of a receiving antenna is the ratio of the power received by the antenna from a given direction with the polarization of the expected maximum power transmission (co-polarization) to the power received from the same distant source with equal power but orthogonal polarization from the same direction. The cross-polarization discrimination of a transmitting antenna is the ratio of the power transmitted in a given direction with the expected polarization (co-polarization) to the power transmitted in the same direction with a polarization orthogonal to the expected polarization. Unless otherwise specified, the cross-polarization discrimination is the discrimination produced at the peak of the co-polarization beam pattern.
If the polarization is linear, the cross-polarization discrimination (XPD) is given by the square of the axial ratio (r). If circular polarization is used, the relationship between φ and XPD is expressed by equation (4):
[r+1i2
Note: The cross-polarization discrimination is defined for each port of a single-polarization antenna or a dual-polarization antenna (e.g., an orthogonally polarized antenna). 7.2.2 Measurement method for linearly polarized antennas
The antenna under test is mounted on the test field and illuminated by a linearly polarized source antenna located in the far field. The two antennas should be nominally co-polarized and the maximum gain position should be accurately left. The received power (Pmax) is recorded. The source antenna is then rotated around its beam axis to the position of minimum power transmission (polarization zero point) and the received power (Pm) is recorded. The rotation angle must be checked to be approximately 90°. The source antenna is then rotated accurately by 90° and checked to verify that there is no significant difference between the received power and the maximum power (Fmax). The cross-polarization discrimination (XPD) is given by equation (5): XPD = r2 -
If the polarization plane of the antenna under test is adjustable, the measurement should be repeated at various positions within the adjustment range. Note: () The cross-polarization discrimination of the source antenna should be significantly greater than the cross-polarization discrimination of the antenna under test. ② The source antenna should be designed so that the peak value of its co-polarization pattern is consistent with the zero value of its cross-polarization pattern, and the beam axis should be consistent with the mechanical axis of rotation. And it should be precisely aligned with the direction of the antenna under test. ③ It is important that the signal level reflected from the test field should be lower than the level that affects the measurement accuracy. 7.2.3 Measurement method of circularly polarized antenna
The antenna under test is installed in the test field and illuminated by a linearly polarized source antenna located in the far area. The two antennas are measured according to 7.2.2 are precisely set at the position of maximum gain. The source antenna rotates at least 180° around its beam axis and observes the maximum received power (Pmax) and the minimum received power (P). 56
The axial ratio is expressed as:
GB11299.6—89
(4) Formula can be used to calculate the cross-polarization discrimination (XPD); Notes (), ②), and ③) of 7.2.2 are also applicable. 6
Note: 7.2.2 and 7.2.3 are not very suitable for the measurement of cross-polarization discrimination of large earth station antennas. It is recommended to use the satellite source method for testing, see Appendix C, and the measurement accuracy is shown in Appendix D.
8 Power gain of antennas
8.1 Definitions and general considerations
The power gain of an antenna is the total gain relative to an isotropic lossless source. It is the sum of the partial gains of two orthogonal polarizations. If a partial gain of a certain polarization is referred to, this polarization should be indicated, for example, "right-hand circular polarization gain" or "horizontal linear polarization gain", etc.
The definition of the receiving linear gain (G) can also be derived from the effective area (A.): AtA
where: A - operating wavelength; A. - effective area of the receiving antenna (see definition in 5.6), that is, Ae
If the antenna is used for transmission or reception at the same frequency from the same port, as long as the antenna is reciprocal, the transmission gain and reception gain defined above are equal.
8.2 Measurement method
One of the main methods for measuring antenna power gain is to compare it with a gain reference antenna. Another method includes:
Determine the directivity coefficient of the antenna by integrating the directional pattern. a.
b. Determine the antenna efficiency by independent measurement or calculation. This method is suitable for measuring low-gain antennas. The main error sources and how to determine their values are given in Appendix A and Appendix B. 8.2.1 Gain measurement by direct comparison with a gain reference antenna The direct comparison method of gain measurement is to compare the signal levels received by the gain reference antenna and the antenna under test from the same distance from the radiator.
To minimize the errors caused by different propagation paths, the gain reference antenna and the antenna under test should be located as close as possible. The gain reference antenna is usually mounted on a large antenna structure to minimize transmission line length and pointing errors, and care should be taken to ensure that the structure of the large antenna does not significantly affect the characteristics of the gain reference antenna. To avoid errors caused by different gains, the gain reference antenna and the antenna under test should use the same set of electronic receiving equipment. To avoid errors related to gain drift in the receiving equipment, a fast comparison device (such as a switch) should be used to connect one antenna to the receiving equipment first and then the other antenna to the receiving equipment. This technique also reduces errors caused by variations in the radiating source itself.
To avoid nonlinear errors in signal detection when the received signal levels differ greatly, it is desirable that the signal levels received from the two antennas be substantially equal. To this end, calibrated directional couplers and/or attenuators can be used. When the signal level from a distant source is low (e.g., signals from a satellite), a calibrated directional coupler is preferred over an attenuator, and the coupler should be terminated with a cold load (see Figure 2). Another method is to alternately insert and remove the attenuator after the low noise amplifier without significantly decreasing the noise temperature of the receiver (see Figure 3). In both methods, it is important to ensure that the low noise amplifier and receiver are linear over the entire signal range during the measurement. 57
GB11299.6-—89
When inserting switches in the receive path, care must be taken to minimize the impedance mismatch in each switch position, because the gain of some electronic components will change with changes in impedance mismatch. The power transfer between the gain reference line and the preamplifier must also be carefully determined, including the impedance matching and power losses of the transmission line switches. The power transfer factor between the gain reference point specified by the line under test and the preamplifier must also be determined. When there is an inevitable difference between the polarization of the antenna and the signal polarization of the far-field radiator, a corresponding polarization mismatch relationship should be established for each pair of antennas. When the wavefront of the incoming radiation source deviates greatly from the flat wave condition with uniform amplitude and phase, a power transfer correction factor is required for each antenna in order to accurately determine the gain of the line under test. After considering all the above factors, the far-field gain of the antenna under test can be determined by equation (9): G. GnLN.(P/P)
muLLeuN
wherein; G is the gain of the antenna under test relative to the isotropic antenna measured at the specified gain reference point; G is the gain of the reference antenna relative to the gain of the isotropic antenna; G
is the polarization efficiency (see Section 7.1);
La is the power transfer ratio between the test receiver input and the output of the antenna under test (<1), excluding the RF variable attenuator (5) in Figure 3;
L is the power transfer ratio between the test receiver input and the output of the reference antenna <)N is the correction factor for non-uniform incident wavefront; 1eg refers to the power transfer ratio (<1) of the RF variable attenuator (5) in Figure 3, or the power transfer ratio (1) of the variable attenuator (11) and the fixed coupler (5) in Figure 2: P./P, - is the ratio of the power received by the antenna under test to the power received by the gain reference antenna when the detector is at the same level. Note: 1) The signal related to the antenna under test is indicated by the subscript a. 2) The signal related to the gain reference antenna is indicated by the subscript 1. 8.2.1.1 Method for measuring gain using amplitude modulated signal When using the direct comparison method to measure antenna gain, it is often impractical to use a waveguide to connect the gain reference antenna and the common test receiver, especially when the gain reference antenna needs to be moved to calibrate the uniformity of the received RF field wavefront. A more practical method is to use two calibrated detectors, which are respectively installed on the output flanges of the antenna under test and the gain reference antenna! The RF signal source is amplitude modulated by a low-frequency (e.g. 1kHz) signal. This measurement device is shown in Figure 4, where the low-pass filter (5) is used to reduce the measurement error caused by the harmonic components of the RF signal source. The gain reference antenna (10) is installed so that the antenna under test can detect the incident electromagnetic field without causing mutual interference. A precisely calibrated RF variable attenuator (16) is used at the output end of the antenna under test to make the signal levels from the measurement branch and the reference branch exactly equal, so that the gains of the two antennas can be compared at the same input level using a low-frequency selective amplifier (12). The gain reference antenna is moved up and down and left and right in a vertical plane to the propagation direction of the distant source wavefront. The low-frequency selective amplifier is connected to the output end of the gain reference antenna. After each movement of the antenna, the output power of the low-frequency selective amplifier is recorded. The purpose is to find such an antenna position that the incident wave from the radiation source is not disturbed by reflections from the ground, the antenna under test, or any other obstacles. The uniformity of the incident wavefront can be obtained from the power difference received by the antenna at each position. If an area with uniform field strength distribution cannot be found, a smooth training line connecting the corresponding points of the antenna coordinates with the received power is drawn and the maximum power is recorded. It must be ensured that the gain reference antenna is not illuminated by the reflection of the antenna under test, for example, avoid being close to the focal position of the antenna under test.
Then switch is switched to connect the recorder to the antenna under test, which should be positioned so that its main axis is pointing toward the source antenna. Adjust the attenuator (16) to the desired value and record the reading P. The attenuator must be adjusted again until PP is very small, preferably zero, at which point the gain (G) of the antenna under test is given by equation (10), which differs from the previous equation in that its various quantities are expressed in decibels: G=Gr+AN+Ca+C
中ee60T)）
武中：G.—Gain of reference line 10); GB11299.6—89
A'——Difference between attenuator reading and recorded value P'P\; N\—…Calibration coefficient for non-uniform wavefront; N' = 10 logo
C\. ——Correction coefficient for attenuator calibration error; C
Correction coefficient for the difference in sensitivity between the two detectors. 8.2.2 Gain Measurement Using Direct Calibration Signal Power Method When measuring gain using direct calibration signal power method, any of the following two techniques can be used: a. A radiating source with known absolute value of equivalent isotropic radiated power (EIRP) is used to transmit, and the signal power received by the antenna under test is measured in the far field.
A signal source with known absolute power is connected to the antenna under test, and then the signal power received by the calibrated gain reference antenna is measured in the far field. b.
Absolute received signal power.
Figure 5 shows a typical measurement equipment configuration for the first case. Both gain measurement techniques require the determination of the propagation loss between the radiating source and the receiving antenna. If the distance is large enough so that the incident wave on the receiving antenna is essentially a plane wavefront, then the propagation loss is simply the free space loss plus the absorption loss and/or scattering loss of the propagation medium.
The free space propagation loss L. is greater than 1, equivalent to the loss between two isotropic antennas, and is given by equation (11): L
where: λ is the wavelength of the received signal;
d is the distance between the radiating source and the receiving antenna aperture (in the same units as λ). (11)
In order to avoid various errors caused by changes in the power of the radiating source, the receiver gain and the propagation loss, it is essential to repeatedly check and calibrate the test circuit during the measurement. If both ends of the line are controlled simultaneously, the use of additional measurement and data transmission equipment and continuous monitoring of the transmitted and received signal powers can help to minimize the errors. Another configuration method is to connect the standard signal generator (8) to the directional coupler at the input of the low noise amplifier (9) in FIG. 5 , and use a spectrum analyzer instead of the indicator (17) to observe the relative levels of the two spectrum lines. When the above method a is used, the measured far-field gain of the antenna under test can be determined by formula (12): PexL.Ls
(EIRP)NL
wherein: na, N. and L. are given in 8.2.1; Ca is the power gain of the antenna under test relative to the isotropic antenna measured at the gain reference point; Px is the power input to the receiver, that is, the input power of the waveguide switch (6) in Figure 5, W; L is the propagation loss in free space (>1); 1 is the absorption/scattering loss of the propagation medium (>1); EIRP.- P,L,G, and,
P is the power of the calibration source, W;
L is the power transfer ratio between the source antenna input and the calibration source output (<1); G
is the gain of the source antenna.
According to the antenna reciprocity theorem, this equation is also applicable to the above 6 methods. (12)
Note: (D) In the equation given above, if the polarizations of the source antenna and the measured antenna are the same within the accuracy required for the measurement, the polarization efficiency (\) can be assumed to be 1. If this condition is not met, two measurements are required for the two orthogonal polarizations of the source antenna, and the gain (G%) is given by the sum of the two measurements, which is:
nG + neG, = (n + n2)G =Ga
(13))
GB 11299.6- 89
wherein the subscripts 1 and 2 refer to the two orthogonal polarizations of the source. ② The standard signal generator (8) shown in Figure 5 is a radio frequency signal generator, and the indicator (17) is calibrated according to the power input to the receiver, that is, the power at the input end of the waveguide switch (6).
③) If a radio star is used to replace the beacon source (2) in Figure 5, the standard signal generator (8) can be used as a calibrated noise source. 8.2.3 Gain measurement using a radio star 2
When measuring gain using a radio star with a known power spectrum flux density, two methods can be used, namely the indirect method and the direct method. The gain of the human line can be derived by the indirect method without disconnecting the earth station equipment. See Section 2 of Part 3 of this series of standards "Measurement of the quality factor (G/T) of the 4~6GH receiving system". First, the G/T value is determined, and then the noise temperature (T) is determined according to the method in Article 9 of this standard. The multiplication of these two measurement results gives the antenna gain. When measuring the antenna gain by the direct method, the following method can be used. 8.2.3.1 Relationship between gain (G) and noise temperature (Ts) introduced by radio stars Radio stars transmit noise power in the microwave frequency band. When the antenna of the earth station points to the star, the increase in the noise power received by the antenna in the narrow band (B) is expressed as follows:
SAB-SGB
Where: Ps
（14）
Assuming that the influence of background noise remains unchanged in the direction that deviates slightly from the direction pointing to the star, the antenna points to the radio star and the antenna The received noise power increment compared to the deviation of the pointing direction from the star by a few degrees, W; the incident power spectrum flux density generated by the radio star, W/m/Hz; S
A---the effective area of the receiving antenna, m\; B--the noise bandwidth of the receiver (assumed to be very small compared to the frequency used to measure the gain), Hz; G--the antenna gain at the measurement frequency; >--the corresponding wavelength, m;
S and G are functions of frequency, but can be considered constant within the limited bandwidth (B). The factor 2 appears in the equation because the receiving antenna system responds to one polarization, while the polarization of the radio star is assumed to be random. If the polarization of the radio star is not random, and the antenna is nominally linearly polarized. The noise power value (P.) must be obtained from the average of the two orthogonal polarizations of the antenna.
Equation (14) is applicable to point source radio stars radiating through a lossless atmosphere. In general, neither of these two conditions can be satisfied: Therefore, the equation must be modified to the following form: Ps = SGBx
8 yuan, K2
where: K, (>1) is the correction factor for atmospheric attenuation; K (≥1) is the correction factor for the angular spread of the radio source. : (15)
If Ts is the increase in noise temperature introduced by the radio star at the reference point of the gain measurement of the receiving system, it can be written in absolute temperature as: KBT's
where is the Boltzmann constant. The gain of the antenna subsystem is given by the following equation: S·G·B?bzxZ.net
8 yuan,,
8 yuan kKkz .Ts
. (17)
Equation (17) shows that when measuring with the radio star method, the gain can be calculated by measuring the noise temperature increase (T) when the antenna is pointed at the radio star.
Noise temperature increment (T) is determined by measurement, while all other parameters in (17) are known. In (17), the power spectrum is used. Note:
2) It is recommended to use the radio satellite method to measure the antenna gain. :60
GB11299.6-89
The value of the flux density depends on the selected radio satellite and the frequency of the measured gain (G). For the calculation of the correction factors K, and K, and the value of the power spectrum flux density (S), see Section 2 of Part 1 of this series of standards "4-6GHz receiving system quality factor (G/T) measurement". 8.2.3.2 Selection of radio satellite and pointing technology For the selection of radio satellite and pointing technology, see Section 4-6 GHz receiving system quality factor (GT) measurement in Part 3 of this series of standards.
8.2.3.3 Typical equipment configuration for measuring gain Figure 6 shows the equipment for accurately measuring antenna gain (G) using a direct method, which consists of three main components: waveguide switch component;
h. RF head component;
c. intermediate frequency detection component.
Component a includes:
i) to A cold standard load (11), such as a liquid carbon tetrafluoride (CF4) container; ii) a refrigerated reference load (7), such as a liquid nitrogen (N4) container; ii) a controlled waveguide switch (3), which connects the receiver input to either the antenna under test or the cold standard load iv) a precisely calibrated variable waveguide attenuator (4) and a variable waveguide attenuator (6), which usually operate at room temperature or near room temperature (approximately 290K);
v) an electronic RF switch (5), which switches the receiver input point to a frequency of about 80Hz frequency, alternately switching between the cooling reference load and the cooling standard load (or the output flange of the antenna subsystem). Component b includes:
i) a low noise amplifier (13);
ii) a narrowband filter (14) with the measurement frequency as the center frequency; i) a frequency converter from RF to IF (16). Component c includes:
i) an IF amplifier (17);
ii) two synchronous gate circuit rectifiers (20 and 22); iii) two low pass filters (21 and 23); iv ) comparator network (24);
v) zero indicator (25);
vi) optional recorder (26).
For accurate measurements, it is important that the temperature of the refrigeration reference load and the refrigeration standard load remain constant. For this purpose, the refrigeration liquid is applied at a boiling point corresponding to the local atmospheric pressure.
Typical boiling temperatures of the same coolant at a pressure of 760 mmHg are as follows: Liquid nitrogen 4.216 K
Liquid nitrogen 77.395 K
Liquid tetrafluorocarbon 145.140K
The boiling point temperature is a function of the purity of the liquid and the local atmospheric pressure at the time of measurement. During the measurement, the temperature of the refrigeration reference load (T) and the physical temperature of the attenuator (6) (T.) must remain constant, but their values do not participate in the calculation. On the one hand, the exact temperature difference T-Tca! (where To is the temperature of the attenuator (4) and Teal is the temperature of the refrigeration standard load) must be determined, because this temperature difference appears in the gain expression (see equation 23). This temperature difference is usually determined by measuring each temperature value separately, for example, using a thermistor bridge in Figure 6, using a platinum sensitive element. The thermistor bridge can also be used to measure the T and T. values and check whether they remain unchanged during the measurement. The working principle of the
test circuit is as follows: the temperature (TY) at point Y is given by formula (18): 61
GB11299.6-89
wherein: Lh (≥1) - loss introduced by attenuator (6): To - physical temperature of attenuator (6):
Tk - temperature of cooling reference load.
Adjusting the introduced loss (L) can make the temperature (T) at point Y vary within the temperature range between Tk and T. The noise temperature at point X depends on the position of the waveguide switch (3), or is determined by the antenna input noise power or by the cooling standard load, and H, in both cases, is determined by the temperature (T) of the attenuator (4) and the loss introduced by the attenuator (4). When the electronic switch (5) is alternately connected to the X port or the Y port at a frequency of about 80 Hz, the noise power at the Z end of the receiver radio head! 11 alternates between levels proportional to the noise temperature Tx and T. FIG. 7a shows the corresponding noise signal waveform at the Z port. This noise is amplified by a low noise amplifier (13), filtered by a bandpass filter (14) with the gain measurement frequency as the center frequency, and finally converted to an intermediate frequency by a mixer (16). The main purpose of the radio frequency bandpass filter (14) is to reduce the image interference of the mixer (16). The noise bandwidth (B) is usually determined by the bandwidth of the intermediate frequency amplifier (17). After the frequency conversion, the noise signal is sent to the intermediate frequency detection component, as shown in FIG. 6, and the intermediate frequency noise signal is added to two rectifiers (20 and 22), which are alternately turned on and off by a rectangular wave signal synchronized with the electronic switch (5). The rectangular wave signal is generated by the "synchronization device" (18). Two independent noise signal waveforms appear at the output of the gated rectifier. FIG. 7b shows the noise waveform at the output of the rectifier (20), and FIG. 7c shows the noise waveform at the output of the rectifier (22). At the output of the two low-pass filters (21) and (23), two DC voltages 8 proportional to the noise temperatures Tx and 7 appear respectively. They are added to the comparator network (24), and the output of the comparator network contains a signal proportional to the temperature difference △T=Tx-T. The measurement process requires adjusting the precision attenuator (4) until the values of the noise temperatures Tx and T are equal to ensure that the accuracy of the gain measurement is independent of the linearity of the receiver and the linearity of its indicator. Because these two are only used to indicate the zero state. 8.2.3.4 Measurement method
The symbols used in this clause are defined as follows (see Figure 6): Teal is the temperature of the refrigerated standard load (11); To is the physical temperature of the attenuator (4);
TR is the temperature of the reference load (7);
Tc is the noise temperature when the antenna is pointed at the background sky; Ts is the increase in noise temperature when the antenna is pointed at a radio star; Tx refers to the temperature at point X;
TY refers to the temperature at point Y:
la is the attenuation (input to output power ratio) of the attenuator (4) in step 1 of procedure (a) or (b); L is the attenuation (input to output power ratio) of the attenuator (4) in step 2; L is the attenuation (input to output power ratio) of the attenuator (4) in step 3. It is assumed in the following measurement method.
TR
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