CN108918966B - Bottom noise cancellation method based on frequency spectrograph - Google Patents
Bottom noise cancellation method based on frequency spectrograph Download PDFInfo
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Abstract
The invention particularly relates to a bottom noise cancellation method based on a frequency spectrograph, which comprises the following steps: (A) setting an attenuation value of a frequency spectrograph and selecting a frequency conversion channel according to the frequency of an input signal; (B) without input signal, in RBW0Bottom noise logarithmic power P of lower measurement frequency spectrograph0(ii) a (C) Calculating the linear normalized power Plin: (D) inputting a signal to be measured at RBWxMeasuring the total power P (x) of the signals and the noise; (E) solving the input signal power P for eliminating the influence of the bottom noiselog(s): (F) if the measurement is finished, the process is exited, otherwise, the next step is executed; (G) and C, judging whether the testing environment of the frequency spectrograph is changed or not, if not, returning to the step C to directly carry out measurement, and if so, returning to the step B to carry out measurement after recalibration. The influence of the bottom noise of the frequency spectrograph on the power measurement of the small signal can be eliminated to the maximum extent through the bottom noise cancellation of the frequency spectrograph, so that the measurement range of the low-power small signal is expanded.
Description
Technical Field
The invention relates to the technical field of electronic test and measurement, in particular to a bottom noise cancellation method based on a frequency spectrograph.
Background
When a general spectrometer or a receiver is used for small signal measurement, the signal power is already relatively close to the noise power in the resolution bandwidth, namely, the RBW bandwidth, so that the measurement of the signal power is influenced by the noise power, and the measurement accuracy is reduced.
To reduce the effect of noise signals, one of the more common ways is to reduce the resolution bandwidth RBW. For the single-tone signal, since the bandwidth of the single-tone signal is quite small and the power is concentrated without being affected by the resolution bandwidth, the method of reducing the resolution bandwidth is feasible and the measuring process is rarely affected. For broadband signals, although the signal power measurement can be more accurately performed by reducing the resolution bandwidth, reducing the noise power in the band, and increasing the signal-to-noise ratio, the method has the following defects: namely, as the resolution bandwidth is reduced, the number of frequency sweeping points is increased, the scanning time is greatly increased, and the measurement efficiency is greatly reduced.
Disclosure of Invention
The invention aims to provide a bottom noise cancellation method based on a frequency spectrograph, which can eliminate measurement errors caused by the bottom noise of the frequency spectrograph under the condition of large-resolution bandwidth measurement.
In order to realize the purpose, the invention adopts the technical scheme that: a bottom noise cancellation method based on a frequency spectrograph comprises the following steps: (A) setting an attenuation value of a frequency spectrograph and selecting a frequency conversion channel according to the frequency of an input signal; (B) without input signal, in RBW0Bottom noise logarithmic power P of lower measurement frequency spectrograph0Wherein RBW0The bandwidth of the intermediate frequency filter is adopted when the bottom noise is calibrated; (C) according to P0And RBW0The linear normalized power P is obtained according to the following formulalin：
(D) Inputting a signal to be measured at RBWxTotal power P (x) of the measured signal and noise, wherein RBWxThe bandwidth of the intermediate frequency filter is adopted during measurement; (E) according to PlinP (x) and RBWxThe input signal power P for eliminating the influence of the background noise is obtained according to the following formulalog(s)：
(F) If the measurement is finished, the process is exited, otherwise, the next step is executed; (G) and C, judging whether the testing environment of the frequency spectrograph is changed or not, if not, returning to the step C to directly carry out measurement, and if so, returning to the step B to carry out measurement after recalibration.
Compared with the prior art, the invention has the following technical effects: the influence of the bottom noise of the frequency spectrograph on the power measurement of the small signal can be eliminated to the greatest extent through the bottom noise cancellation of the frequency spectrograph, so that the measurement range of the low-power small signal is expanded, and the measurement method can be realized under the condition of large resolution bandwidth measurement, so that the measurement efficiency is very high, and the method has high practicability.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a measurement flow diagram of a spectrometer;
fig. 3 is a plot of the power of the signal from the test after noise floor cancellation.
Detailed Description
The present invention will be described in further detail with reference to fig. 1 to 3.
Referring to fig. 1, a bottom noise cancellation method based on a frequency spectrograph includes the following steps: (A) setting attenuation values of a frequency spectrograph, selecting a frequency conversion channel, and determining the frequency conversion channel according to the frequency of an input signal, wherein when the input signal is a low-power signal, the attenuation is required to be as small as possible or is directly set to be zero. (B) In case of no input signal, in RBW0Bottom noise logarithmic power P of lower measurement frequency spectrograph0Wherein RBW0The bandwidth of the intermediate frequency filter is adopted when the bottom noise is calibrated; the step is used for measuring bottom noise of the frequency spectrograph, the measuring process is full-band small RBW scanning, and scanning results are normalized; before measurement, power averaging must be performed to make the measured background noise power spectral density closer to reality, otherwise the effect of background noise cancellation is deteriorated due to the strongly jittered background noise power spectral density. Because the display point number of the frequency spectrograph is not enough, the noise level of each frequency point can be recorded in a segmented scanning mode. In the process of calibrating the bottom noise, the noise must be normalized to obtain the noise power spectral density of each frequency point. For example, a measurement can be performed using RBW 1kHz as the minimum step to obtain a table of background noise measurements. (C) After normalization, the logarithmic power spectral density is:
Plog＝P0-10·lg(RBW0)；
the corresponding linear normalized power is:
inputting a signal to be measured, measuring the total power of the signal and the noise, obtaining the total power P of the signal and the noise corresponding to each frequency point of the equipment, and calculating a power spectrum. According to the current attenuation value and the scanned frequency points, the normalized background noise of each frequency point of the instrument is inquired, the noise power corresponding to each frequency point is subtracted from the total power of the measured signal and noise, and the signal power after background noise cancellation is obtained, wherein the process is as follows during actual measurement:
(D) inputting a signal to be measured at RBWxTotal power P (x) of the measured signal and noise, wherein RBWxThe bandwidth of the intermediate frequency filter is adopted during measurement; the power p (x) is the total power of the input signal power spectrometer after superposition of the bottom noise power. In RBWxThe linear power of the background noise under the bandwidth is:
from this the linear power P of the actual signal can be calculatedlin(s) is:
(E) according to PlinP (x) and RBWxThe input signal power P for eliminating the influence of the background noise is obtained according to the following formulalog(s) calculating the logarithmic power P of the signallog(s), i.e. the input signal power to eliminate the effect of background noise:
according to RBWxAnd a window function shape factor, wherein the signal power value of each display point for eliminating the influence of the background noise is calculated by using the original measured signal power sequence (namely, P (x) sequence).
In the above steps, steps B and C are actually to calibrate the bottom noise of the spectrometer, and when measurement is needed, only one calibration is needed as long as the test environment of the spectrometer is not changed, and steps D and E are the actual measurement process.
(F) If the measurement is finished, the process is exited, otherwise, the next step is executed;
(G) because the bottom noise of the spectrometer is changed when the testing environment of the spectrometer is changed, if the measurement is continued, whether the testing environment of the spectrometer is changed or not is judged, if the testing environment of the spectrometer is not changed, the step C is returned to for direct measurement, and if the testing environment of the spectrometer is not changed, the step B is returned to for recalibration and then the measurement is carried out.
The influence of the bottom noise of the frequency spectrograph on the power measurement of the small signal can be eliminated to the maximum extent by the bottom noise cancellation of the frequency spectrograph, so that the measurement range of the low-power small signal is expanded, and the measurement method can be realized under the condition of large-resolution bandwidth measurement, so that the measurement efficiency is high, and the method has practicability.
Preferably, in the step a, if the input signal is a low-power signal, the attenuation value of the spectrometer should be as small as possible or set to zero directly, so as to enable the measurement result to be more accurate.
In the step G, the change of the attenuation value at the front end of the frequency spectrograph, the change of an input frequency conversion channel, the change of the use environment of the frequency spectrograph or the time of executing the step B last time exceeds the set time belongs to the change of the test environment of the frequency spectrograph. The attenuation value and the frequency conversion channel change, the setting of the input frequency spectrograph changes, and the bottom noise changes at the moment, so that re-calibration is needed; the using environment of the spectrometer changes, generally refers to the ambient temperature and the like; the final time setting can be actually understood as that the bottom noise of the frequency spectrograph needs to be measured and calibrated again at intervals, so that the measurement precision can be improved. In practical use, when any one of the above changes, steps B and C need to be performed again to ensure the accuracy of the measurement process.
Referring to fig. 2, the spectrometer measures power values according to the following steps: (S1) attenuating the input signal according to the set attenuation value and mixing the attenuated input signal with the local oscillation signal to obtain an intermediate frequency signal; (S2) filtering the if signal by a low pass filter to obtain a complex signal; (S3) the complex signal is analog-to-digital converted and the power thus obtained is outputted. In the above steps, no signal is input and the bandwidth of the low pass filter is set to be RBW0Then, the power output in step S3 is stepP in step B0(ii) a In the above steps, the signal to be measured is input and the bandwidth of the low-pass filter is set as RBWxThen, the output power of step S3 is p (x) in step D. The bottom noise cancellation technology is based on hardware of a frequency spectrograph, and carries out bottom noise calibration on the frequency spectrograph according to the bandwidth of an RBW filter when different input attenuation values and variable frequency channels are set.
The rationality of the above method steps is verified by analyzing the signal power measured by the spectrometer.
The real signal-to-noise ratio of the small signal is assumed to be enough and is influenced by the noise bottom of the instrument, so that the measurement signal-to-noise ratio of the signal is reduced, and the measurement error of the signal power is caused.
(1) Taking the input signal expression as:
where n (t) is the noise in the input signal.
(2) Attenuating the input signal with local oscillator signal sLO(t) mixing to obtain an Intermediate Frequency (IF) signal sIF(t)：
Wherein
(3) The intermediate frequency signal is filtered by a low pass filter, namely a RBW filter, to obtain a complex signal s0(t)：
(4) Calculating power measured by a spectrometer
Including actual signal power, input noise power, and measured work of spectrum analyzer background noiseRate:
(5) in order to ensure the measurement accuracy, the measured results are subjected to power averaging. Because when the power is calculated by using small RBW, the bandwidth B of the intermediate frequency filter is small as RBW value, and simultaneously (w-w)c)<(B/2), then (w-2 w)c)≈-wcTherefore, the error due to the last term of the power calculation is averaged over the power:
wherein, P(s) is the input signal power, P (n) is the noise power in the RBW to be tested, and P (e) is the power calculation error, and the powers are calculated by taking the logarithmic power. From this equation, we can know that the input signal power is basically the rest after eliminating the bottom noise power of the spectrometer.
Fig. 3 is a plot of the power of a signal obtained from a test after performing background noise cancellation. In the figure, the uppermost curve is the received signal power of the spectrometer, i.e. the input signal power, and the spectrometer noise power is superimposed; the curve with points at the lower part is the power of the input signal when the bottom noise of the frequency spectrograph is not superposed; the curve without dots below is the signal power obtained after performing the bottom noise cancellation. It can be seen that after performing background noise cancellation, the measured power is close to the input signal power.
Claims (4)
1. A bottom noise cancellation method based on a frequency spectrograph comprises the following steps:
(A) setting an attenuation value of a frequency spectrograph and selecting a frequency conversion channel according to the frequency of an input signal;
(B) without input signal, in RBW0Bottom noise logarithmic power P of lower measurement frequency spectrograph0Wherein RBW0The bandwidth of the intermediate frequency filter is adopted when the bottom noise is calibrated;
(C) according to P0And RBW0The linear normalized power P is obtained according to the following formulalin：
(D) Inputting a signal to be measured at RBWxThe total power P (x) of the measured signal and noise, wherein RBWxThe bandwidth of the intermediate frequency filter is adopted during measurement;
(E) according to PlinP (x) and RBWxThe input signal power P for eliminating the influence of the background noise is obtained according to the following formulalog(s)：
(F) If the measurement is finished, the process is exited, otherwise, the next step is executed;
(G) and C, judging whether the testing environment of the frequency spectrograph is changed or not, if not, returning to the step C to directly carry out measurement, and if so, returning to the step B to carry out measurement after recalibration.
2. The spectrometer-based bottom noise cancellation method of claim 1, wherein: in the step a, if the input signal is a low-power signal, the attenuation value of the spectrometer should be as small as possible or set to zero directly.
3. The spectrometer-based bottom noise cancellation method of claim 1, wherein: in the step G, the change of the attenuation value at the front end of the frequency spectrograph, the change of an input frequency conversion channel, the change of the use environment of the frequency spectrograph, or the time from the last execution of the step B exceeding the set time belong to the change of the test environment of the frequency spectrograph.
4. The spectrometer-based bottom noise cancellation method of claim 1, wherein: the frequency spectrograph measures the power value according to the following steps:
(S1) attenuating the input signal according to the set attenuation value and mixing the attenuated input signal with the local oscillation signal to obtain an intermediate frequency signal;
(S2) filtering the if signal by a low pass filter to obtain a complex signal;
(S3) performing analog-to-digital conversion on the complex signal and outputting the calculated power;
in the above steps, no signal is input and the bandwidth of the low pass filter is set to be RBW0When the power output in step S3 is P in step B0；
In the above steps, the signal to be measured is input and the bandwidth of the low-pass filter is set as RBWxThen, the output power of step S3 is p (x) in step D.
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