CN112653526A - Device and method for testing nonlinear distortion of frequency hopping transmitter - Google Patents
Device and method for testing nonlinear distortion of frequency hopping transmitter Download PDFInfo
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Abstract
The invention discloses a device and a method for testing nonlinear distortion of a frequency hopping transmitter, and belongs to the technical field of communication. The invention provides a frequency hopping signal testing device, which mainly comprises a broadband receiver based on a multi-tone local oscillator and a baseband signal processing unit; the method uses a multi-tone local oscillator to carry out down-conversion on the frequency hopping signal, so that the ADC can carry out undistorted sampling on the frequency hopping signal through a lower sampling rate. The invention can reduce the sampling rate requirement of ADC in the sampling loop, and in addition, the local oscillator signal and the frequency hopping signal do not need to have strict frequency and phase synchronization relationship, thus greatly reducing the hardware test cost of the frequency hopping signal.
Description
Technical Field
The invention belongs to the technical field of communication, and relates to a testing device and a testing method for testing output distortion of a frequency hopping transmitter by using a broadband receiver based on a multi-tone local oscillator as a sampling loop.
Background
In the conventional communication mode, the transmission frequency is fixed and unchanged, so the interference and the interception are easy to happen. The frequency hopping communication technology is rapidly developed by virtue of the advantages of strong anti-interference performance, easy realization of code division multiple access and strong anti-fading capability. Meanwhile, with the development of modulation technology, after the signal is modulated, non-constant envelope and high peak-to-average power ratio characteristics appear, which cause strong nonlinear distortion of the signal through a power amplifier, and great influence is caused on the correct transmission and reception of the signal. Accurately testing the non-linear characteristics of a frequency hopping transmitter is therefore crucial to the design of frequency hopping communication systems.
In order to test the nonlinear characteristics of a frequency hopping transmitter, an ideal receiver is used to obtain the output signal of the transmitter, and then nonlinear index analysis is performed in combination with a known test input signal. The conventional receiver is designed in the following two ways: (1) and the local oscillation frequency of the receiver and the frequency hopping transmitter synchronously hop, and received signals are directly converted to a baseband for processing. However, the method has the disadvantages that when the number of frequency hopping points is large or the frequency hopping rate is high, the local oscillation frequency of the receiving end must be strictly and completely synchronized with the hopping of the local oscillation frequency of the transmitting end, otherwise, the frequency cannot be correctly converted; meanwhile, the frequency hopping of the local oscillator at the receiving end may cause distortion, so that the nonlinearity introduced by the frequency hopping of the receiver cannot be distinguished. (2) And the receiver samples all signals in a certain frequency band, the frequency band comprises all hopping points, and then the sampled data is processed to obtain an output signal corresponding to the input signal. The method has the disadvantages that when the number of frequency hopping points is large or the frequency hopping interval is wide, the bandwidth required to be sampled is wide, high requirements are provided for the design of modules such as an ADC sampling rate and a frequency conversion channel, the cost is very high, and the realization is difficult.
Therefore, designing a nonlinear testing device for a frequency hopping transmitter is a very challenging problem in terms of system implementation cost, local oscillator signal synchronization problem, testing accuracy and the like.
Disclosure of Invention
The traditional distortion testing device for the frequency hopping transmitter often has the problems of difficult synchronization of local oscillation signals and high requirement on ADC sampling rate. In order to solve the problems, the invention provides a device and a method for testing nonlinear distortion of a frequency hopping transmitter based on a multi-tone local oscillator broadband receiver.
A nonlinear distortion testing device of a frequency hopping transmitter comprises a broadband receiver based on multi-tone local oscillators and a baseband signal processing unit.
The input of the broadband receiver based on the multi-tone local oscillator is a frequency hopping test signal generated by a frequency hopping transmitter, the frequency hopping test signal in a broadband frequency range is subjected to down-conversion by using the multi-tone local oscillator, so that sampling is carried out by using an ADC (analog to digital converter) with a low sampling rate, and finally, a digitized baseband output signal is output;
the baseband signal processing unit comprises a channel compensation module, a frequency spectrum shifting module, a digital filtering module, a time delay alignment module and a distortion analysis module which are sequentially cascaded; and the digitalized baseband output signal is input to a baseband signal processing unit for digital processing, and then nonlinear distortion analysis is carried out on the distortion of the frequency hopping transmitter by combining the baseband input signal of the frequency hopping transmitter to obtain an index representing the nonlinear distortion of the frequency hopping transmitter.
The broadband receiver based on the multi-tone local oscillator comprises a multi-tone local oscillator generating unit, a frequency mixer, an analog low-pass filter and an analog-to-digital converter.
The multi-tone local oscillator generating unit is used for generating multi-tone signals in a specific frequency range. Specifically, the multi-tone local oscillator generating unit comprises three modules, namely an FPGA, a band-pass filter and a power amplifier, which are sequentially cascaded. The FPGA is used for generating periodic square wave signals; the band-pass filter is used for filtering multi-tone signals in a specific frequency range in periodic square wave signals; the power amplifier is used for amplifying multi-tone signals so as to meet the power requirement of the mixer on local oscillator input.
The input of the frequency mixer is a multi-tone signal generated by a multi-tone local oscillation generating unit and a frequency hopping test signal, and the frequency mixer is used for carrying out down-conversion on the frequency hopping test signal to obtain a frequency hopping test intermediate frequency signal; the frequency hopping test intermediate frequency signal is filtered through the analog low-pass filter, so that the analog-to-digital converter can sample without aliasing; and the analog-to-digital converter performs analog-to-digital conversion on the filtered frequency hopping test intermediate frequency signal to obtain a digitized baseband output signal.
The baseband signal processing unit comprises a channel compensation module, a frequency spectrum shifting module, a digital filtering module, a time delay alignment module and a distortion analysis module which are sequentially cascaded.
The channel compensation module is used for compensating signal distortion caused by non-ideal characteristics of devices of a previous-stage broadband receiver based on multi-tone local oscillation through a digital method.
The frequency spectrum shifting module is used for eliminating frequency offset between the baseband output signal and the baseband input signal.
And the digital filtering module is used for filtering redundant signals of which the center frequency is not at a zero frequency point after the frequency spectrum is moved.
The delay alignment module is used for eliminating time delay between the baseband output signal and the baseband input signal.
And the distortion analysis module is used for carrying out distortion analysis on the baseband output signal in a time domain and a frequency domain respectively by combining the baseband input signal to obtain an index representing the nonlinear distortion of the frequency hopping transmitter.
Based on the testing device, the invention also provides a method for testing the nonlinear distortion of the frequency hopping transmitter, which comprises the following steps:
s1, setting parameters of a testing device:
the parameter setting of the testing device is used for determining the frequency variation range and the bandwidth of a frequency hopping test signal output by the frequency hopping transmitter to be tested, the frequency interval of a multi-tone signal in the broadband receiver, the bandwidth of an analog low-pass filter and the filtering bandwidth of a digital filtering module in the baseband signal processing unit.
The parameter setting of the testing device comprises four steps:
s1.1, a baseband input signal of a frequency hopping transmitter to be tested is given.
S1.2, estimating the bandwidth of a frequency hopping test signal output by the frequency hopping transmitter.
The bandwidth of the frequency hopping test signal is 5 times of that of the baseband input signal, and the bandwidth of the frequency hopping test signal is the bandwidth of the signal to be processed by the broadband receiver.
S1.3, calculating the frequency range and frequency interval of the multi-tone signal in the broadband receiver.
And determining the frequency range of the multi-tone signal according to the frequency change range of the frequency hopping transmitter to be tested, wherein the frequency range of the multi-tone signal is larger than the frequency hopping range. And determining the frequency interval of the multi-tone signal according to the frequency hopping test signal bandwidth determined in the step S1.2, wherein the frequency interval of the multi-tone signal is wider than 2 times of the frequency hopping test signal bandwidth.
And S1.4, calculating the bandwidth of the filter of the analog low-pass filter and the digital filtering module.
And D, determining the bandwidth of the analog low-pass filter according to the frequency interval of the polyphonic signals determined in the step S1.3, wherein the cut-off frequency of the analog low-pass filter is consistent with the frequency interval of the polyphonic signals. And determining the filter bandwidth of the digital filtering module according to the frequency hopping test signal bandwidth determined in the step S1.2, wherein the filter bandwidth of the digital filtering module is equal to the frequency hopping test signal bandwidth.
S2, acquiring a baseband output signal:
and building a specific test device according to the test device parameters obtained in the step S1, and acquiring a baseband output signal by passing a frequency hopping test signal generated by the frequency hopping transmitter to be tested through a multi-tone local oscillator-based broadband receiver.
S3, performing digital processing on the baseband output signal:
and processing the baseband output signal in a digital domain, and determining a specific index of nonlinear distortion of the frequency hopping transmitter through the processed signal. The method specifically comprises the following steps:
s3.1, channel compensation.
And signal distortion caused by non-ideal characteristics such as unbalanced local oscillator amplitude and uneven in analog low-pass filter band in the broadband receiver is compensated.
And S3.2, frequency shifting of the baseband output signal.
And carrying out frequency spectrum shift on the baseband output signal to enable the center frequency of the baseband output signal to be zero frequency.
And S3.3, digitally filtering the baseband output signal.
And filtering the baseband output signal after frequency shift through a digital filtering module to filter out redundant signals with the frequency not at zero frequency.
And S3.4, aligning the time delay of the baseband signals.
And aligning the baseband output signal and the baseband input signal in a time domain, and eliminating the time delay between the baseband output signal and the baseband input signal.
And S3.5, testing signal nonlinear distortion.
And combining the baseband input signal, and performing nonlinear distortion analysis on the baseband output signal on a time domain and a frequency domain respectively to obtain an index representing the nonlinear distortion of the frequency hopping transmitter. Including analyzing Normalized Mean Square Error (NMSE), Error Vector Magnitude (EVM), magnitude-to-magnitude transform distortion (AM-AM), and magnitude-to-phase transform distortion (AM-PM), etc., in the time domain; adjacent Channel Power Ratio (ACPR) in the frequency domain, etc.
The invention has the beneficial effects that:
in a broadband receiver based on a multi-tone local oscillator, because a multi-tone signal is used to down-convert a frequency hopping test signal in a broadband range, the sampling rate of an analog-to-digital converter in the broadband receiver depends only on the bandwidth of the frequency hopping test signal. Compared with the traditional broadband receiving test method, the method has the advantages that the sampling rate requirement of the ADC is greatly reduced, the hardware design cost is saved, and the power consumption of a receiving loop is reduced. In the baseband signal processing unit, because the frequency difference between the frequency hopping test signal and the polyphonic signals in the broadband receiver is eliminated through frequency shifting, the broadband receiver does not need to use local oscillation signals strictly synchronous with the frequency hopping transmitter, so that the applicability of the scheme is greatly improved. In addition, compared with the traditional frequency hopping receiving test method, the method can effectively avoid the problem that the nonlinear distortion introduced by receiving frequency hopping is difficult to distinguish, so that the nonlinear distortion of the frequency hopping transmitter can be more accurately analyzed.
Drawings
Fig. 1 is an overall block diagram of a nonlinear distortion testing apparatus of a frequency hopping transmitter according to the present invention.
Fig. 2 is a flowchart of a method for testing nonlinear distortion of a frequency hopping transmitter according to the present invention.
Fig. 3 is a block diagram of a structure of a wideband receiver based on a multi-tone local oscillator according to the present invention.
Fig. 4 is a basic schematic diagram of a multi-tone local oscillator generating unit.
Fig. 5 is a functional block diagram of a baseband processing unit according to the present invention.
Fig. 6 is a signal diagram of each key module in embodiment 1 of the present invention.
Detailed Description
To demonstrate the feasibility and technical advantages of the present invention, the present invention is described in further detail below with reference to the accompanying drawings and specific embodiments.
The nonlinear distortion testing device of the frequency hopping transmitter comprises a broadband receiver based on multi-tone local oscillator and a baseband signal processing unit, and the whole block diagram of the device is shown in figure 1. A frequency hopping test signal generated by the frequency hopping transmitter is used as the input of the broadband receiver based on the multi-tone local oscillator to obtain a baseband output signal; the baseband output signal is input to a baseband signal processing unit to perform a series of digital processing, including channel compensation, spectrum shifting, digital filtering and time delay alignment, and then the nonlinear distortion of the frequency hopping transmitter is analyzed from the aspects of time domain and frequency domain by combining the baseband input signal of the frequency hopping transmitter.
The basic principle of the broadband receiver module based on the multi-tone local oscillator is that the multi-tone local oscillator is used for carrying out down-conversion on frequency hopping signals in a broadband frequency range, so that sampling can be carried out by an ADC (analog to digital converter) with a low sampling rate. The broadband receiver module based on the multi-tone local oscillator comprises: the device comprises a multi-tone local oscillator generating unit, a frequency mixer, an analog low-pass filter and an analog-to-digital converter.
The multi-tone local oscillation generating unit filters the square wave signal by using a band-pass filter, so as to obtain the multi-tone signal in a specific frequency range. The unit comprises three modules of an FPGA, a band-pass filter and a power amplifier as shown in FIG. 4; the FPGA is used for generating periodic square wave signals, the band-pass filter filters multi-tone signals in a specific frequency range in the periodic square wave signals, and the power amplifier amplifies the multi-tone signals to meet the power requirement of the mixer on local oscillator input.
The input of the frequency mixer is a multi-tone signal generated by a multi-tone local oscillation generating unit and a frequency hopping test signal, and the frequency mixer is used for carrying out down-conversion on the frequency hopping test signal to obtain a frequency hopping test intermediate frequency signal; the frequency hopping test intermediate frequency signal is filtered through the analog low-pass filter, so that the analog-to-digital converter can sample without aliasing; and the analog-to-digital converter performs analog-to-digital conversion on the filtered frequency hopping test intermediate frequency signal to obtain a digitized baseband output signal.
As shown in fig. 5, the baseband signal processing unit includes five modules, namely a channel compensation module, a spectrum shift module, a digital filtering module, a delay alignment module and a distortion analysis module, which are sequentially cascaded.
The channel compensation module compensates distortion caused by non-ideal characteristics of devices of a previous stage broadband receiver based on multi-tone local oscillation through a digital method, wherein the distortion comprises signal distortion caused by unbalanced local oscillation amplitude, unevenness in a low-pass filter band and the like.
The frequency spectrum shifting module is used for eliminating frequency offset between the baseband output signal and the baseband input signal. Because the frequency of the multi-tone signal is not completely consistent with the frequency of the frequency hopping test signal, after the baseband output signal is obtained through one-time down-conversion, the frequency deviation of the central frequency from the zero frequency point still exists, the frequency deviation is not more than 0.5 time of the frequency interval of the multi-tone signal, so that the frequency spectrum shifting is needed, and the further distortion analysis is conveniently carried out by subsequently combining the baseband input signal.
The digital filtering module is used for filtering out redundant signals of which the center frequency is not at a zero frequency point after the frequency spectrum is moved.
The input of the delay alignment module further comprises a baseband input signal, and the delay alignment module is used for performing time domain alignment processing on the baseband output signal and the baseband input signal to eliminate time delay between the baseband output signal and the baseband input signal.
And the distortion analysis module is used for carrying out distortion analysis on the baseband output signal in a time domain and a frequency domain respectively by combining the baseband input signal to obtain an index representing the nonlinear distortion of the frequency hopping transmitter. The indexes comprise Normalized Mean Square Error (NMSE) in time domain, Error Vector Magnitude (EVM), magnitude-to-magnitude conversion distortion (AM-AM), magnitude-to-phase conversion distortion (AM-PM) and the like; adjacent Channel Power Ratio (ACPR) in the frequency domain, etc.
Based on the above testing device, the present invention further provides a method for testing nonlinear distortion of a frequency hopping transmitter, the basic flow of which is shown in fig. 2, and the method comprises the following steps:
s1, setting parameters of a testing device:
the parameter setting of the testing device aims to determine the frequency change range and the bandwidth of a frequency hopping test signal output by a frequency hopping transmitter to be tested, the frequency interval of a multi-tone signal in a broadband receiver, the bandwidth of an analog low-pass filter and the filtering bandwidth of a digital filtering module in a baseband signal processing unit.
The parameter setting of the testing device comprises four steps:
s1.1: and giving a baseband input signal of the frequency hopping transmitter to be tested.
S1.2: and estimating the bandwidth of the frequency hopping test signal output by the frequency hopping transmitter. Considering the expansion effect of the nonlinear system on the signal bandwidth, without loss of generality, the frequency hopping test signal bandwidth is 5 times of the baseband input signal bandwidth. The frequency hopping test signal bandwidth is the signal bandwidth which needs to be processed by the broadband receiver.
S1.3: the multi-tone signal frequency range and frequency interval are calculated. Determining a multi-tone signal frequency range according to the frequency change range of the frequency hopping transmitter to be tested, wherein the multi-tone signal frequency range is larger than the frequency hopping range; the frequency interval of the multi-tone signal is determined according to the frequency hopping test signal bandwidth determined in step S1.2, which should be wider than 2 times the frequency hopping test signal bandwidth.
S1.4: the filter bandwidths of the analog low-pass filter and the digital filter module are calculated. Determining the bandwidth of the analog low-pass filter according to the frequency interval of the polyphonic signals determined in the step S1.3, wherein the cut-off frequency of the analog low-pass filter is consistent with the frequency interval of the polyphonic signals; and determining the filter bandwidth of the digital filtering module according to the frequency hopping test signal bandwidth determined in the step S1.2, wherein the filter bandwidth of the digital filtering module is equal to the frequency hopping test signal bandwidth.
S2, acquiring a baseband output signal:
and building a specific test device according to the test device parameters obtained in the step S1, and acquiring a baseband output signal by passing a frequency hopping test signal generated by the frequency hopping transmitter to be tested through a multi-tone local oscillator-based broadband receiver.
S3, digitally processing the baseband output signal:
the baseband output signal is processed in the digital domain, and the processed signal is used for determining a specific index of nonlinear distortion of the frequency hopping transmitter. The method specifically comprises the following steps:
s3.1: and (5) channel compensation. And signal distortion caused by non-ideal characteristics such as unbalanced local oscillator amplitude and uneven in analog low-pass filter band in the receiver is compensated.
S3.2: the baseband output signal is frequency shifted. Since the center frequency of the baseband output signal obtained in step S2 has a frequency offset not exceeding 0.5 times of the frequency interval of the multi-tone signal from the zero frequency point, the baseband output signal needs to be subjected to frequency spectrum shifting, so that the center frequency of the baseband output signal is zero frequency.
S3.3: the baseband output signal is digitally filtered. And filtering the baseband output signal after frequency shift through a digital filtering module, and further filtering out redundant signals of which the frequency is not at zero frequency in the signal.
S3.4: the baseband signals are time delay aligned. And aligning the baseband output signal and the baseband input signal in a time domain, and eliminating the time delay between the baseband output signal and the baseband input signal.
S3.5: and (5) testing signal nonlinear distortion. Combining with a baseband input signal, respectively carrying out nonlinear distortion analysis on the baseband output signal in time domain and frequency domain, wherein the nonlinear distortion analysis comprises the analysis of Normalized Mean Square Error (NMSE), Error Vector Magnitude (EVM), amplitude-to-amplitude conversion distortion (AM-AM), amplitude-to-phase conversion distortion (AM-PM) and the like in the time domain; adjacent Channel Power Ratio (ACPR) in the frequency domain, etc.
In order to more clearly illustrate the method for testing the nonlinear distortion of the frequency hopping transmitter, the present invention will be further described with reference to an embodiment. This section is for illustration purposes only as to the specific configuration of the modules therein and should not be construed as limiting the patent.
Example 1
As shown in fig. 6, in this embodiment, the frequency hopping points of the target frequency hopping test signal are 1.2GHz, 1.23GHz, and 1.26GHz, and the signal bandwidth is 60M. In order to enable the signal output by the mixing to be located near the baseband, the local oscillator signal of the mixer is adjusted to be a multi-tone signal with a frequency close to the frequency hopping frequency point, for example, the frequency points are 1080MHz, 1200MHz, 1320MHz, 1440MHz, where the multi-tone signal is determined through step S1.3 in the test method, so as to avoid the mixing output from generating spectrum aliasing. At any moment, the output of the mixer has a complete copy of the original signal with frequency offset from zero frequency not more than 0.5 times the frequency interval of the multi-tone signal, as shown in fig. 6(c), and an analog low-pass filter with a certain bandwidth can be used to filter out the out-of-band signal that is not considered, as shown in fig. 6(d), and ensure that the ADC can sample at a lower sampling rate without aliasing. However, since there is a spectrum offset between the multitone signal and the frequency hopping test signal at some time, such as at time t2 and time t3 in fig. 6(d), this center frequency point difference can be eliminated by spectrum shifting, as shown in fig. 6 (e). After the frequency spectrum is shifted, the out-of-band signal is filtered by a digital filtering method, so as to obtain the frequency hopping distortion signal located at the baseband at different times, as shown in fig. 6 (f). By the mode of using the multi-tone signal to carry out down-conversion on the frequency hopping test signal, the sampling rate requirement of an ADC (analog to digital converter) in a sampling loop is greatly reduced, and meanwhile, the local oscillator signal does not have the requirement of strict synchronization of frequency and phase, so that the method has wide application prospect.
Claims (9)
1. A frequency hopping transmitter nonlinear distortion testing device comprises a broadband receiver based on multi-tone local oscillator and a baseband signal processing unit;
the input of the broadband receiver based on the multi-tone local oscillator is a frequency hopping test signal generated by a frequency hopping transmitter, the frequency hopping test signal in a broadband frequency range is subjected to down-conversion by using the multi-tone local oscillator, so that sampling is carried out by using an ADC (analog to digital converter) with a low sampling rate, and finally, a digitized baseband output signal is output;
the baseband signal processing unit comprises a channel compensation module, a frequency spectrum shifting module, a digital filtering module, a time delay alignment module and a distortion analysis module which are sequentially cascaded; and the digitalized baseband output signal is input to a baseband signal processing unit for digital processing, and then nonlinear distortion analysis is carried out on the distortion of the frequency hopping transmitter by combining the baseband input signal of the frequency hopping transmitter to obtain an index representing the nonlinear distortion of the frequency hopping transmitter.
2. The apparatus of claim 1, wherein the wideband receiver based on multi-tone local oscillator comprises four modules of a multi-tone local oscillator generating unit, a mixer, an analog low pass filter, and an analog-to-digital converter;
the multi-tone local oscillator generating unit is used for generating multi-tone signals in a specific frequency range;
the input of the frequency mixer is a multi-tone signal and a frequency hopping test signal, and the frequency mixer is used for carrying out down-conversion on the frequency hopping test signal to obtain a frequency hopping test intermediate frequency signal;
the analog low-pass filter is used for filtering the frequency hopping test intermediate frequency signal so as to ensure that the analog-to-digital converter can sample without aliasing;
and the analog-to-digital converter performs analog-to-digital conversion on the filtered frequency hopping test intermediate frequency signal to obtain a digitized baseband output signal.
3. The apparatus of claim 2, wherein the multi-tone local oscillator generating unit comprises three modules of an FPGA, a band-pass filter, and a power amplifier, which are sequentially cascaded;
the FPGA is used for generating periodic square wave signals;
the band-pass filter is used for filtering multi-tone signals in a specific frequency range in periodic square wave signals;
the power amplifier is used for amplifying polyphonic signals.
4. The apparatus as claimed in claim 1, wherein the channel compensation module is configured to digitally compensate for signal distortion caused by device non-ideal characteristics of a wideband receiver based on multi-tone local oscillator in a previous stage;
the frequency spectrum shifting module is used for eliminating frequency offset between a baseband output signal and a baseband input signal;
the digital filtering module is used for filtering redundant signals of which the center frequency is not at a zero frequency point after the frequency spectrum is moved;
the time delay alignment module is used for eliminating time delay between a baseband output signal and a baseband input signal;
and the distortion analysis module is used for carrying out distortion analysis on the baseband output signal in a time domain and a frequency domain respectively by combining the baseband input signal to obtain an index representing the nonlinear distortion of the frequency hopping transmitter.
5. A method for testing nonlinear distortion of a frequency hopping transmitter comprises the following steps:
s1, setting parameters of testing device
The parameter setting of the testing device is used for determining the frequency variation range and the bandwidth of a frequency hopping test signal output by the frequency hopping transmitter to be tested, the frequency interval of a multi-tone signal in the broadband receiver, the bandwidth of an analog low-pass filter and the filtering bandwidth of a digital filtering module in the baseband signal processing unit;
the test device parameter setting comprises the following steps:
s1.1, giving a baseband input signal of a frequency hopping transmitter to be tested;
s1.2, pre-estimating the bandwidth of a frequency hopping test signal output by a frequency hopping transmitter;
s1.3, calculating the frequency range and frequency interval of multi-tone signals in the broadband receiver;
s1.4, calculating the filter bandwidth of the analog low-pass filter and the digital filtering module;
s2, collecting baseband output signals
Building a specific test device according to the test device parameters obtained in the step S1, and acquiring a baseband output signal by passing a frequency hopping test signal generated by a frequency hopping transmitter to be tested through a multi-tone local oscillator-based broadband receiver;
s3, performing digital processing on baseband output signals
Processing the baseband output signal in a digital domain, and determining specific indexes of nonlinear distortion of the frequency hopping transmitter through the processed signal; the method specifically comprises the following steps:
s3.1, channel compensation:
compensating for signal distortion in the wideband receiver due to non-ideal characteristics of the devices;
s3.2, frequency shift of baseband output signals:
carrying out frequency spectrum shifting on the baseband output signal to enable the center frequency of the baseband output signal to be zero frequency;
s3.3, digital filtering of baseband output signals:
filtering the baseband output signal after frequency shift through a digital filtering module, and filtering out redundant signals with the frequency not at zero frequency;
s3.4, aligning the time delay of baseband signals:
aligning the baseband output signal and the baseband input signal in a time domain, and eliminating time delay between the baseband output signal and the baseband input signal;
s3.5, testing nonlinear distortion of signals:
and combining the baseband input signal, and performing nonlinear distortion analysis on the baseband output signal on a time domain and a frequency domain respectively to obtain an index representing the nonlinear distortion of the frequency hopping transmitter.
6. The method of claim 5, wherein said nonlinear distortion analysis in step S3.5 includes analyzing normalized mean square error, error vector magnitude, magnitude-to-magnitude conversion distortion, and magnitude-to-phase conversion distortion in the time domain; adjacent channel power ratio in the frequency domain.
7. The method of claim 5, wherein in step S1.2, the frequency hopping test signal bandwidth is 5 times the baseband input signal bandwidth.
8. The method as claimed in claim 7, wherein in step S1.3, a multi-tone signal frequency range is determined according to a frequency variation range of the frequency hopping transmitter to be tested, the multi-tone signal frequency range being greater than the frequency hopping range; and determining the frequency interval of the multi-tone signal according to the frequency hopping test signal bandwidth determined in the step S1.2, wherein the frequency interval of the multi-tone signal is wider than 2 times of the frequency hopping test signal bandwidth.
9. The method of claim 8, wherein in step S1.4, the analog low pass filter bandwidth is determined according to the frequency interval of the polyphonic signals determined in step S1.3, and the cut-off frequency of the analog low pass filter is consistent with the frequency interval of the polyphonic signals; and determining the filter bandwidth of the digital filtering module according to the frequency hopping test signal bandwidth determined in the step S1.2, wherein the filter bandwidth of the digital filtering module is equal to the frequency hopping test signal bandwidth.
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