Intermodulation distortion analyzer
阅读：172发布：2021-06-20
专利汇可以提供Intermodulation distortion analyzer专利检索，专利查询，专利分析的服务。并且An intermodulation distortion analyzer generates two pairs of sinusoidal test tones to serve as a test signal for the channel under test. The two pairs of tones simulate two respective noise band test signals but eliminate the long time averaging required for measurements when noise bands are used. A highly linear AGC circuit employs sampling at an output-controlled duty cycle to maintain a constant reference level for the analyzer. This reference level permits automatic distortion measurements to be read out directly in db below the test signal. An RMS detector circuit for second order intermodulation products employs feedback control to maintain the input signal to a squaring circuit constant. Squaring of the constant level sinusoids produces RMS DC components which can be separated for direct measurement. A distortion circuit provides known levels of second and third order intermodulation in the test signal to permit accurate check out of the analyzer.，下面是Intermodulation distortion analyzer专利的具体信息内容。
1. A distortion analyzer for a communication channel under test having a known frequency passband, said distortion analyzer comprising: means for generating at least four alternating test signals; wherein the frequencies of more than one second order intermodulation product and more than one third order intermodulation product of said test signals reside within the passband of said communication channel under test; means for simultaneously applying all of said test signals to said communication channel under test; input means for receiving signals from said communication channel under test; means for monitoring selected intermodulation distortion products of said test signals present in said signals received by said input means; wherein said means for monitoring comprises: an automatic gain control circuit for automatically adjusting the level Of signals received by said input means to a constant reference level in response to a relatively wide range of received signal levels; filter means for separating said selected intermodulation products from said signal of constant reference level; and means for providing a visual indication representing the amplitude of said separated selected intermodulation products.
2. The distortion analyzer according to claim 1 further comprising means for providing an indication in response to the level of said received signal being outside said relatively wide range of signal levels.
3. The distortion analyzer according to claim 1 wherein said last-mentioned means comprises: detector means for converting said separated intermodulation products into detected DC levels proportional to the amplitude of said intermodulation products; means for converting said DC levels to further DC levels which are logarithmically related to the detected DC levels; and meter means calibrated in db for directly measuring said further DC levels.
4. The distortion analyzer according to claim 3 wherein said detector means includes an RMS detector having input and output signal terminals, said RMS detector comprising: a squaring circuit for squaring the amplitudes of signals applied thereto; gain control means for automatically adjusting signals applied to said input signal terminal to a signal at a substantially constant level, said gain control means including feedback means for rendering said substantially constant level an inverse function of signal amplitude at said output signal terminal; means for applying said signal at substantially constant level to said squaring circuit; and low pass filter means for eliminating all but DC components in the squared output signal from said squaring circuit and applying said DC components to said output signal terminal.
5. The distortion analyzer according to claim 4 wherein said low pass filter means further includes a high gain DC amplifier for applying said DC components to said output signal terminal.
6. The distortion analyzer according to claim 5 wherein said gain control means comprises: a high gain operational amplifier having an input terminal and an output terminal which provides an inverted version of the signal at said input terminal; an operational transconductance amplifier having an input signal terminal, a bias control terminal, and an output terminal which provides an output current proportional to the product of the voltage at said input signal terminal times the current delivered to said bias control terminal; means connecting the output signal from the output terminal of said operational amplifier to the input signal terminal of said operational transconductance amplifier; a voltage to current converter for providing a control current proportional to the voltage at the output signal terminal of said RMS detector; means for applying said control current to said bias control terminal; and means for applying the current delivered from the output terminal of said operational transconductance amplifier to the input terminal of said operational amplifier.
7. The distortion analyzer according to claim 5 wherein said squaring circuit comprises: an operational transconductance amplifier having an input terminal, a bias control terminal and an output terminal which provides an output current proportional to the product of the voltage applied to said input terminal times the current applied to said bias control terminal; means for applying said substantially constant level to said input terminal of said operational transconductance amplifier; and means for applying a current proportional to said constant level to said bias control terminal.
8. The distortion analyzer according to claim 1 wherein said automatic gain control circuit comprises: an input terminal; an output terminal; sampling means fOr passing discrete periodic samples of input signals applied to said input terminal, said samples having a fixed frequency; low pass filter means for passing to said output terminal only components of the sampled signal having frequencies below said fixed frequency; and feedback means for controlling the duty cycle of said sampling means as a function of the amplitude of signal components passed by said low pass filter means, wherein the interval of said samples is increased in response to decreases in the amplitude of signal and at said output terminal and is decreased in response to increases in amplitude of signals at said output terminal.
9. The distortion analyzer according to claim 8 wherein said sampling means comprises: a fixed frequency signal generator for providing a repetitive triangular wave signal; a comparator responsive to said triangular wave signal and the voltage at said output terminal for providing an output pulse once during each cycle of said triangular wave for an interval corresponding to that during which said triangular wave signal is at a lower level than the voltage at said output terminal; and switch means for passing said samples only during the intervals of said output pulses.
10. The distortion analyzer according to claim 8 wherein said sampling means comprises: a first switch capable of being switched on and off to alternately pass and block said input signals; means for generating a fixed frequency alternating signal having a voltage which varies linearly with time between two levels; detector means for converting the output signal at said output terminal to a DC level proportional to the amplitude of said output signal; and comparator means for maintaining said first switch off when the voltage of said alternating signal exceeds said DC level and for maintaining said first switch on when said DC level exceeds the voltage of said alternating signal.
11. The distortion analyzer according to claim 10 wherein said feedback means includes bandpass filter means for passing only output signal components at the frequencies of said test signals to said detector means.
12. The distortion analyzer according to claim 11 wherein said sampling means further comprises a second switch connected in series with said first switch and arranged to be maintained on by said output pulses; and wherein said low pass filter means includes a first low pass filter connected between said first and second switches, and a second low pass filter connected between said second switch and said output terminal.
13. An intermodulation distortion analyzer for a communication channel under test having a known frequency pass-band, said analyzer comprising: means for simulating a first band of noise by generating a pair of first and second alternating test signals each having a frequency lying in said passband of said communication channel; means for simulating a second band of noise by generating a pair of third and fourth alternating test signals each having a frequency lying in said passband of said communication channel; means for simultaneously applying said first, second, third and fourth test signals to said communication channel; input means for receiving from said communication channel said test signals and intermodulation products of said test signals which are introduced by said communication channel; a first bandpass filter arranged to receive signals received by said input means and having a bandwidth so positioned and sufficiently wide to pass at least two discrete second order intermodulation products of said test signals, which products reside in the passband of said communication channel; and means for monitoring the amplitude of signals passed by said first bandpass filter.
14. The analyzer according to claim 13 further comprising: a second bandpass filter to receive signals received by said input means and having a bandwidth so positioned and sUfficiently wide to pass at least two discrete third order intermodulation products of said test signals, which products reside in the passband of said communication channel; wherein said means for monitoring includes additional means for monitoring the amplitude of signals passed by said second bandpass filter.
15. The analyzer according to claim 14 further comprising: a third bandpass filter arranged to receive signals received by said input means and having a bandwidth so positioned and sufficiently wide to pass at least two additional discrete second order intermodulation products of said test signals, which products reside in the passband of said communication channel; wherein said monitoring means includes means for summing the amplitudes of signals passed by said first and third bandpass filters and for monitoring the summed amplitudes.
16. The analyzer according to claim 15 wherein said communication channel is a telephone line and said known passband is in the audio frequency range; wherein the average of the frequencies of said first and second test signals is approximately 860 Hz; and wherein the average of the frequencies of said third and fourth test signals is approximately 1,380 Hz.
17. The analyzer according to claim 16 wherein the frequency of said test signal is approximately 856 Hz, the frequency of said second test signal is approximately 863 Hz, the frequency of said third test signal is approximately 1,374 Hz and the frequency of said fourth test signal is approximately 1385 Hz; wherein said at least two second order intermodulation products passed by said first bandpass filter have frequencies of 511 Hz and 529 Hz; wherein said at least two third order intermodulation products passed by said second bandpass filter have frequencies of approximately 1,885 Hz and 1,914 Hz; and wherein said at least two additional second order products passed by said third bandpass filter have frequencies of approximately 2,230 Hz and 2, 248 Hz.
18. The analyzer according to claim 15 wherein the frequencies of the second order intermodulation products passed by said bandpass filter including the difference between the frequencies of said fourth and first test signals and the difference between the frequencies of said third and second test signals; and wherein the frequencies of the additional second order intermodulation products passed by said third bandpass filter include the sum of the first frequencies of said first and third test signals and the sum of the frequencies of said second and fourth test signals.
19. The analyzer according to claim 18 wherein the frequencies of third order intermodulation products passed by said second bandpass filter include: a frequency equal to twice the frequency of said fourth test signal minus the frequency of said first test signal; and a frequency equal to twice the frequency of said third test signal minus the frequency of said second test signal.
20. The analyzer according to claim 15 wherein the frequency of said second test signal exceeds the frequency of said first test signal by no more than 50 Hz; wherein the frequency of said fourth test signal exceeds the frequency of said third test signal by no more than 50 Hz; wherein the frequency of said third test signal exceeds the frequency of said second test signal by a frequency residing within said known passband of said channel; and wherein the sum of the frequency of said second test signal and the frequency of said fourth test signal resides in said known passband of said channel
21. The analyzer according to claim 20 wherein the difference between twice the frequency of said fourth test signal minus the frequency of said second test signal is a frequency which resides within said known passband of said channel.
22. The distortion analyzer according to claim 13 further comprising: a non-linear circuit for receiving said foUr test signals and providing known levels of specific intermodulation products of said test signals; and means for selectively applying said known levels of specific intermodulation products to said means for monitoring to establish operability of said distortion analyzer.
23. The distortion analyzer according to claim 22 wherein said non-linear circuit comprises: a circuit input terminal; a circuit output terminal; an operational amplifier having an amplifier input terminal and an amplifier output terminal; resistive means connected in series between said input terminals of said circuit and said amplifier; a first relatively low-resistance path connected between said circuit output terminal and said circuit input terminal; a second path including a resistance and a diode connected in series between said circuit output terminal and said circuit input terminal, said diode being connected to conduct positive current toward said circuit terminal; and a third path comprising: a resistance connected between said circuit input terminal and said amplifier input terminal; a relatively high resistance negative feedback path connected between said amplifier output terminal and said amplifier input terminal; and variable resistance means connected in series between the output terminals of said amplifier and said circuit.
24. The distortion analyzer according to claim 13 further comprising: means for sensing the level of said first pair of test signals received by said input means; means for sensing the level of said second pair of test signals received by said input means; and means for providing an indication when the difference in the two sensed levels exceeds a predetermined difference.
25. The distortion analyzer according to claim 13 further comprising: means for establishing a test mode by applying only one of said pairs of test signals to said channel under test at an increased power level corresponding to the total power level of the two pairs of tones when both are applied to the channel under test during a distortion measurement; whereby measurement for distortion components at said means for monitoring provides an indication of residual noise in the channel under test.
26. An intermodulation distortion analyzer for a communication channel under test having a known frequency pass-band, said analyzer comprising: means for simulating a first band of noise by generating a pair of first and second alternating test signals each having a frequency lying in said passband of said communication channel; means for simulating a second band of noise by generating a pair of third and fourth alternating test signals each having a frequency lying in said passband of said communication channel; means for simultaneously applying said first, second, third and fourth test signals to said communication channel; input means for receiving from said communication channel said test signals and intermodulation products of said test signals which are introduced by said communication channel; a first bandpass filter arranged to receive signals received by said input means and having a bandwidth so positioned and sufficiently wide to pass at least two discrete third order intermodulation products of said test signals, which products reside in the passband of said communication channel; and means for monitoring the amplitude of signals passed by said first bandpass filter.
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