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Address:No.191 Guangzhong Road (West) Shanghai
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AM signal spectrum analyzer measurement and analysis
Release time:2016-08-15   Clicks:1030

AM signal spectrum analyzer measurement and analysis
I. Overview
  With the continuous development of technology, both in the aerospace, aviation, navigation, communications, and other aspects are inseparable from the analysis of the signal spectrum. Spectrum analyzer is mainly used for spectral analysis, but also measurable frequency, level, attenuation, modulation, distortion, jitter, etc., is also widely used in communications, radar, navigation, electronic warfare, space technology, satellite ground stations, frequency management, signal in monitoring, EMI diagnosis, EMC measurements, is to play a military electronic components, military and other departments to the whole system of scientific research, production, testing, testing, measurement of essential equipment.
?Second, the spectrum analyzer and working principle
  Input signal via an attenuator to limit signal amplitude, low-pass filter to remove without input frequency, then through a mixer with a local oscillator (LO) signal mixer convert the input signal to an intermediate frequency (IF). LO frequency is controlled by the sweep generator. With the change, the mixer output signal LO frequency (which includes both the original signal, their sum, difference and harmonic,) by the resolution bandwidth filter filter out LO higher than the input signal frequency, and logarithmic scale enlarged or compressed. Then use the detector signals through the IF filter is rectified to obtain a DC voltage for driving the display vertical portion. With the sweep generator sweeps a range of frequencies, the screen will draw a trace. The trace shows the input signal frequency range within the display frequency components.
Signal analysis Third, the role of the various parts of the spectrum analyzer and display
  Input Attenuator: ensure spectrum analyzer maintained over a wide range of good matching characteristics, in order to reduce the mismatch error; protection mixers and other IF processing circuit to prevent component damage and excessive nonlinear distortion.
  Mixer: complete spectrum shifting signals of different frequencies corresponding input signal into an intermediate frequency. At low frequencies (<3GHz) mixing the use of high and low pass filter suppresses image interference; at high frequencies (> 3GHz) using a band-pass tracking filter suppresses interference mirror.
  Local oscillator (LO): It is a voltage-controlled oscillator whose frequency is controlled by the sweep generator. Its frequency stability of phase-locked to the reference source.
  Sweep generator: In addition to controlling the local oscillator frequency, but it can also control the display of horizontal deflection sawtooth scan from left to right so that the screen is displayed on the spectrum analyzer signal, and then repeat the scanning traces continuously updated. Sweep width (Span) is left fstart to the right fstop10 grid frequency difference, for example: Span = 1MHz, then 100kHz / div.
  IF amplifier: gain and attenuator settings linked to work, that is, when the input attenuation 10dB, then the IF gain while increasing 10dB, the input signal level remains unchanged. Top of the screen grid line reference level set indirectly IF gain value. When the reference level ↑ (or ↓) 10dB, the gain ↓ (or ↑) ↓ shift the signal (or ↑ Shift) 10dB, namely to change the display position signal, but the signal amplitude remains unchanged. When the input 10dB attenuation increases when the SNR decreases 10dB. This is because the input attenuation before the mixer, only the signal level attenuation, and noise is generated after mixing, in order to ensure the input signal level unchanged , the mixed signal to a corresponding amplification 10dB, so that noise will follow accordingly enlarged, i.e., the noise level is increased 10dB.
  IF filter: also known as the resolution bandwidth filter to distinguish between signals of different frequencies. It only allows the local oscillator signal by subtracting the difference between the input signal frequency is equal to the intermediate frequency, the order by the IF filter, and finally displayed on the screen. Other interfering signals will be suppressed out.
  3dB bandwidth of the IF filter is also called the resolution bandwidth (RBW). The reason why the signals can not be displayed on the spectrum analyzer for the wireless thin line, but there is a certain width, because when tuning by signal, the shape itself is a spectrum analyzer resolution bandwidth (IF filter) the shape of the display. When changing RBW, to change the width of the display response.
  RBW is smaller, the higher the frequency resolution. RBW is reduced 10-fold, 10-fold decrease of the noise energy reaches the detector, and the displayed average noise level will be reduced 10dB, and vice versa contrary. Noise level changes ΔdB = 10log (RBWnew / RBWold) = 10log (1kHz / 10kHz) = - 10dB.
  Detector: it converts the input signal power to output video voltage, and the voltage value corresponding to the input signal power. When the spectrum analyzer sweep width Span = 0, the envelope detector input signal will be demodulated, as working at the center frequency (CF), the receiver bandwidth RBW (time domain mode), the display signal envelope waveform .
  Video Filters: smooth noise displayed.
  It is a video detector output signal low-pass filtering averaging process to smooth display. Reduce video bandwidth (VBW), can show the noise spectrum jitter smoothing, which reduces display jitter range. Such favorable spectrum analyzer found drowned in the noise of the low power continuous wave (CW) signal, but also improve the measurement repeatability.
  Show: equivalent to AM radio in speakers. The amplitude of each frequency we see exactly when the amplitude of each frequency tuned AM radio we heard, the signal in the frequency domain as a rotating vector display, the vector length = peak amplitude of the sine wave, vectors in which position on the frequency axis = signal frequency.
Fourth, the AM signal measurement and analysis
  Features carrier amplitude modulated wave is modulated with the envelope signal changes, the carrier frequency fC unchanged. AM signal represented by the following formula for the U (t) = AC [1 + macos (2πfmt)] cos (2πfct) (4-1). Where Ac = constant total signal amplitude decision; ma = AM depth (0≤ma≤1); cos (2πfmt) = normalized modulation signal, fm = modulation frequency; AC [1 + macos (2πfmt)] decided carrier envelope amplitude. fc = carrier frequency. Carrier waveform in the time domain, having a sinusoidal amplitude modulation is shown in Fig. Envelope variation signal is modulated signal, so that the envelope waveform analysis may reflect the amplitude modulation parameters. Minimum and maximum waveform envelope called Umin and Umax.
  Redistribution ma can be calculated from these two parameters. When the modulation sine wave is at its maximum positive or a negative value that is ± 1, will appear maximum envelope voltage Umax = 1 + ma;. And the minimum envelope voltage Umin = 1-ma AM coefficient obtained ma = (Umax-Umin ) / (Umax + Umin) (4-2) type, or ma = (1-Umin / Umax) / (1 + Umin / Umax) (4-3) formula. The (4-1) where expansion was U (t) = Accos (2πfct) + maAc / 2 [cos2π (fc + fm) t + cos2π (fc-fm) t] (4-4) formula. It can be seen, in the frequency domain, amplitude modulated signal U (t) by the amplitude Ac of the carrier and two sideband composition; one at fc + fm, another at fc-fm, both amplitude are maAc / 2, the spectrum shown in FIG. Fm is the modulation frequency of the carrier and one sideband frequency difference (symmetrical sidebands relative to the carrier). The magnitude of the difference coefficient corresponding to the amplitude ma. That sideband amplitude relative to the carrier in dB is expressed as dBc = 20log (ma / 2), then ma = 2 × 10dBc / 20.
V. Conclusion
  Spectrum analyzer can be used in the frequency domain and time domain are two ways to measure the AM signal. By principle and the measurement signal analysis tells spectrum analyzer, correctly understood that the use, operation and application spectrum analyzer. Due to limited space, only the AM signal was measured analysis, spectrum analyzer can also harmonic distortion, third-order intermodulation, stimulus-response, phase noise and other types of signal frequency, power, bandwidth, modulation parameter measurement analysis.
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