[Reprint] Spectrum Analyzer Fundamentals: How the Heterodyne spectrum Analyzer works

Spectrum Analyzer is the use of frequency domain to analyze and study the signal of a measuring instrument, for signal analysis it is indispensable, with the rapid development of communication technology, more and more field operations need to support the spectrum instrument (Spectrum Analyzer type), such as communication transmitters and interference signal measurement, spectrum monitoring, The characteristics of the device and so on, the application of a wide range of applications, and all walks of life, various sectors of the spectrum Analyzer application focus is not the same. So how does the spectrum analyzer work? Generally speaking, the working principle of the Spectrum Analyzer (digital multimeter) can be used in the following two aspects:

The first is the time domain acquisition of the signal, then Fourier transform it to convert it into a frequency domain signal. We call this method a mathematical method for dynamic signal analysis. Features are relatively fast, with higher sampling rates and higher resolution. Even if the two signal intervals are very close, they can be distinguished by Fourier transforms. However, because the analysis is digital sampling, the highest frequency of the signal can be analyzed by its sampling rate, which limits the analysis of the high frequency.

For now, the highest analysis frequency is at 10MHz or dozens of MHz, which means that the measurement range is from DC to dozens of MHz.

Science has developed to this day, we can measure a signal in many ways, no matter what signal it is. The most basic instrument commonly used is the oscilloscope, which observes the waveform, frequency, amplitude, etc. of the signal. But the signal changes are very complex, many of the information is not detected by the oscilloscope, if we want to restore a non-sinusoidal signal F, theoretically speaking, it is the frequency F1, voltage V1 and frequency of F2, voltage for the V2 signal vector superposition (see figure 1).

From the analysis means, the oscilloscope horizontal axis represents the time, the longitudinal shaft is the voltage amplitude, the curve is to indicate the voltage amplitude that changes over time. This is the time domain measurement method, if you want to observe the frequency of the composition, to use the frequency domain method, its horizontal axis is the frequency, the longitudinal shaft is the power amplitude. In this way, we can see the spectrum of the two (or more) signals by the distribution of the power amplitude at different frequency points. With the spectrum of these individual signals, we can reproduce and reproduce complex signals. This is very important.

For a cable TV signal, it contains many images and sound signals, and its spectral distribution is very complex. In the satellite monitoring, can receive a plurality of channels, each channel occupies a certain spectrum components, each frequency point occupies a certain bandwidth. These signals need to be obtained from the angle of the spectrum analysis to obtain the required parameters.

This method is commonly used for analysis of low frequency signals, such as sound, vibration, and so on.

The second is to rely on the hardware of the circuit to achieve, rather than through the mathematical transformation. It is directly received, called an Heterodyne spectrum analyzer.

How the Heterodyne Spectrum Analyzer works

Now the spectrum analyzer used more than the Heterodyne type, and the use of multiple frequency conversion (three to four times), in order to reduce the frequency, narrow pass band and high resolution. The basic working principle of the Heterodyne spectrum Analyzer is as follows:

Basic working principle of Heterodyne Spectrum analyzer

The input signal is mixed with the local oscillator (LO), resulting in the intermediate frequency (IF) signal being sent to the envelope detector by a narrow band, the detector output signal is amplified and the screen display is vertically deflected, and the scan generator ensures that the screen display is synchronized with the horizontal and native oscillator tuning, which simultaneously drives the horizontal deflection of the tuned LO.

In the measurement of high-frequency signal, the Heterodyne spectrum Analyzer mixed wave after the medium frequency amplification, it can get a higher sensitivity, and change the frequency band width of the If filter, can easily change the resolution of frequencies, but because the Heterodyne spectrum Analyzer is in the frequency band scanning, therefore, unless the scan time to near zero, Cannot get the real time response of the input signal, so we want to get the same kind of heterodyne spectrum Analyzer as the performance of real-time analyzer, the scanning speed is very fast, if the scan time is smaller than the time constant of the intermediate frequency filter, the correct amplitude of the signal is not obtained. Therefore, to improve the frequency resolution of the spectrum analyzer, and to be able to get an accurate response, the appropriate scanning speed.

From the above, we can learn that the Heterodyne spectrum Analyzer cannot analyze the spectrum of instantaneous signal (transientsignal) or pulse signal (Impulse Signal), and its main application is to test the spectrum of periodic signals and other spurious signals (Random Signal).

The Heterodyne spectrum Analyzer is used to realize spectrum analysis by using the Heterodyne receiver method. The most basic part of the core is its mixer. The basic function is to change the measured signal to the if 21.4MHz, and then the intermediate frequency processing, to obtain the amplitude. In the process of lower frequency, it is by the local oscillator to achieve the lower frequency. The local oscillator signal is scanned, and the range of the local oscillator scan covers the frequency range of the signal to be analyzed. So the tuning is done in the local oscillator. All the signals to be analyzed are down-converted to if and the spectral frequency is obtained.

Using the method of Heterodyne receiver to realize the principle of spectrum analysis

This is the same principle as the daily television and radio. But the cable output signal range is very wide, for example, there are 50 channels to play. These 50 signals are simultaneously entered into the receiver, and the total power is superimposed. And the TV show can only be one of them. In the same vein, the input port signal sent to the spectrometer is the sum of the collected signals, including the specific signal to be analyzed, and the power input to the spectrum meter is the total power. This is to introduce a parameter-maximum burnt power.

This value is 1 watts or +30dbm. This means that the sum of the signal power entered into the spectrometer must not exceed 1 watts, otherwise the attenuator and mixer of the instrument will be burnt.

For example, we want to monitor a satellite signal, assuming that its frequency is 12GHz, its power may only be about -80DBM, which is very small. But to know that the input signal is composed of a lot of signals, if you include a strong signal at some other frequency, even if you do not see this high-power signal, if the sum of the input signal power is greater than 1 watts, but also to burn the spectrum meter, and the high-power signal is not the signal you want to analyze. This is what we need to be careful in our daily work because the cost of replacing the mixer is very high.

Of course, the spectrometer does not directly access the mixer when it enters the signal, but instead first accesses an attenuator. This does not affect the final measurement results, purely for the internal coordination of the instrument, such as matching, optimal working point and so on. Its attenuation value is stepping, 0dB, 5dB, 10dB, the maximum is 60dB.

There is also a spectrum analyzer can not input DC, otherwise it will damage the device. In addition, it should also be noted that there is no static electricity, because the instantaneous voltage is very high, easy to penetrate the active device. The daily work of the instrument grounding will have a good effect, of course, it is better to have a protective grounding.

In the IF, the power amplitude value of all signals is linearly related to the power of the input signal. The input signal power increases, it also increases, and vice versa. So we detect if the intermediate frequency signal is feasible. In addition, in order to detect effectively, an internal if signal amplification. The mixer itself has a differential attenuation, this frequency and RF mixing it is not only a single if out, its if signal is very rich, all of these signals will be output from the mixer. In many of the harmonic components, only one intermediate frequency is of interest. This is the 21.4MHz that was mentioned earlier. This is done in the instrument device, with a band-pass filter to set the center frequency of 21.4MHz, filtering out other signals, extracting 21.4MHz if signal. The signal that is output through the IF filter is the signal that we want to detect.

There are several factors in the operation of the filter: the center frequency is 21.4MHz, fixed, and its 30dB bandwidth can be changed. For example, for broadcast signal, its bandwidth is generally dozens of khz, if the signal bandwidth is 25kHz, if the bandwidth must be greater than 25kHz. In this way, all the signals can be brought in. If it is too wide, it will mix with other signals, and if it is too narrow, the signal comes in part, or the low-frequency component, or the high-frequency component. So the signal is not demodulated.

The If bandwidth setting is determined according to the needs of the actual work. Of course, it affects many other factors, such as bottom noise, signal demodulation distortion and so on.

The IF signal power after the intermediate frequency filter is the power of the input signal. The detection method is to use a detector to turn it into a voltage output, reflected in the amplitude of the longitudinal axis. Of course, we have to go through D/A conversion and some data processing, plus some corrections and some logarithmic, linear transformations. This is enough to bring us a lot of convenience in signal analysis.

This article is from: Sai Microelectronics-Electronic engineer Community original address: http://www.srvee.com/test/apply/ppfxyjczs_wcsppfxydgzyl_68283.html

[Reprint] Spectrum Analyzer Fundamentals: How the Heterodyne spectrum Analyzer works