1. How to use the signal generator
A signal generator, also known as a signal source, is an instrument used to generate an oscillating signal, providing users with a stable and credible reference signal, and the characteristic parameters of the signal are completely controllable. The so-called controllable signal characteristics mainly mean that the frequency, amplitude, waveform, duty cycle, modulation form and other parameters of the output signal can be artificially controlled and set. With the development of science and technology, more and more signal forms are actually applied, more and more complex, and the frequency is getting higher and higher, so there are more and more types of signal generators, and the circuit structure of signal generators is also changing. Continuously develop towards intelligence, software and programmability.
Application and Significance of Signal Generator
The signal generated by the signal generator is often used in the circuit to replace the actual signal of the front-end circuit to provide an ideal signal for the back-end circuit. Since the characteristic parameters of the signal source signal can be set manually, it can easily simulate signals with different characteristics in various situations, which is especially useful for product development and circuit experiments. In the circuit test, we can judge whether the function and characteristics of the signal processing circuit meet the design requirements by measuring and comparing the input and output signals. For example, use a signal generator to generate a sine wave signal with a frequency of 1kHz, input it to a signal processing circuit under test (the function is sine wave input, square wave output), and use an oscilloscope to check whether there is compliance at the output end of the circuit under test. The square wave output required by the design. The high-precision signal generator can also be used as a standard signal source (reference source) in the field of metrology and calibration, and the instrument to be calibrated is adjusted based on the reference source. It can be seen that the signal generator can be widely used in electronic research and development, maintenance, measurement, calibration and other fields.
For amateur radio enthusiasts, the signal generator is used the most to measure the sensitivity of radio stations and walkie-talkies. Many HAMs believe that it is most effective to use a comprehensive tester to test the sensitivity of the walkie-talkie. In fact, using a comprehensive tester to test the sensitivity of the walkie-talkie mainly uses the two functions of the signal generator and the signal-to-noise ratio meter of the comprehensive tester. The signal generator sends out a signal with the same modulation mode as the walkie-talkie (usually FM or AM), and then gradually reduces the amplitude (strength) of the output signal, and at the same time monitors the noise level of the walkie-talkie's audio output through a signal-to-noise ratio meter. When the ratio of effective audio signal to noise output by the walkie-talkie deteriorates to a certain extent (general walkie-talkie sensitivity test signal-to-noise ratio standard S/N=12dB), the level value output by the signal generator is equal to the sensitivity of the measured walkie-talkie. In this test, the signal generator simulates the air signal actually received by the walkie-talkie, and the simulated signal strength can be adjusted manually.
Some HAMs do not have a signal generator at hand, and the sensitivity of the tuning station can only be based on listening to the actual signal in the air as a reference. This approach is obviously not as convenient as using a signal generator. Use a comprehensive tester or a signal generator to measure the sensitivity of the walkie-talkie. The standard connection method is: the signal output of the signal generator is connected to the antenna input of the walkie-talkie through a cable, and the audio output of the walkie-talkie is connected to the signal of the comprehensive tester or the signal-to-narrative ratio meter. Audio input. The author saw in some walkie-talkie repair sites that when measuring the sensitivity of the walkie-talkie, only the signal generator is connected, and then the signal-to-noise ratio is roughly judged by listening to the sound quality of the walkie-talkie speaker, which is obviously not standardized. In addition, we also need to pay attention to the different test standards, that is, the expected signal-to-noise ratio threshold of the output audio of the walkie-talkie is different, such as S/N=10dB or S/N=20dB, even if the sensitivity value measured by the same walkie-talkie is also different will be different. Reminiscent of the names such as "squelch opening sensitivity" and "usable sensitivity" that appear in some walkie-talkie technical specifications, they all use different test standards. Sensitivity values obtained by different test standards are not meaningful to compare with each other.
The signal generator can be used to adjust the sensitivity of the radio station and the walkie-talkie. The basic principle is to minimize the attenuation of the useful signal transmission by the filter tank in the receiving channel of the walkie-talkie, so that the walkie-talkie has a higher sensitivity. This is improved in some amateur radio stations. It is widely used in frequency transformation and self-made radio stations. The signal generator here also plays the role of simulating an over-the-air signal. If the walkie-talkie itself has a receiving signal strength S meter or a test point, you can use the signal generator to input a signal that makes the signal meter of the machine indicate a strength of about 30% (it is easy to see the effect of the adjustment change), and then adjust according to the instructions in the walkie-talkie maintenance manual. In the receiving tank circuit, make the signal meter indicate to the maximum. If the signal meter is full during the adjustment process, you can adjust the signal amplitude of the signal generator to be smaller. Usually, in order to ensure the sensitivity of the entire frequency band, it is necessary to use multiple frequency points at the high end, low end, and center of the target frequency band as reference points for "adjustment" on average. For walkie-talkies without signal strength indication feedback, the receiver channel can only be adjusted by monitoring the improvement and deterioration of the SNR in the low SNR state. In fact, in addition to adjusting the receiving sensitivity of the walkie-talkie, the signal generator can also be used to adjust the radio and TV, as long as the signal generator can generate the same type of signal.
The signal generator can be used to find faults in the receiving channel of radio stations and walkie-talkies. The basic principle is: from the front stage to the next stage, measure each stage of amplification and filter in the receiving path one by one, and find out which stage of the amplifier circuit does not reach the designed amplification or which stage of the filter circuit has too much attenuation. The signal generator here plays the role of a standard signal source. The signal source inputs a signal with a known amplitude at the antenna input of the walkie-talkie, and then measures the gain from the antenna input port step by step through an ultra-high frequency voltmeter or an oscilloscope with a sufficiently high frequency to find out the unit with abnormal gain, and then check further , and finally diagnose the faulty parts.
Signal generators can be used to debug filters, typically bandpass filters and duplexers used on radio. One of the main functional components of network analyzers and frequency sweepers, the ideal instrument for debugging filters, is the signal generator. In the absence of these advanced instruments, the signal generator can barely debug the filter with high-frequency voltage measurement tools, such as ultra-high-frequency millivoltmeters, oscilloscopes with high enough frequencies, and measurement receivers. The basic principle is to measure The attenuation of the signal inside and outside the pass band of the filter. The signal generator plays the role of a standard signal source here. The signal generator generates a relatively strong signal of known frequency and amplitude, which is input from the INPUT terminal of the filter or duplexer, and the signal attenuation at the output terminal is measured. The band-pass filter requires the in-band attenuation to be as small as possible, and the out-of-band attenuation to be as large as possible, while the notch filter is just the opposite, the greater the attenuation of the notch frequency point, the better. Because ordinary signal generators transmit at a fixed single-point frequency, multiple test points are required for "uniform tuning" when debugging the filter. If there is a frequency sweep signal source and a matching spectrum analyzer, you can graphically see the overall frequency characteristics of the filter, which is very convenient for debugging.
The signal generator can be used to calibrate the signal strength meter of the walkie-talkie and the receiver, and the signal generator plays the role of the standard signal source here. According to the requirements of the maintenance manual of each model, input a signal with a specific strength at the calibration frequency point, and then correct the actual indication of the S signal strength meter. In the actual adjustment, we can see that although there are reference field strength standards indicated by the S signal meter of the receiver in the world, many manufacturers now implement their own standards, making the S meter indication too large and the indication range too small, giving users The feeling is that the S meter indicator is easy to be full, implying that the user has a high receiving sensitivity.
In addition to applications in radio frequency, signal generators also have a wide range of applications in the audio field.
The signal generator is used for commissioning of the voice circuit and modulation circuit of the walkie-talkie. The signal generator replaces the electret pickup and sends a 1kHz monotone signal that meets the requirements to the "MIC in" of the walkie-talkie (the input amplitude requirement will be marked in the maintenance manual), and then the FM walkie-talkie is in the transmitting state. Under normal circumstances, 1kHz audio will be heard in the receiver, and the modulation amplitude of the tested walkie-talkie can be measured through the modulation meter. Thus, the voice modulation circuit of the FM walkie-talkie can be detected and adjusted (the modulation degree is generally adjustable inside the walkie-talkie). Generally, the 25kHz interval FM modulated walkie-talkie requires the modulation degree to be around 4.5kHz at 1kHz audio. If the modulation of the FM walkie-talkie is too small, the voice will be light; if the modulation is too large, it will affect the voice and increase the occupied bandwidth. Some walkie-talkies that transmit without voice failure can also measure the voice signal status step by step from the MIC in through a similar method.
The signal generator is used for the repair of the audio power amplifier. The signal generator plays the role of ideal signal source here. The signal source produces an audio sinusoidal signal of appropriate amplitude, which is used as the signal input of the audio power amplifier. By measuring the output amplitude and waveform of the audio power amplifier, we can judge whether the audio power amplifier circuit is basically working normally, including whether there are abnormal states such as self-excitation and distortion.
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