Swan Signal Source, Signal Source of Frequency Scanner
Keywords: homemade sweep signal source, signal source production
When the RF Electronic Aob is making a sweep meter, a spectrometer and other tools, it will definitely use a sweep signal generator, which describes a sweep signal source.
The simplest sweep signal source controls the empty oscillator with a selected periodic signal, so that its frequency is transmitted according to the cycle of the control signal. The control signal may be a sine wave, or may be a sawtooth wave, and may be triangular waves, and the waveform is determined as needed. There is no special place in the voltage controlled oscillator, that is, the device in the radiosity circuit of the oscillator is replaced by voltage control, and the current common practice is to change C in the LC selection circuit to the variable diode, change The tuning voltage of the diode changes, the capacitance of the diode changes, and the oscillation frequency also changes corresponding to the voltage controlled oscillator. The following figure is a very practical circuit.
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The circuit parameters in the figure are suitable for working in 200-500 MHz. If the frequency of 500 MHz can be replaced by 6.8P capacitance to 3.3P, and the above 800MHz should be replaced with 2P. As for the series of capacitors on the valence tube, the attributes of the modulation signal can be selected in a larger range. If the modulation signal is a video, the figure above has a very practical FM TV modulator. If the modulated signal is a serrated wave, it is a sweep signal generator.
However, the use of such a sweep signal generator is limited because the width of the sweep is too small. To achieve a wide scan source, it is difficult to implement with such a simple oscillator.
For the frequency adjustable LC resonant circuit, the adjustable range is very limited. At lower frequencies, such as medium wave radios, the highest frequency can do more than 3 times more than the lowest frequency. When the frequency is high, the distribution capacitance has a large influence, which gradually decreases, so the variable frequency that is made with an oscillator is very limited. The relative range is not easy to increase, the absolute range is increased, for example, 1000-2000 MHz oscillator, the lowest frequency 1000MHz, the highest frequency 2000MHz, has a tunable range of 1000 MHz, is very easy to implement. Therefore, if it is mixed with a fixed 1000 MHz oscillator, a variable frequency oscillator of 1000-2000 MHz is mixed, and the difference between 0-1000 MHz is very easy, as shown in the following figure.
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It is easy to think that it is easier, if you really need such a set of devices, the workload of design and production is still not small. For product design, this workload is not countless, but it is a bit not budget for enthusiast experiments. It is good to use a lot of finished products, such as the UHF vibration of the TV, at least (470 + 38) MHz - (860 + 38) MHz electronically adjustable range; satellite receiver tuning demodulator The absolute adjustable range is larger, most of which guarantees (850 + 480) MHz - (2250 + 480) MHz electronically adjustable range. Even with old-fashioned tuning demodulators, its local oscillator also has a small adjustable range. For example, we have some very old-fashioned tuning demodulators now, the local scope is (920 + 612) MHz - (( 1450 + 612) MHz, there is no less than 500 MHz adjustable range, with the appropriate fixed frequency oscillator, making a 0-500 MHz sweep signal source should be no difficult.
The first step is to test the screen tuning demodulator. First test the lowest oscillation frequency, the tuning voltage terminal is used to the location, the measured frequency is; the maximum frequency is the highest frequency, the tuning voltage is connected to 12V, and the measured frequency is calculated as the highest frequency. The reason why it is called "calculation as the highest frequency", because improving the tuning voltage can also further increase the oscillating frequency, but the sweep control voltage is often generated with an operational amplifier, and there is a difficulty of the frequently used op amp and higher voltage, and Further increase the frequency of its linearness, it is also a bit wrong.
It is found that it is found that its minimum oscillation frequency is below 1300MHz. The maximum oscillation frequency is more than 2000MHz, and most of the adjustable range is more than 800MHz, and the signal source used to make 0-500 MHz is certainly no problem.
There is no crystal resonator for 1300 MHz or more, and there is only 670 MHz replacement, and 670 MHz multiplier is 1340 MHz, and it is also very suitable. Its fundamental ratio is 170 MHz higher than the maximum frequency of the resulting signal source, and can be easily filtered with a low pass filter. The experimental circuitry is simultaneously connected.
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At first, the experimental results were not ideal. When changing the tuning voltage, there were two lines on the spectrum, one is the difference in the difference we have, and the other is opposite to the movement direction of 0-500 MHz, it is estimated 670MHz three harmonics 2010MHz and satellite tuning demodulator vibration difference, measured calculations, it is true, it seems that the fixed frequency signal of the input mixer must suppress three harmonics. This problem is not difficult to solve, there are still many old-fashioned satellite TVs in the same year, the nominal frequency range is 950-1450 MHz, which is exactly the 670 MHz second harmonic. After this measure, the 0-500 MHz oscillation signal obtained was clean and the experiment was successful.
Look at the ingredients of the output signal end 670MHz, less than 50 decibels, completely don't consider it, other ingredients are more no mention.
This experiment is to use several unit devices, it is not practical. When there is time in the future, it makes it a practical device. As for what signal to control this signal source, what kind of sweeping is made, this is afraid I can't manage it.
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