Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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Viewing the frequency response of the receiving path of the transceiver using the GKCh is convenient, fast and clear. But for this, at least, you need to have a GKCh. However, even without such a device, you can see the desired picture by using an oscilloscope and a simple home-made crystal oscillator for this. Of several possible measurement options, the choice was made on the option with the least interference in the circuit of the device under study.
The connection diagram of devices for viewing the frequency response of the transceiver is shown in the figure.
The author used the S1-67 oscilloscope as an indicator. The sawtooth voltage from the corresponding output of the oscilloscope is applied to the variable resistor R1, which regulates the size of the horizontal section of the characteristic being viewed. From the slider of the resistor R1, the "saw" voltage is applied to the GPA transceiver detuning varicap. The transceiver detuning control circuits are disconnected from the varicap.
The antenna input of the transceiver receives a signal from any quartz local oscillator with the frequency of the selected range (for example, with a frequency of 14,2 MHz for the 14 MHz range). The output of the LF transceiver amplifier through the LF signal voltage rectifier, consisting of the VD1 diode and the capacitor C1, is connected to the input of the vertical deflection amplifier (Y) of the oscilloscope.
The oscilloscope is switched to the DC voltage measurement mode. The input sensitivity is set to 0,2 V/div. The oscilloscope clock is internal (+). Sweep duration -10 ms.
The vertical size of the image is set by the transceiver's "LF Gain" knob. The shift of the image "right-left" is produced by changing the tuning frequency of the transceiver. The bandwidth of the main selection element is determined by the digital scale of the transceiver with a slow swing. By manipulating the resistor R1 and the transceiver tuning knob, the calibration grid of the oscilloscope is matched to the corresponding frequency values (on the digital scale of the transceiver). AGC of the transceiver must be turned off.
On the oscilloscope screen, the frequency response of the main selection element of the transceiver will be observed, since the remaining selection elements and various amplifying stages (input circuits, UFC, ULF, ULF) have a much wider bandwidth. All the main parameters of the filter under study are determined by the same method as when using the GKCh.
Author: V.Rubtsov (UN7BV), Astana, Kazakhstan
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