With the crystal calibrator, it is possible to calibrate the scale of the receiver even without a counter. It is a crystal oscillator whose output signal contains a large part of harmonic multiples of the fundamental frequency. With a regular calibrator equipped with a 100 kHz crystal, we have a record in the receiver at 100 kHz, but also 200, 300, 400, etc.
Described calibrator it uses a divider, which makes it possible to obtain a record of 1000, 500, 100, 50 and 10 kHz. Due to the rectangular course of the output signal, sounds can be picked up to at least 30 MHz.
Connecting the calibrator
Connecting the calibrator is very simple. Field-effect transistor T1 is a crystal-controlled oscillator followed by an amplifier with transistor T2. The signal is fed to a divider with a 74LS390 circuit. From its outputs, we select signals from 500 to 10 kHz with a switch, the 1 MHz signal is taken before the divider. The supply voltage is stabilized for all circuits at 5V.
The calibrator must work for the first connection. Using trimmer C1, it is possible to adjust the oscillator frequency to exactly 1 MHz.
Calibrator parts list
| R1 | 100 k |
| R2, R4 | 1 pc |
| R3 | 39 k |
| C1 | 5-65pF trimmer |
| C2 | 33 pF |
| C3 | 100 pF |
| C4, C5, C6 | 100nF |
| T1 | BF245 and so on. |
| T2 | KC508 |
| IO1 | 78L05 |
| IO2 | 74LS390 |
| PR1 | 5-position switch |