Crystal calibrator
With the crystal calibrator, it is possible to calibrate the receiver scale without a counter. It is a crystal oscillator, whose output signal contains a large fraction of harmonic multiples of the fundamental frequency. With a regular calibrator installed 100 kHz crystal we have recorded in the receiver at 100 kHz, but also 200, 300, 400, etc..
Described calibrator uses a divider, by which it is possible to obtain a record after 1000, 500, 100, 50 a 10 kHz. Due to the rectangular course of the output signal, the sounds can be captured at least up to 30 MHz.
Connecting the calibrator is very simple. The field controlled 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 po with a switch 500 to 10 kHz, signal 1 MHz is taken even before the division. The supply voltage is stabilized for all circuits at 5V.
The calibrator must work for the first connection. With the help of trimmer C1, it is possible to set the frequency of the oscillator exactly to 1 MHz.
Parts list:
| R1 | 100k |
| R2, R4 | 1k |
| R3 | 39k |
| C1 | 5 – 65pF trimer |
| C2 | 33pF |
| C3 | 100pF |
| C4, C5, C6 | 100nF |
| T1 | BF245 a pod. |
| T2 | KC508 |
| IO1 | 78L05 |
| IO2 | 74LS390 |
| PR1 | 5-position switch |
