Imagine you have a radio receiver and you wish to display the receiving frequency. As you know the oscillator in a radio receiver works 455 kHz or 10.7 MHz above the receiving frequency. The 455 kHz and 10.7 MHz difference is called IF Intermediate Frequency).
Hardware and schematicMost HF frequency counters need a prescaler in front of the counter unit.
In this case I use a circuit called LMX2322 which has prescaler function.
Of course you can use any prescaler as long as it divides by 64.
I have chosen this circuit since I have them and they are very sensitive and easy to work with.
The sensitivity is so good that in many cases you will need a attenuator based on a few resistors to make it work properly.More details later.
The prescaler has two differential inputs called Fin and /Fin.
If you wish you can connect /Fin to ground and use only Fin going to pin 8. Two 100 resistor forms 50ohm for impedance input matching.
The output of the prescaler (CPo) is a TLL lever where the frequency is divided by 64.
The signal then enters the microcontroller into its counting register (RC0). The microcontroller count the input pulses during 64mS.
The accuracy of this counter is set by the frequency of the 13MHz crystal.
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I advice you to choose a good and stable crystal. In my case I use a very stable and accurate SMD 13.000MHz crystal.
The crystal frequency can be fine tuned by a variable capacitor C11 to obtain very high accuracy.
The frequency is displayed on the 6 LED display in multiplexed way. It means that only one LED is turned on at the same time. First LED 1 is lightning, then LED 2 and so one. This goes so fast that it appears that all are lightning. During the 64mS counting, the LEDs are off to decrease noise on the power line. When the display are updated the info is also sent on the RS232 line (RC6).
A PNP transistor convert the TTL voltage to RS232 voltage. The connector is a standard 9-pol female dsub which will fit into any computers comport. If you are not going to use this part you don’t have to build it.
Frequency offset by SW1 and SW2.
When SW1 and SW2 is not activated the counter show actual frequency.
When SW1 is activated (short circuit) the counter subtract the measured frequency with 455 KHz.
When SW2 is activated (short circuit) the counter subtract the measured frequency with 10.7 MHz.
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