Monday, November 29, 2010

Clarifier Calibration Idea

Inexpensive receivers as the HF3 need occasional calibration of the clarifier since such receiver to exhibit some temperature drift. One option is zero-beat a known carrier, another option is adjusting a known carrier to create a beat close to the audio frequency we want to use/listen to, e.g. 2Khz, and display it on a spectrum display.

And here would be my planned approach to the second option. A micro-controller (PIC, PICAXE, ATMEL, etc.) programmed as frequency counter of particular kind. Lets count as precisely as possible around 2kHz.
Here are some ideas how to display the deviation from 2kHz:
  1. 5 LEDs: red (+/-10 to -+/-5Hz), amber (+/-4 to +/-2Hz), green (from -1Hz to +1Hz)
  2. 7 segment display: deviation in Hz, decimal dot a negative sign indicator
In both cases, one may want to indicate grater deviations. In the first case I would suggest having the left hand read and amber LEDs illuminated together with the green one when the frequency is below the range, right hand side for above. In the second case, a suggestion could be to have segments C,D, E and G  on for "too low", and hence, A, B, G and F for "too high".

The following idea is for the more advanced builder. It should also be seen as a modification to the receiver. The clarifier-potentiometer could be replaced by some circuitry creating/controlling the clarifier varactor. PWM could be an idea here. The concept would be to "visit" a known carrier, activate the counting and adjust the varactor voltage such that the counter counts a 2kHz beat. Now deactivate counting and keep the varactor voltage constant. Actually, in a grabber setup, one may consider to have such "calibration visits" periodically (maybe every 3h) executed by the micro-controller, e.g. by switching between memory channels.