Drive varicap with pic ?

[Giorgio]

I have a receiver where a pot drive a varicap for tuning a ic td7000.

Anyone have used a pic for tuning and scaning a reicever ?

It is ok to use PWM + filter or better use a DAC ?

There is available any sample for manager a receiver ?

Thank in advance de Giorgio

[SherpaDoug]

All you need to do is generate a DC voltage. How you do it doesn't matter. I would look at how stable the voltage needs to be. If you don't have a spec then consider how stable the frequency needs to be (Hz) and multiply by the conversion factor of the receiver (V/Hz). That will tell you how much filtering a PWM output will need.
If that approach takes too much filtering, or if you must slew the voltage quicker that a filter will allow, you then need either a higher order filter, or a DAC.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[BrianS]

I couldn't find info on the TD7000. There are a couple of problems in using varicap tuning with any method of generating tuning voltage. Primary is the problem of staying on frequency.

If you're varicap-tuning an analog FM Broadcast receiver subsystem, it's best to use a chip that can produce an AFC output. This can be read using the PIC's A/D system, and the value used to adjust the PWM or DAC output to lock the receiver on channel. The PWM/DAC voltage is usually too coarse an adjustment for accurate step control, so you'll probably have to dither the PWM/ADC and average the control output using filters. Filter characteristics are critical to getting smooth control with no audio artifacts.

If you're tuning an AM receiver at Broadcast frequencies, you may be able to use varicap tuning directly without AFC, but you'll have to include in your software some means of calibrating the voltage/frequency relationship of your oscillator...varicaps do not have tightly-controlled voltage/frequency specs.

A better (simpler, more stable) overall method is to use a frequency synthesizer chip that can be controlled digitally by the PIC to produce a local oscillator frequency that can drive the External Oscillator input of the receiver subsystem chip. Motorola and others make inexpensive synthesizers. Another method is to use a DDS chip such as those made by Analog Devices. Output of the DDS is also used as the local oscillator for the receiver subsystem. These chips can give good performance through UHF without AFC, if a stable reference oscillator is used (think TCXO).

For microwave downconverter applications, you'll probably still find some form of frequency feedback (AFC) necessary. Block Downconverters are not stable enough for anything but very broadband reception without AFC, due to drift caused by temperature variations.

Good luck!