On Wed, 19 Nov 1997 07:26:33 +0000 Nigel Goodwin writes: >In message , >Dave Celsnak writes >>Example: I am looking for a circuit that can turn my PIC PWM output >into >>a nice steady voltage (no ripple). Is there a discussion list just >for >>hardware? > >If you look at the circuit of almost any non-digitally tuned TV from >the >last 10-15 years (one that uses search tuning, and even some digitally >tuned ones) they use a low-pass filter to covert a PWM output to a >0-30 >volt tuning voltage. The ripple on these is extremely low - even small >amounts of ripple can be seen on the picture as the tuning varies >slightly. It's usually just a very low low-pass. I suppose an advanced design would use a band-reject at the PWM frequency. The TV signal being AM, a mistuning of a couple hundred KHz is not noticeable. Even this seemingly wide tolerance is only 1/1800 of the width of the UHF band, so a good stable voltage is needed. There is very useful feedback taken from the AFC detector, primarily to combat drifting of the tuner (for a given voltage input, the frequency tuned will vary), but it's also likely fast enough to buck any ripple. The tuning filter in digitally tuned TVs is often a discrete circuit with a JFET input and a bipolar output, and a large capacitor for the feedback loop making it work as an ideal integrator. Often an op-amp is built into one of the chips for this function. The amplifier/filter needs to boost the 5V output from the tuning chip up to a maximum of 30V or so. Even the digitally-tuned sets use feedback from the AFC. Usually the CPU sets the PLL somewhat below the nominal channel frequency then steps the tuning up in small steps until a comparator attached to the AFC voltage trips. After this lock is obtained the AFC is often ignored. The actual frequency tuned is not so precisely related to the crystal frequency. I suppose this is needed for compatibility with off-frequency cable TV signals or imprecise RF modulators in VCRs and games.