On Wed, 11 Aug 1999 11:58:48 -0400 Vincent Deno writes: >In case anyone is interested, this is for a audio/video application. >Problem originates in these cheap boxes which are coming out that pay >very >little attention to their clock. Therefore, we are forced to deal >with >horrible digital timing--which is where the +-1000 PPM pull comes in. You've talked about "pull" a lot but that's only part of the picture. I can imagine at least 3 key specifications: Pull-- If the control voltage is varied from minimum to maximum, how much does the frequency change? Accuracy-- If a particular control voltage is applied, how close is the frequency to the expected value? Stability-- With the control voltage held constant, how much does the frequency change due to other factors? Poor stability of course implies poor accuracy. If you need a circuit with large pull and high accuracy, it's going to be expensive. Most likely you don't, especially if a constant input signal is present that your circuit can phase or frequency lock to. An R-C oscillator can pull over a tremendous range. Stability is extremely low. R-C oscillators phase locked to crystals seem to be in all computers to generate the CPU and video clocks. Rather large "pull" (at least tens of percent) can be acheived with an L-C oscillator. Since this is also a cheap circuit, the two out of 3 law dictates that accuracy/stability will be poor. It may be necessary to adjust each unit at the factory to a frequency close enough that lock can be achieved. If the application requires less that extreme pull it is much better than an R-C circuit. I recommend using an IF transformer as the L-C circuit since they have a capacitor temeperature compensated for the inductor built in. Next on the list of decreasing pull but increasing accuracy is a ceramic resonator. Most late-model TV sets use a ceramic resonator circuit for horizontal and vertical timing. It is locked to the incoming video's sync. The pull range is much larger than a crystal, probably enough for 1000 ppm, and the accuracy is not terrible. However you will need to work closely with the resonator manufacturer to make sure they will continue to make units that work in your circuit. Resonators designed for "pull" circuits may be custom parts. Finally there's the VCXO. It has high accuracy and stability but very limited pull. Because of the linited pull, it's limited to situations where the input frequency is already quite precise, such as a GPS receiver or the color decoder of a TV. In the former case it's stability is a great advantage because the input signal may be lost for a while. The receiver's local clock can stay close enough that the signal can be re-synchronized quickly when it returns. Without knowing about your application, I couldn't say if various "hybrid" techniques could be used. For example, an oscillator with poor accuracy could be periodically re-calibrated to a fixed crystal oscillator. Or the pull of an oscillator could be increased by using digital techniques to modify the output frequency. A "pretty good feedback circuit" can be very useful. Feedback can make many "impossible" things possible. ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: http://dl.www.juno.com/dynoget/tagj.