10-08-2015, 08:47 AM
(09-08-2015, 08:02 PM)Geordie McBoyne Wrote: The modulated sound carrier will be above the vision carrier which is incorrect for Sytem A.
No! This IF, remember! With a local oscillator running at 79.75MHz, for example, Fv = 79.75 - 34.75 = 45MHz and Fs = 79.75 - 38.25 = 41.5MHz.
As for generating the AM sound at 3.5MHz, 34.75 + 3.5 = 38.25MHz, which is the required sound IF. It might complicate things if the sound IF was generated at 38.25MHz which means that it would have to be introduced between the vision modulator and the SAW - my idea is to keep the modifications as simple as possible and mixing both before the vision modulator is much easier - particularly if the schematic for the original modulator is not available!
SAWs can do unpredictable things if you upset the impedance matching!
