I've been lurking in the background like many on this list, trying to absorb all the intelligence being thrown about... and this includes doing some study of data sheets on some of the proposed parts. Looking at the FX2 USB data sheet I find it needs 24.000 MHz. Looking at the TI PCM4202 data sheet I find it needs 24.576 MHz. And Phil H. has been looking for something higher than 24 MHz. for Altera Cyclone FPGA clocking. And there has been some discussion (on other lists) about the accuracy and stability of the clocks in sound cards that we're intending to replace... After a little exercise with the good old calculator, I would like to throw out a possible idea to kick around. Attached is a PDF file of a block diagram. In case it does not make it to you via this email, I'll also post a link to it on our Xylo-SDR web page a little later this evening. Referring to the diagram... I propose using a 96 MHz. oscillator. This is quite a common frequency for some microwavers and can be optionally disciplined by the "reflock" circuitry. (It might serve also as a multiplier chain oscillator for those interested in 1296 and up microwave work.) The reflock can be referenced to an available 10 MHz., 1 pps, or other stable reference standard. The 96 MHz. neatly divides by 4 to produce the 24.000 MHz. we need for FX2 USB. Further dividing by 125 yields 192 Hz which is fed into a PLL phase detector. A VCO in the PLL produces the 24.576 MHz required for the PCM4202 (and other chips). The PCM4202, if programmed for 192 Hz sampling, yields a nice 192 Hz. output on the LRCK (left/right clock) which might be used in the PLL feedback to the phase detector in lieu of an actual outboard divider. This proposal also gives Phil H. his higher clocking frequency for the Cyclone chip. The cyclone FPGA should be able to easily handle the various logic necessary for the reflock, the divisions, and the PLL except for the VCO. It also provides very accurate and stable clocking to the parts, eliminating some of the problems caused by off frequency clocks and wandering frequencies...especially if the reflock and a GPS or other reference is used. OK, shoot holes in it ... I'm learning! 73, Don AE5K PS - this discussion list is now up to 62 participants.
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