[time-nuts] Soundcard sampling

[Bruce Griffiths]

Lux, James P wrote:
>
> There's low phase noise oscillators intended for the SONET market from Valpey Fisher, etc. that have fairly good performance. A VF161 is a <$50 sort of part and is speced at 1ps jitter (1 sigma, fj>1kHz).  VF has "jitter attenuators" too, which are a low jitter pll with <0.18ps jitter (RMS 12kHz-20MHz)..
>
> Pick a XO at 200-300 MHz or so (whatever frequency is readily available), then set your DDS for a "spur free" frequency (where it divides nicely into the SIN table size)  (on the AD9584 running at 200MHz which has a 48 bit phase accumulator, 17 bit phase table, and a 12 bit DAC, these are about every 3 kHz or so)
>   

An AD9835 based synthesizer clocked at 50MHz (conveniently generated by multiplying a low phase noise 10MHz source by 5 using a diode or equivalent performance odd order frequency multiplier) is readily available at relatively low cost.

Since the phase is truncated to 12 bits within the AD9835, phase
truncation spur free outputs are available at harmonics of 50MHz/4096
(12207.03125Hz).
Conveniently the 819 harmonic of 50MHz/4096 (9.997558.59375MHz) produces
a beat frequency of about 2441.40625 Hz when mixed with a 10MHz signal.
When the input frequencies to be compared are 5MHz then either the above
frequency can be divided by 2 to produce a nominal 1220.703125 Hz beat
frequency or the 410th harmonic of 12207.03125 Hz can be employed to
produce a 4882.8125Hz beat frequency.

All of these beat frequencies are well within the frequency range of a
sound card.

Bruce