From: John Larkin
Subject: Re: Binary Sampler
Date: Sun, 19 Jan 2003 11:56:13 -0800
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On Sun, 19 Jan 2003 16:47:33 GMT, "Mike" wrote:
>"Mike Monett" wrote in message
>> Mike wrote:
>> > Ugh. That's not good. Okay, then, what is your sample rate for the
>> > sine and square waves?
>> > -- Mike --
>> I'll have to check the software, but I'm pretty certain the sampling was
>> at the same frequency.
>Okay then, let's try this: you input a sine wave at frequency fo, and you
>sample at frequency fo-df, and your output frequency is df, where df << fo.
>Precisely the same result would be obtained if your input frequency was df.
>You get (fo-df)/df samples per cycle of the df frequency sine wave. The
>samples are passed through a limiter and low pass filtered to average out
>-- Mike --
I *think* that MM is proposing to use a dds to generate a sampling
timebase for his 'binary sampler'. If the signal frequency is known,
you can in theory synthesize a slightly different sampling clock and
thus sample the waveform down to a low, sorta-baseband I guess you
could call it, frequency. The LeCroy patent I cited above does just
My observations here are that
1. You're adding the signal reprate jitter to the measurement, unlike
a triggered-timebase scope that ignores this. So if the reprate has
any jitter, the waveform is trashed. Lots of interesting signals have
jittery reprates but contain picosecond detail: lasers, radar, most
any laboratory pulse generator, most anything triggered by an
affordable crystal oscillator.
2. It's very hard to make a DDS with picosecond edge jitter over wide
time spans; hell, even over narrow time spans (he says, with chagrin.)
Has to do with the price of perfect dacs and ideal lowpass filters.
3. It's about a jillion times cheaper and easier to do a conventional
time-delay-ramp timebase for a sampler.
LeCroy is the modern Philco: always doing strange stuff.