Lux, Jim (337C) wrote:
On 10/1/09 7:24 PM, "Magnus Danielson" <[email protected]> wrote:
Brian,
Some quick comments...
The Op Amps at Point #2 would be a something like LT1000s or so and they
b
Since getting rid of essentially all of the 20 MHz sum signal can be
done using trivial passive networks close to the mixer, you can
concentrate on the beat note or 100 Hz difference frequency. Notice that
the beat note output level actually depends on the loading network.
Essentially the trick is to let the 20 MHz see a short (cap) while the
beat note sees high impedance. The loss in level is essentially due to
the traditional 50 Ohm impedance, which doesn't match the mixers
effective output impedence and besides, for that frequency we don't
really care about reflections as lumped parameter models may be used and
we count highest voltage and not highest power.
So, let's say you end up with 50 V/s and you want the counter to see say
5 V/us (really 5 MV/s or 5 MVHz) to avoid jitter-trigger then the
slew-rate gain you need becomes 100.000.
What if you're going to run the beat note into an A/D (e.g. High performance
audio interfac) and sample it, then do the sinewave fit to calculate where
the zero crossing would have been? Just a good low 1/f noise opamp to get
the level up where the A/D performance is good?
That would do it nicely. Then it can be debated wither fitting the curve
to find a suitable zero crossing is the best method. Doing that will
defeat much of the point in doing DMTD since the cancelation of the
transfer oscillator reduces as time difference between channels
zero-crossing occurs. De-correlation may be done through multiplication
with a complex digital transfer oscillator, complement one of them and
then do complex multiplications sample by sample. That would keep
correlation loss down to sample length level instabilities. Another
method would be cross-correlation by means of FFT methods. This has the
benefit of producing decorrelations for various time-shifts between the
oscillators.
Cheers,
Magnus
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