[time-nuts] WWVB BPSK Receiver Project? (fwd)

J. Forster jfor at quikus.com
Sun Mar 18 18:27:49 UTC 2012


I'd not use a vertical antenna at all. Far too prone to EMI.

My choice would be a center tapped, shielded, air core loop, running into
a low noise instrumentation amp. Center tap of loop to twinax shield,
grounded at preamp.

The instrumentation amp has fixed gain, and very high CMRR and PSRR. It
also does the differential to single ended conversion properly and has a
low output impedance.

YMMV.

-John

================



> The circuit in question doesn't appear to be in the PDF. You need to use
> a lot of caution with Lankford's theories. I don't want to get into a
> pissing contest, so I will leave it at that.
>
> Push pull with transformers goes back to the tube days. It is a
> convenient scheme to kill 2nd harmonic distortion while at the same time
> biasing the single sex amplifier.
>
> Take the center tap and tie it to a positive voltage. Feed the other two
> inputs to the transformer with a differential signal. You have blocked
> DC from the output, restoring a ground referenced single phase. (as
> opposed to differential) output.
>
> These active whips are prone to picking up electrical noise. Fine if you
> live in the boonies. Not so good for urban dwellers.
>
> If the antenna is just a wire in the air, I'm not sure what good it does
> to capacitively couple the input. Who cares if some DC is floating on a
> wire just poking in the air.
>
> While some people think of transformers are bandlimiting devices, note
> that all those coupling caps have series inductance. There is no free
> lunch.
>
> Just meditating out loud, if you were to go push pull with a ferrite
> antenna AND you are winding it yourself, you could avoid the biasing
> resistors by putting a center tap in the antenna itself, then tie that
> center tap to an appropriate bias voltage. I haven't seen this done, so
> their may be a gotcha with that scheme, but the science is good.
> Generally you will get a lower noise circuit if the input device is an
> amplifier rather than a buffer.
>
> Lanksford's input stage is essentially a push pull buffer, but I don't
> see that cancelling 2nd harmonics like a push pull amp. But for a whip,
> which is a single ended input, I don't see a way to get a differential
> input. Not true for a ferrite antenna.
>
>
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