[Elecraft] K4: superhet vs. direct sampling

[Richard Corfield]

Which is interesting. We were taught that you can recover signals up to the
Nyquist frequency (half sampling frequency) before aliasing becomes an
issue. People low pass filter well before then to avoid aliasing issues,
but it means you need a sampling frequency sufficiently more than twice the
highest frequency you want to record. So presumably for the 6m band that's
sufficiently greater than 110,000,000 samples per second.

What gets me though, is when you look at the waveforms and the sampling
waveforms, a signal at the Nyquist frequency could have magnitude from 0 to
full size depending on its phase with respect to the sampling. Assuming
regular sampling (do any dither the sampling?). Move just below Nyquist and
you'll see a beat frequency come in as it moves in and out of phase. I
guess the maths is based on infinite time - Fourier being the integral from
-infinity to +infinity - but we've not got infinite time to listen to an
infinitely long SSB signal.

You can buy a reasonably cheap digital oscilloscope (Tektronix TBS1000 -
£600) which boasts 1GS/s so a Nyquist frequency of 500MHz. They claim 50MHz
analogue bandwidth.  (If you're thinking of buying this double check the
figures first rather than take my work for it. The marketing material was a
little opaque).

A 2nd order filter (cheap, simple) with a 3dB point at 50MHz and
40dB/decade would be about 40dB down by the Nyquist frequency. In the lower
bands, say 10MHz, the first aliases are from 990MHz which is 55dB down if I
remember my maths right. I'd assume someone would use better than a 2nd
order filter or a faster sampling ADC.



On Thu, 6 Jun 2019 at 10:38, David Woolley <forums at david-woolley.me.uk>
wrote:

> On 05/06/2019 13:23, W2xj wrote:
> > Direct sampling has no image issues.
>
> That's because they are called aliasing issues!
>
> --
> David Woolley
>
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