[time-nuts] Down-conversion to IF and sampling

[Tim Shoppa]

It would be (as you point out) a bad idea to have the the ADC sampling rate
to be exactly the same as the downconversion oscillator.

In many cases they would both be derived from the same master oscillator.
But the ratios would be consciously chosen to not be simple integer
multiple relations so beating between the ADC and the downconversion
oscillator would get washed out.

This issue of avoiding simple harmonic relationships between receiver
oscillators (and samplers) long predates digital radios, it is a goal in
any superhet to avoid "birdies".

Tim N3QE

On Sat, Dec 23, 2017 at 9:46 AM, Stephan Sandenbergh <ssandenbergh at gmail.com
> wrote:

> Hi All,
>
> Consider the following very common scenario: A perfect RF signal is
> heterodyne down-converted to baseband using an offset oscillator. Let's
> assume this oscillator has x(t) = xo + yot. This produces a time and
> frequency offset baseband signal. Then, this baseband signal is coherently
> ADC sampled using that same offset oscillator.
>
> What would the effect of this coherent ADC sampling be?
>
> See attached diagram. Here I assumed the ADC timebase is a time-dependent
> function of the oscillator offset. However, it feels like I'm making a
> logic error? I can't remember ever seeing anyone accounting for the ADC
> time-base errors in coherent heterodyne down-converter stages. I have
> limited experience though.
>
> Regards,
>
> Stephan.
>
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