[Skimmertalk] Red Pitaya 1:14 impedance transformator + SV1AFN pre-amplifier

Bob Wilson, N6TV n6tv at arrl.net
Tue Oct 24 13:44:12 EDT 2017

On Tue, Oct 24, 2017 at 5:53 AM, W3OA <w3oa at roadrunner.com> wrote:

> Comparing test 3 and 6, both with a pre-amp, adding the transformer
> improved the S/N by 5 dB.
> Does this make sense?

Perhaps it does, if the pre-amp, like a filter, is designed to work with a
50 ohm load, not the approximate 1K ohm load (or higher) of the Red Pitaya
inputs.  My theory is that if you present a preamplifier with a "really bad
SWR," it does not operate linearly, and can generate lots of
intermodulation products (spurious signals caused by sums and differences
of two large input signals).

Please note that I have no EE degree (I'm really a software guy), so I'd
really appreciate it if someone with a better understanding of
pre-amplifier performance can add to this discussion.  I'm just trying to
explain my observations.  I saw huge FM mixing products almost disappear
when I added a transformer between the pre-amp and the Red Pitaya.  Compare
the two spectrum plots at the bottom of this page:


Bob, N6TV

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