[hfsig] 20m WSPR shield for Raspberry Pi

KB3CS - Chris kb3cs at arrl.net
Wed Apr 10 08:25:56 EDT 2019

it's the WSPR software driving the Pi clock, and the transmitter is
amplifying the modulated clock signals, actually.  .. but we understand
what you meant.  :-P

in a previous post, i noted the signal purity improved with a better power
supply for the Pi. i suspect the earlier power supply had too much

i note upon inspection of the 30m WSPR board schematic, it may also be
helpful to add a bypass capacitor to ground at the mutual junction of R5
(15k) and R1 (20k pot). there is no decoupling from the Pi power pins save
for C3 (47uF). better decoupling would have an inductor prior to C3. absent
this, a bypass capacitor at the R5-R1 junction would keep power supply
ripple/hash from modulating the gate of the FET (Q2). an alternate location
for this new bypass cap might be the mutual junction of the wiper of R1 and
R3 (20k). i am not sure what the design impedance at the gate of Q2 should

i will eventually test this, but someone may get to it before i do. if so,
please report results. thanks!

 - 0x49 -

On Mon, 25 Mar 2019 20:44:45 -0500, Bruce Raymond <bruce at raymondtech.net>
I think the 30m WSPR board should work with the same software that the 20m
version uses. I haven't tried it, but the 20m and 30m transmitters are
doing the same thing (driving the Pi clock to generate WSPR symbols).

I discovered something strange today. I have the 30m WSPR board hooked up
to a Pi 3 with the experimental software loaded. I hooked my scope up to
the output (FFT on scope) and I'm seeing a whole bunch of spectral garbage
several MHz wide below the carrier. It's ~-30 dBc. It goes away when the
transmitter shuts off and comes back when the transmitter turns on.

I don't recall seeing this before, so I'm wondering if it's the new
software load causing the problem? I'll go back to the previous software
load and see if it's doing the same thing. If it is the software I'm
suspecting the new version of WsprryPi is the culprit.
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