10 Sep 2012

TX only WSPR beacon

UPDATE 11.9.12: see later post for improved version

WISPY is a full 2-way WSPR transceiver but there may be occasions when just a simple WSPR TX is needed. This is the (current) schematic of the WSPR TX beacon for 10m, but the very same schematic with a different crystal and values should work on any HF band. On lower bands the doubler could be removed and a simple on-frequency crystal substituted.  The doubler approach was the cheapest way for 10m WSPR.

The schematic needs rationalising and some further simplification, so treat this (as always) as a design starting point.

5 comments:

Tiégui said...

Hello Roger !
I'm happy to see this kind of QRP schematic for the WSPR. Actually, people don't know how to use WSPR has they are always with power around 5 to 10 W or more !!!!

I remember this modulation is based on phase shifting. So why not using the final transistor in a class C ?

73, from Thierry.

Roger G3XBM said...

Thierry,

With a normal SSB WSPR signal a class C PA is fine.

When the signal is a DSB one (two tones being emitted as in this design) then any non-linearity will result in a signal that will splatter over many tens of kHz either side of the carrier.

73s Roger

Tiégui said...

Hello Roger,
on wiki they said it's a 4 tone FSK. But I still do not understand why class C PA is not suitable here.

Because here, there is no amplitude modulation, just frequency modulation, class C should be ok even if it's DSB or SSB. Am I wrong ?

Roger G3XBM said...

Thierry,

If you put two FSK signals spaced about 2.5kHz apart into a non-linear amplifier then the transfer characteristic (non linear) will result in additional signals (intermod products) spaced at 2.5kHz intervals either side of the original ones. This is why a 2-tone test is done to check linearity of an SSB PA). With a single FSK signal this will not happen but with DSB there are TWO FSK signals present and this will be an inevitable outcome unless a linear PA is used.

Tiégui said...

Of course ! Thank you for the explanation, in fact it's like a two tone test because there is always two tones present, one in LSB and an other one in USB.
I didn't think to that and that is why it was confusing to me :))

73 from Thierry.