9 Sep 2012

WISPY-10 schematic (draft A)

OK, I promised to show the schematic of WISPY-10 (there may be versions for lower bands later) today so here it is. I have breadboarded and tested the TX and the RX parts on-air and both work well. In the transceiver I have used the oscillator with an additional buffer (x2) stage on TX and this has still to be tested but I don't expect any issues at all. Also shown is the TX-RX change-over using the back-to-back diodes in the middle of a series tuned circuit on the RX input. On TX the diodes conduct and the capacitor forms part of the output low pass filter.
Updated schematic (an error spotted by G3TFX)
At the moment I am not planning any further work as I want to crack on with the 472kHz work but I expect to return to this in a few weeks' time.

Click on the schematic for a larger, more readable, version. Please let me know if you spot any obvious errors (e.g. where I have put grossly wrong values of components in) so I can update the schematic.

Please feel free to use this as a starting point for your own version. All in, I doubt the components would cost more than £5 (7 euros or 8 dollars) even if all the parts were obtained new.

The 2N3866 runs quite hot in class A and I want to change this biasing in the final version. I may also susbstitute the 2N3866 for a couple of paralleled 2N3904s to save further cost.

The same circuit will work well on PSK31 (pull the crystal to the right frequency of course) but both with WSPR and PSK31 do not over drive the PA. This is a DSB transmitter and there will be splatter either side of the carrier for tens of kHz if driven too hard. Adjust the drive so the output from the PA is a clean, DSB signal free from obvious audio distortion.

7 comments:

Anonymous said...

Nice design. Thanks
PA2RF

threeme3 said...

Roger, nice compact design!

Maybe possible to have SSB capability by phase shifting 45 degrees of Fc/2 oscillator signal and feed in another mixer; this translates to quadrature signals. Thus sound cards can feed I/Q signal, or maybe a simple 90 degree phase shift network in the audio path will do the job as WSPR signals having small bandwidth.

Original idea here:
http://draaggolf.blogspot.com/2010/01/30m-subharmonic-iq-sdr-receiver.html


73, Guido

Martin G7MRV said...

Roger, I have finally finished building the Wispy transceiver and am now testing and aligning.

Two quick questions if I may -

How sensitive was your receiver?

I have used now a different, 7-pole LPF, how much influence does the trimmer cap have at that point?

Cheers
Martin

Roger G3XBM said...

Martin,

Never measured sensitivity but it hears most that the FT817 does. It is pretty sensitive.

If you mean the trimmer before the Polyakov mixer it should still be there.

73s Roger G3XBM

Martin G7MRV said...

Hi Roger,

Ive been right through the design and still cant get any better than -50dB or so LDS.

One thing I have discovered, is that the drive level from the oscillator is just 24mV pk-pk, yet a stand alone colpitts I have on 14MHz is well over 5v pk-pk!

Ive played about with the oscillator but even with no load on it I cant get much out of it? Not sure whats going on there, the Tx side seems fine, but its the same oscillator.

I might jury rig just the mixer and oscillator as a separate circuit and see how it performs.

Roger G3XBM said...

Sorry to hear this Martin. The oscillator level should be healthy so investigate why this is. You could add a buffer between the oscillator and RX mixer, but this should not really be needed. Try tweeking osc values to see if it needs more feedback.


Martin G7MRV said...

Sorry, that level in my last post should have read 0.5v!

It does seem very low. What would you say is an ideal level of LO drive for this type of mixer? Ive not been able to find that out anywhere so far.

Ive given up for tonight!