Wednesday, April 6, 2011

17m XTAL Controlled QRSS / WSPR / QRP / SDR

Sorry for having been silent for a while. Some new thought, although not entirely mine, I believe sharing it this way would be more than appropriate.
The idea about the frequency generation is borrowed from DJ1ZB. Ha-Jo uses CB crystals on their fundamental and doubles the result.

Doubling sounds very much like two things I previously disclosed on this blot, namely, the subharmonic direct conversion receiver (e.g. this) and the subharmonic I/Q-SDR (see earlier posts on the SDR and possible frequencies).
Both designs rely on a local oscillator on half the operating frequency.

Concentrating what could be interesting for QRP, QRSS and WSPR, lets have a closer look to the available crystals.
  • 27.105MHz (12T) => 9.035MHz x 2 = 18.070MHz
  • 27.135MHz (15T) => 9.045MHz x 2 = 18.090MHz
  • 27.155MHz (16T) => 9.051667MHz x 2 = 18.103333MHz
  • 27.165MHz (17T) => 9.055MHz x 2 = 18.110MHz
The frequency generated with a 15T-XTAL appears to be ideal for I/Q-SDR since the whole CW & Data & beacons range will be covered by 48kHz sampling rate. Have a look: difference 18.090-0.024=18.066 and sum 18.090+0.024=18.114.
Additionally, 18.090MHz is the quite close to the QRP frequency, a subharmonic direct conversion receiver would be an obvious choice, so would be a transmitter with a doubler...

Very obviously, the other frequencies are directed more to QRSS and WSPR. I am not going into the QRSS feature here, it is kinda trivial (see above).

Remaining topic: WSPR. The frequency produced by the 16T-XTAL is very very close to the WSPR "dial frequency" 18.1046MHz. With a Pierce oscillator the tiny amount of 633Hz upwards pull should not be a big deal; remember, on half the frequency, one only needs to pull half the distance.
RX: subharmonic direct conversion.
TX: subharmonic mixing of the local oscillator with an audio signal should create a DSB signal... that's the theory... I have not yet tried it yet, however, I fail to see any reason why this should not work.

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