Saturday, November 6, 2010

SW+ Transceivers for QRSS

I was shopping for QRSS kits once again. And, once again at K1SWL.
The RockMites work great for QRSS, however, this time it was the SW+ Series Transceivers.
The SW+ design employs a three pole crystal ladder filter and a VFO tuned by a varicap.
Lets look at the different models and what would get us to a QRSS range.

SW+ 80
This model uses a 8.000MHz intermediate frequency. Subtractive mixing with a 4.500MHz crystal (standard but maybe harder to find) gets us to 3.500MHz.
The color-burst range is reached by subtractive mixing with a 4.433618MHz crystal. The result will be 3.566382MHz, somewhat low, however, the 4.433618MHz xtal can easily penned down to 4.420MHz. Mixing would then result in 3.580MHz.

SW+ 40
This model uses a 4.000MHz intermediate frequency.
Additive mixing with a 3.000MHz crystal gets us to 7.000MHz.
Subtractive mixing with a 11.000MHz crystal gets us to 7.000MHz.
Subtractive mixing with a 11.059MHz crystal gets us to the novice range 7.0599MHz.
With the help of crystal-penning, the novice range is reachable by additive mixing with a 3.072MHz standard xtal results in 7.072MHz, with some penning 7.0599MHz should not be any problem.


SW+ 30
This model uses a 7.68MHz intermediate frequency. I can't think of any cheap crystal to match up with that frequency. However, there is help!
The 7.68MHz i.f. could be changed to 6.144MHz. From here on, with penning again, additive mixing with a 4.000MHz xtal-oscillator will result in 10.140MHz.

SW+ 20
This model uses a 9.000MHz intermediate frequency. Additive mixing with a 5.000MHz crystal gets us to 14.000MHz
Non-qrss bonus mod: 5.0688MHz super-VXO for regular QRP work.
Further option: use a 10.000MHz intermediate frequency. With a 4.000MHz oscillator we arrive at 14.000MHz. An (additional) alternative 4.096MHz XTAL will result in the vicinity of 20m WSPR.
A 80m QRP-XTAL will result in 22m hifer gear at 13.560MHz.

Possible Mods
  • Preserve the original QRP-TRX: toggle between the LC-VFO and an external XO.
  • QRSSify the TRX: replace the VFO's RC-network by the crystal.
Both solutions have their advantages.
The first option keeps the original function, that's nice! Secondly, since the 4.500MHz crystal may be hard to get, one may consider using a 9.000MHz oscillator, based on a 27MHz standard crystal, and divide the frequency by two before, as to obtain 4.500MHz.
The second option renders all VFO parts essentially obsolete. However, the VFO is realized by means of a varicap, which could be used to pull the crystal for coarse frequency adjustment.

FSK modulation can easily be generated with the help if some sort of diode pulling the TX converter's XTAL in the usual fashion.

Audio for the sound-card could be tabbed off just at the AF final's input, just behind the muting transistor.

Due to the 100% duty cycle of FSK-QRSS, the TX output should be reduced.


Conclusions
  • SW+ 80: subtractive mixing cancels temperature drift partially
  • SW+ 40: best choice: subtractive mixing 11.000 & 11.059MHz
  • SW+ 30: there is a better kit for 30m: the PSK-30
  • SW+ 20: the 10MHz i.f. mod will offer the most

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