Sunday, January 31, 2010

QRSS RM80 final verdict (QRT)

Here's what you wanna do when going for a Rock-Mite-80-QRSS-mept/grabber.
  1. get a Rock-Mite 80 kit from Dave
  2. get two 3.579545MHz crystals
  3. get one 4.7pF and one 22pF capacitor
  4. build the kit, leaving out C8 (beep beep)
  5. for C12, use 4,7pF in place of 47pF (reduces chirp)
  6. for C2, use 22pF in place of 33pF (lifts filter response)
  7. get a 8-pin PIC (to replace U3) and program it as wanted

The reduced coupling from the oscillator to the (keyed) buffer results in lower output power. 4.7pF to me seems the working compromise. I tried all of the following values (in order of the experimentation timeline): 47pF (original - severe chirp, PA becomes hot), 27pF (still intolerable chirp, PA warms slightly up), 2.2pF (no chirp at all, nearly no power), 18pF (chirp), 10pF (chirp), 7.5pF (still too much chirp) and finally 4.7pF.

Optional, a 10V Zener diode could be dropped into D5's empty place. This will lower the "upper" qrg by about 200Hz, further reducing chirp.

For the mode, I would advice A1A with QSK, for timing, QRSS6 will do just fine. 

Programming: With no further hardware modification, an 8pin PIC, such as the 12F629 can be used.
GPIO4 low will provide the varicap diode with 12V, resulting in the higher frequency, which is used for TX.
GPIO5 high will key the buffer and the PA.
I found that it is sufficient to low GPIO4 just before keying the transmitter. Returning GPIO4 to high directly after transmission allows for full QSK. No additional chirp in the TX signal.
To also receive, e.g. as a grabber, my program receives for 10min before TXing the message again.

I did all that, it works, concept proven.... hope to see more signals on the color burst frequency and more grabbers on the internet in the close future.

PS: Somewhat obvious... this mod will work on any frequency we got crystals for... e.g. 30m.