Q and A's
1. Why are there two R112's?
The microphone amplifier originally had two diodes that acted as a limiter. When the ARRL tested that radio we discovered that these diodes were causing distortion so I removed them and put a 39K resistor in place of one of the diodes then added AGC to the TX audio path in the DSP. Builders can either do what I did and put a leaded 39K in D1 or D2 on the PCB or change R6 to a 27K ohm and leave D1 and D2 open.
2. Where can I get RL1 - RL4?
These relays are no longer available at All Electronics. The manufacturer is COTO and the part number is 2200-2302 and they are available new from several retailers for $8 - $10 each. Don't buy them!!! The new off-board PA doesn't need them. Build the DSP-610 PA.
3. What's up with the silk screen for R60 and C103?
The component on the PCB labeled R60 is actually C103 and vice verse. The transmitter won't work if these parts are reversed.
4. Why is my PA oscillating in the VHF range?
It turns out that the RF16HHF1's need a little gate inductance, if you build the new PA, it's taken care of. If you build the PA on the main PCB you'll need to cut the traces going to each of the gates and put in a 68nH inductor.
5. What additional bypassing is needed to get the on-board amplifier to work?
There needs to be a 0.1 uf cap across the relay coils RL1 - RL4. Also cut the control line traces TX_6, RX_6 and TX_10 where they go to the AS-179 switches and insert a SMT ferrite bead in the line. There also SMT ferrite bead in the 5VD line that feeds RL1 - RL4, finally a 0.1 uf cap to ground on pin 7 of U9.
6. Why the PIC18 clock having an issue?
Try decreasing R112 to 390 ohms. You could also use a real level translator there instead of a MOSFET.
7. Can I build the DSP-610 for other bands maybe 80, 40 and 30M?
There's a fairly complicated switching scheme in the low level RF sections that use Skyworks AS179 RF switches, they are spec'ed down to 20 MHz. I'm using one in the IF at 10.7 MHz and it seems to work OK but I don't know about 3.5 MHz. The LNA's are probably not a good idea for the lowbands either they have really high gain and very low noise figure which is important on 10 and 6M but not 80 and 40. You could probably trash everything after the SBL-1 mixer and make band pass filters for 80 and 40, a single 10 dB gain preamp, build the off-board PA for 80 and 40M and you'd be running.
30M is going to be a problem because of the 10.7MHz IF. It's nowhere near 43 dB down (FCC spec) so the RF filter has to knock it down further and on 40 and 80 the LPF in the PA can help with a few more dB. On 30M, designing a filter that passes 10.15 MHz and rejects 10.7 is going to he touchy. Lot's of thing to think about if you are rebanding.