FM'ing the PM MASTR II

FM'ing a PM GE MASTR II
By Jeff DePolo WN3A

The best/easiest way to Frequency Modulate (FM) a Phase Modulated (PM) Mastr II UHF or Highband exciter is to graft an FM ICOM (ICOM=GE for channel element) into it. Take a run-of-the-mill transmit ICOM from a highband PLL Mastr II, or for UHF if you can find one, a UHF FM ICOM, and just wire it up. I buy the male pin strips and female sockets that mate with the pins on the exciter board and the PLL ICOM and make up a little 4" "harness" to make it a plug-and-play operation. The pins/sockets are a standard Molex/Waldom part available from Mouser et al (they are the 0.156" spaced series).

The only mods you have to do the FM ICOM are very minor:

1. Ground the ICOM enable line via a jumper between pins 7 and 8 inside the ICOM (or just flow solder between the pads for pins 7 and 8).

2. Bias pin 6 to 6.1V (or thereabouts, not critical as long as it is very stable). I use a 10K/15K resistor voltage divider off the 10V line. Inside the ICOM, I put 10K between pins 1 and 6, and 15K between pins 6 and 7. 1/8 watt resistors fit inside the can on the solder side of the board, or you can do it on the component side with larger resistors if necessary. Resistors should be very tight tolerance and thermally-stable (not Radio Shack carbon composition), otherwise you risk additional carrier frequency drift.

3. Feed AC-coupled audio into pin 3. Limit/filter it externally (shame on you if you don't).

4. Connect +10, RF out, ground, and compensation lines back to the ICOM plug on the exciter board via a little harness. Use shielded cable for the RF connection.

Remember, if you use an EC PLL ICOM (why are, by far, the most readily available) that you still need to put a 5C ICOM on the exciter board to provide compensation. Or if you're lucky enough to find one, use a 5C or 2C PLL/FM ICOM. (Note: this applies to M2 stations. On mobiles, the Rx and Tx compensation lines are tied together).

For reference, here are the pinouts for the PM and FM ICOM's so you can see how to make the harness. Note that the highest-numbered pin is the one that is offset from the others:

PM ICOM
-------
1 - +10V
2 - Compensation
3 - RF out
4 - Ground
5 - Ground (not used in my harness)
6 - Select <offset pin> (not used in my harness)

FM ICOM
-------
1 - +10V
2 - RF out
3 - Audio in
4 - Compensation
5 - Ground
6 - +6.1V bias (not used in my harness)
7 - Ground (not used in my harness)
8 - Select <offset pin> (not used in my harness)

So, to recap, the harness should connect 1->1, 2->4, and also 3->2 using a shielded cable (ground shield to pin 5 on both ends). If the lead length is relatively long, use shielded cable for the audio input as well because it is fairly high Z.

Note that the select lines are the offset pins on both the PM exciter board and FM ICOM. Because a standard Molex plug won't fit on that offset pin, I don't bother wire to wire the select line and instead just short it out inside the ICOM. So even though there are 6 pins on the PM exciter board, you use a 4-pin Molex socket. Likewise you use a strip of only 5 male pins to plug into the FM ICOM.

Oh yeah, you can do this on a *highband* Mastr II as well, but you obviously don't get the advantage of lower transmitter noise like you do on a real highband PLL exciter.

Please please please, if someone tells you that FMing a Mastr II is as easy as AC-coupling audio into a regular PM ICOM compensation line, make sure you look at it on a spectrum analyzer if you try it. I've seen it done (and tried it myself several times), sometimes it comes out clean, more times than not it makes a big huge ugly mess. To do it right takes additional surgery inside the ICOM, including adding a good modulation varactor. Not worth the aggravation IMHO. Aside from the risk of spectral pollution, the compensation varactor in the ICOM's is not very good for modulation. The additional varactor in the PLL/FM ICOM's is good for audio (after all, that's what it's there for).