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 high band exciter is to graft an FM ICOM (ICOM=GE for channel element) into it. Take a run-of-the-mill transmit ICOM from a high band PLL MASTR II, or 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 inch "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 ICOMs 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 impedance.

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 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 high band MASTR II as well, but you obviously don't get the advantage of lower transmitter noise like you do on a real high band 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 ICOMs is not very good for modulation. The additional varactor in the PLL/FM ICOMs is good for audio (after all, that's what it's there for).

Burt I. Weiner - K6OQK offers the following cautions as well:

Regarding using the temperature compensation Varactor for direct FMing a Master II... I may have missed it, but I've never seen any write-up on just what the problems are, just comments saying, "Don't do it." So, if I may be allowed to expand on this... I work in the broadcast engineering world and have experience with broadcast FM exciters, and again, not finding anything with any specifics I decided to try this for myself. I have a PM exciter board for Two-Meters that I used for my 'speriments. As a result of my 'speriments I will tell you what I learned.

1. The temperature compensation Varactor diode is biased by the associated temperature sensitive resistors. This bias does not lend itself for good modulation linearity of the diode. Also, since the temperature affects the bias of the diode, it also affects the "modulation sensitivity" of the Varactor, and since the diode is not necessarily operating in the linear portion of its curve, modulation can really go all over the place with changing temperature, regardless of how well you control the audio you are feeding into the diode. If you properly bias the Varactor diode for better linearity you will destroy the temperature compensation curve. None of this is a good thing.

2. Depending on how you feed the audio into the Varactor diode, it is possible to conduct all sorts of stuff into the diode, which in turn modulates the diode and will produce grunge in the sidebands. Keep in mind that the deviation, grunge and all, at the oscillator is multiplied however many times to get to the final frequency. This grunge, which you may not hear on your receiver, can extend out many tens of kHz from your desired carrier frequency.

Of course it's possible to properly filter and decouple the audio input to the Varactor, but then you still have the problem stated in issue #1 above. Some of the early FM broadcast direct-FM exciters used the same diode for modulation and AFC. This was also true for many of the early direct-FM 940 MHz STL transmitters. These exciters were prone to having peak modulation control issues. They discovered that you shouldn't try to use the same diode for AFC and modulation because the AFC correction voltage (diode bias) would change the modulation sensitivity and linearity of the Varactor diode.