Here's some useful info pulled from usenet posts:

The allmode transceivers tend to use a heterodyne scheme for transmit so that you modulate a crystal oscillator stage rather than the PLL. As I recall, an early Azden FM transceiver did this as well and had reasonable 9600 performance.

Otherwise there is a trade-off between poor frequency response (fast PLL time-constant) and poor T/R switching times (slow PLL time-constant). There are ways to compensate for the frequency response, but it's a bit tricky and may not be all that stable in a production-line radio.

TEKK and some other data radios work fairly well, as do some converted crystal-controlled commercial radios, such as Motorola Mitrek.

Bob Nielsen, W6SWE

There are a few syth'd radios which can be modded to run very well., these are the ones which use an interpolation oscillator to mix the vco to the required frequency. As the interpolation oscillator is outside of the pll, it can be fm'd without problems. further more these radios are usually fairly cheap, with filters like barn doors. Please dont ask for type as I've forgotten !!!, but they were low end market japanese 70cms rigs. these rigs are easy to spot if you can get hold of the circuit diagrams, you know those funny diagrams with lots of squiggles on !!!

73 richard g8jvm

What is a "true FM" modulator, Steve? Are you talking about the difference between a phase modulator (which produces PM) and a frequency modulator (which produces FM)? This is potentially quite confusing, since there are a great many radios out there that indeed have "true" (or "direct") FM that still aren't suitable for 9600 baud. Yup, that's right, even a "direct" or "true" FM radio may not be suitable for 9600 baud. How is this so?

Common PLL synthesized VHF/UHF radios use a "single-loop" design, in which the frequency of a VCO is controlled by the PLL feedback. VCOs are typically a conventional LC oscillator where some part of the resonant circuit is controlled by a varactor diode. FM modulation is introduced directly into the VCO, sometimes on the very same control line used by the PLL, sometimes into a separate varactor circuit specifically designed for modulation rather than wide-range frequency control. The modulation *directly* and *truly* modulates the frequency of the VCO.

The problem with this and 9600 baud is that the PLL feedback will cancel out the lowest frequency portion of the modulation. Modulation inside the loop bandwidth will be cancelled out by the loop (and, not surprisingly, appear on the feedback control line).

So, it sounds like the "True FM" radios you're talking about are non-PLL radios that use very stable oscillators (i.e., crystal oscillators) and are directly modulated by varying a reactance attached to the crystal. In other words, it sounds like you mean "crystal-controlled, direct-FM" when you write "true FM", right?

By the way, not all crystal designs are direct-FM, there are a great many phase-modulated (PM) crystal transmitters. These are useless for 9600 baud, since they do not have a DC response.

Furthermore, there *are* PLL designs that offer DC response. This is done either by using a crystal-controlled, direct-FM oscillator at an intermediate frequency and mixing it onto the desired frequency with a PLL-controlled local oscillator, or by using "two-point" modulation, where both the VCO and the PLL reference oscillator are direct-FM modulated (inside the loop bandwidth, changes in the reference show up as direct modulation of the VCO).

So, all this longish response is intended to point out that it isn't as simple as finding a radio with "True FM". What is required for 9600 baud operation is an FM (direct-FM, not PM) radio with response down to DC. Sure, this is mildly confusing, which explains why 9600 baud packet operation is so challenging to most amateurs.

Dana H. Myers K6JQ

Dave G wrote:

>Sporting an easy to use dedicated data connector, we thought that the >new Yaesu FT-2600M just might be useful for 9600 baud packet in our >local TCP/IP network. The connector provides separate and isolated >inputs and outputs for 9600 and 1200, plus Yaesu includes their DCS or >Digital "PL" in the radio, a characteristic shared by the commercial >radios we've used successfully (Ritron & Motorola).

>Unfortunately we were disappointed with the performance of this radio at >9600. As near as we can tell, Yaesu fell short in two important areas:

This is the usual practice for Japanese FM radios... they put the connector and the label on it because the buyer wants to see that, but otherwise they are just as unsuitable for packet as the earlier models.

I have an FT-212RH and an FT-712RH and the characteristics are exactly as you describe. Only you have to make the external connection yourself before you'll find out!

BTW, 80ms txdelay is LONG. A local node uses an AEG/Telefunken "Telecar" commercial trx and it works well at 20ms, txdelay set to 30ms to allow for some extra delay at the receive end. It also features DC-coupling of the modulation! A variable DC voltage on the input lets you tune it a few KHz up and down. Ideal for 9k6!

Tekk transceivers are similarly fast. 10ms would probably work as well but the timing is done in 10ms steps and has a 10ms uncertainty.

Rob Janssen PE1CHL

When I started fooling with 9.6 kb, I thought the same. But the TEKK radios aren't a panacea. The receivers are pretty deaf, yet they have wide open front ends. They are IMD generators on a high RF site without cavities in front of them, and if you add cavities, that just makes them deafer. So they are really only useful at electrically quiet sites, and paired with a station capable of delivering a strong enough signal to overcome their lack of sensitivity. Thus they can serve in some cases as user radios, but they won't do in general as network radios.

You need a more robustly designed radio to service the network. Now you *can* convert commercial surplus such as a Micor, Mitrek, or GE Mastr II, but that's not *off the shelf*, and I've seen a resistance by you and Charles to the idea that you might actually have to do some radio work. So that limits you to appliances, and the only appliances sold for amateur radio which work adequately out of the box are the multimodes.

You still have the problem of dealing with HDLC encoded signals too. So ordinary serial interfaces won't do either. You either need a HDLC hardware card in the computer, or some sort of software bit banger. You wouldn't want to use a TNC (how quaint) to handle HDLC to serial RS232 conversion because you double latency when you do that. You can't afford that when you start out with such a low speed.

Trying to make a G3RUH encoded 9.6 kb system work well is neither simple nor cheap when you begin to look at the problem from a network perspective (and limit yourself to appliance solutions).

Gary Coffman KE4ZV