3 HOW 9600 BAUD PACKET IS DONE With mirrors, of course :-) 3.1 MODEM The PacComm NB96 G3RUH modem board plugs inside many TNC,s, such as TNC2 clones (MFJ 1270B, etc). There is a special modem available for the MFJ 1278T (and maybe it'll work with other MFJ's, too???), from MFJ. It takes the place of the 2400 baud modem. The K9NG modem kit is cheaper than the G3RUH modem, but is a little fussier about receivers. If the radio you want to use has a pretty broad receiver passband, it'll work almost as well as the G3RUH. If the receiver is tight, the K9NG will be _very_ noticeably poorer. However, this can be somewhat ameliorated by widening the K9NG's receive filter, as detailed below. If you need to externally mount your modem and use a ribbon cable, mount the modem header on the opposite side of the PCB, or use a second short ribbon cable. Ribbon cables "flop" the pins, i.e.: 1 3 5 7 9 ___________________________ 2 4 6 8 10 2 4 6 8 10 1 3 5 7 9 By placing the connector on top rather than underneath, the flop is reversed. Normally, you'll want to use the TNC's keying circuit. With the G3RUH, you have no choice. 3.2 TNC PacComm, Kantronics, Gracillis, MFJ and probably others have TNC's with 9600 baud G3RUH modems built in. Check the ham mags, call the manufacturers, or contact your local dealer for information. Many of us will take the route of installing our own modems. These may often be installed internally. Sometimes it may be necessary or desireable to install these externally. As with all RF environment installations, shielding is important. If an external box is desired, it really should be a metal one. Shielded cable to the radio is mandatory - it will not work without it. Some kind of RFI- proofing of the modem disconnect cable is needed as well. This should be kept as short and unexposed as possible. The G3RUH modem requires you to use the TNC's keying circuitry, and this is recommended for the K9NG as well. If you do this, don't cut the PTT line on the modem disconnect. 3.3 RADIO Many modern rigs are true FM, and it's very simple to add a 9600 baud port. You need only a couple of internal connections and decoupling components to bring out the "raw" transceiver: the direct detector output and direct FM modulator input. Most modern rigs use receiver chips, such as TA7761F/P, LA5006M, LC7532M, TK10420, TK10424, MC3357P, and others. Note: ALL of the chips mentioned here use pin 9 for the detector output. Others may or may not, so check first! Transmit "audio" is injected through a 5 to 10 uF non-polarized capacitor through a 5K resistor into the modulator. This is typically a varactor diode in a transmit oscillator stage, and is pretty simple to find, by simply tracing forward from the microphone. If this is "over your head", see the free offer at the end of this manual. 3.3.1 CRYSTAL CONTROLLED PHASE MODULATED RADIOS We have found that synthesized, true FM multimode rigs using varactor modulation in a crystal oscillator stage tend to work best. They tend to have a tad wider IF filter (i.e. CFW455E) and pass data with less distortion. Most synthesized rigs can be made to work, most with extremely simple mods. However, if your rig is crystal controlled, it can be made to work. If it's a true FM rig, it's simply a matter of connecting directly to the FM modulator. This will always be in a crystal oscillator stage.