I would hate to see your insightful comment missed. I haven't seen anyone else pick up on the part of your message about the linear transponder. If someone wants to pass many different modes through a repeater, you can't do better than a transponder. I've been on the transmitting and receiving end of them for years with the satellite world, and we haven't had to wholesale change out them with the change from FM video to digital modes. They just work.
Not being an expert on designing them, my understanding of the process is as follows. On a block level, they're much simpler than they seem. Take a tightly bandpass filtered IF from just about any quality radio, AGC it really well, frequency convert it to the desired output frequency, run it through a LINEAR power chain, and you now have the transponder. A simple RF envelope detector can act as the "squelch" so that the output power chain isn't constantly running to amplify ambient noise that wanders into the system's input bandpass. The only other challenge is to be able to ID the repeater. A simple MCW oscillator somewhere in the chain, either at the IF frequency or output fundamental, and you're legal. FM, D-Star, P25, AM, CW, SSB and virtually any mode that'll stay within the bandpass of the IF will go through unimpeded ( and undistorted.) One possible minor down side is that the user's radio needs to be somewhat on-frequency. If they come in 1 KHz low, they go out 1 KHz low. In theory, it's not the repeater owner's problem, but can be noticed on the output. I remember someone a few years back doing mods on a standard repeater amplifier to "linearlize" it for ATV and SSB with some bias network additions. As long as the PA is reasonably linear, slight distortions in the linearity curve shouldn't create huge issues except with modes like QAM or other modes with serious amplitude-dependent requirements. I wish I had the time. Being an RF guy, not a software type, it sounds like a great project. 73 de N1FNE --- In Repeater-Builder@yahoogroups.com, Kris Kirby <[EMAIL PROTECTED]> wrote: > > On Tue, 24 Apr 2007, Steve Bosshard (NU5D) wrote: > > Took a couple of antique GE Phoenix SX mobile radios. Programmed for > > 442.0/ 447.0. With TOT. Carrier Squelch - Took Receiver Un Squelched > > lead to PTT thru a one transistor keying transistor. > > > > Took VOL / SQ Hi and ran it thru a single common emitter stage - > > bipolar amp and applied the collector output to the high side of the > > TX deviation control and set for +/- 1 Khz TXD. Did have to bypass > > the emitter leg of the single stage amp and wallah - DSTAR Repeater - > > sort of. Maybe P25 repeater too? Simple - really do need to > > regenerate the data signal and key on detected data with a CCD chip to > > give the preamble tone time to get thru. > > Excellent work Mr. Bosshard. This is one of the things that I've wanted > to explore with D-Star but haven't had the time. This approach also > works for most other modes based on FM (9600 BPS packet, Motorola DES, > etc.). > > Truly, to support multiple modes or technologies, we need only look at > the world of linear transponders for a clue -- there's really not much > of a point in having too much of a radio there when you're just > repeating in the input RF. > > -- > Kris Kirby <[EMAIL PROTECTED]> > "The illegal we do immediately. The unconstitutional takes > a bit longer." -- Henry Kissinger >
