A linear translator for something like D-Star wouldn't work out real well
since you are dealing with mobile users. The first issue to overcome is the
constantly-varying amplitude of the incoming signal - on the order of tens
of dB's in level change doezens of times per second at highway speeds (60
MPH @ 2m = around 15 wavelengths per second). The second is that there is
no opportunity to error-correct at the repeater before retransmission,
unlike a real (digital) repeater. So, in a mobile environment, the bit
error rate at the receiving station is always going to be higher than it was
at the repeater site. If all of the users were at fixed locations where the
AGC issue wasn't a big deal, and likewise, if they were all solid into the
receiver such that the BER was low enough, then a translator might work.
But for mobile-to-mobile use, probably not the way to go...
--- Jeff
> -----Original Message-----
> From: [email protected]
> [mailto:[EMAIL PROTECTED] On Behalf Of Rod Lane
> Sent: Thursday, April 26, 2007 11:02 AM
> To: [email protected]
> Subject: [Repeater-Builder] Re: Repeating D-Star
>
> 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 [email protected], 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
> >
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.5.446 / Virus Database: 269.5.7/771 - Release
> Date: 4/21/2007 11:56 AM
>
>
