Hi Daniel,
I like the idea of TDD for a repeater instead of FDD. On HF it would give you
break-in, but at the cost of some mostly-wasted bandwidth.
Since HTs are power-limited, a modem that does not require linear amplification
might work better for them. GMSK or C4FM, etc. There is a lot of talk about
"Class D" amplifiers that can be linear and power efficient, but building one
and getting the spurious emissions more than 60 dB down is probably still a
research project.
Existing radios will have keying delays based on, among other things, the
lock-up time for the synthesizer PLL. It may be that the same PLL makes the LO
frequency for receive, which is different due to the IF, so the lock-up delay
between T and R can be significant.
Often this isn't even specified.
Thanks
Bruce
On April 20, 2014 5:44:41 PM PDT, Daniel Mundall <[email protected]> wrote:
>David,
>Thanks so much for your input!
>That's the idea a super cheap VHF repeater(no duplexers), also thinking
>of
>doing something with and SDR to allow one repeater to manage like 10
>channels in a 25khz of spectrum. But that's if I can get the timing
>figured
>out.
>Yes it does seem to be an issue on some radios...
>
>If I can I'm trying to avoid noticeable delay, but it's an option if
>all
>else fails.(I may well have to do that..)
>One idea was to drop some of the carriers and run a higher symbol rate,
>but for some reason I can't get the filtering to work right when I
>change
>the sample rate above 16khz, and for anything higher than 100
>samples/second I seem to need a higher sample rate to keep the
>noise/harmonics down.
>
>2. Great idea, I think I can get that to work! I doubt any clocks will
>drift too much in that amount of time.
>I guess the other thing is to have the first RX symbol after TX to know
>what the last RX symbol was to compare that *differential QPSK.*
>
>*Would it be possible to just pause the demod program some how every
>two
>symbols and revive it just where it left off? *
>
>3. Thanks I'll give that a try.
>Thanks,
>Daniel va7drm
>
>Sent from Daniel's iPhone
>
>On Apr 20, 2014, at 5:03 PM, David Rowe <[email protected]> wrote:
>
>Hi Daniel,
>
>That sounds very cool. Another nice application is single channel
>repeater work for VHF. Need a radio that can tx/rx switch fast enough.
>
>Couple of ideas:
>
>1/ Have longer timeslots than 20ms, like say 200ms, estimate the rx
>symbol timing over that window. The timing is probably constant for
>200ms anyway, as it changes only slowly.
>
>2/ Have a square wave shaped window for the timing estimation, that is
>set to zero when you are transmitting, that way you get several of your
>rx timeslots included in the timing estimate.
>
>3/ Test any changes by measuring the BER at an Eb/No point, as per the
>Octave simulations. It's really easy to introduce an error that will
>degrade the demod performance.
>
>Cheers,
>
>David
>
>On Sun, 2014-04-20 at 14:58 -0700, Daniel Mundall wrote:
>
>Hello Guys,
>
>I've been playing with the FDMDV modem to see if I can get it to
>
>support Time Domain Duplexing(TDD), with the end of goal of making
>
>full-duplex voice in around 2KHz.
>
>
>
>Currently I have the modem configured at 100 symbols/second with 14
>
>carriers spaced at 125Hz it works out to 2800bits/s. From my testing
>
>that end of things seems to be working pretty well. I'm planning on
>
>splitting this up into 20ms chunks (2 symbols at a time).
>
>
>
>What I'm trying to figure out now is the best way to keep the timing
>
>when I never have more than two consecutive symbols.
>
>Does anyone have a good idea to keep the timing correct while having
>
>20ms interruptions every 20ms?
>
>Thanks,
>
>Daniel Mundall VA7DRM
>
>
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