[aprssig] 9600 baud capable radios
Henk de Groot
henk.de.groot at hetnet.nl
Wed Feb 2 16:32:44 CST 2005
Ralph Milnes schreef:
> Henk seems to be saying most radios billed as 9600-capable are NOT
They used to be, maybe things have changed. As I showed it can be fixed by
design so it is not a law that what ever was will be so in the future. So
I don't know if this is still true for today's radios. Just adding a 9600
in/out to a traditional PLL radio design doesn't work and this is what
used to happen.
> George says the V7A and D700 are OK. Henk do you agree with that?
Sorry, I never looked at their designs. It's pritty sad but in the past
radios came shipped with their schematics. Now I would not neven know how
my TH-D7E is put together...
Usability also depends on who is the receiver. I just send a private reply
to Bob to discuss this topic, I will repeat it below since I guess it is
interesting for others too.
> George says V7A and D700 do need a good signal path, and Gerhard points out
A strong signal always helps, especially when the detection margin is low.
So the need for a good signal is not an indication that the radio is
suitable, its when the signal becomes weaker that the differences show. So
even with a not so good 9k6 radio you may get away with it when the signal
is strong enough.
By the way, the G3RUH modem has a Rom with profiles to predistort the
signal to compensate for the distortion introduced by the radio. Selection
of the right profile can give a good quality link. Experts say however
that this way you tune the radio to work with one specific other station
and that this is no guarantee at all that you can work with any station.
When you are at home and the other station is a node or BBS than this is
okay, but for APRS every body needs to be able to decode your signal.
> Henk, do you have any examples of the #2-type radios you mention below?
I mentioned my modified car phone, but I guess that's not what you have in
mind. I think the TS2000 is such a radio, when you have to cover a lot of
bands then a simple design doesn't work anymore, so this is one of the
ways to build it and get good 9k6 performance as a bonus. Since the D700
and D7 are multiband radios they also may have a similar design. One has
to examine the schematics to know. I'm not sure about the TS2000 but I
seem to remember this was the case. Better verify before you buy..
Thanks John for the pointer to the V7A measurements. The TX curve looks
very nice, I just wonder how low it goes before the PLL spoils it, unless
of course they took care of that by design. The curves in the picture stop
at .25 kHz, that is still 250 Hz and you need to be able to get as low as
10 Hz. Any radio does at least 67 Hz to be able to use CTCSS. It is a
pitty the reception is not equally flat, but if you read below then maybe
a soundcard receiver could fix that!
Ok, below is the reply I promised, it was send to Bob earlier this
evening. I hope Bob doesn't mind a I quote his part too to define the
Robert Bruninga schreef:
> I thought in the receiver, there was LF phase noise because the PLL
still is a closed loop that will hunt back and forth as it
> maintains lock and these excursions will add to the
> data coming off the discrimintator?
Oh, yes, thanks! That's another factor but this highly depends on the
quality of the VCO. If it wanders only a little then the frequency
variations will be small, if it is cheap then you get more jitter and it
becomes harder for the FSK modem to lock to the zero crossing and detect
This problem also exists with the other synthesiser designs, a pure
crystal tranceiver works best. But the major problem is on the TX side
when the PLL starts hunting to counteract the "drift" caused by the low
frequency components in the 9600 baud signal.
> comment? (I just accepted that as fact, but may be remembering
> the wrong fact...)...
No, this is correct, it *is* an issue with PLL designs, a designer has to
solve this after he solved the first problem: how to modulate without the
PLL distorting the signal. A good quality VCO is what is needed, something
a lot of japaneese sets do not have (they just fully rely on the PLL to
keep the frequency stable). You can hear this in the audio too, it a kind
of "rumble" in the background.
Also here G3RUH modulation certainly has its drawbacks, a lot of it can be
overcome with another type of modulation, however at the expense of
bandwith and complexity. There is no free lunch.
In a digital cell phone system the detector in the receiver is much
smarter than a simple bit slicer to determine if the symbol is a "1" or a
"0". The receiver tries to predict what way the signal was distorted
because knowing that the receiver can just compensate (eqaualize). Not
only simple amplitude distortion but also the accompanying group delay
differences rip the signal apart.
Usualy those advanced receivers are used in the basestations. The idea is,
the basestation send a very clean signal so the receiver in the cell phone
can be kept simple, the phone has a simple transmitter which does not send
a very high quality signal and the basestation has an advanced receiver to
decode the signal. This way the cell phone can be a very cheap mass product.
The soundmodem of Thomas Sailer, HB9JNX/AE4WA, also uses some of these
advanced reception techniques, as written in his paper for TAPR in 2000
(http://www.baycom.org/~tom/ham/dcc2000/soundmodem.pdf). He writes:
Because the frequency response of the transceiver introduces intersymbol
interference, the receiver has to be designed to cope with it. I have used
a constraint length 3 viterbi equalizer to combat the ISI. Constraint
length 3 captures most of the channel energy, and is about the maximum
that can be implemented with reasonable CPU consumption. For every symbol,
8(power)3 = 512 trellis branches have to be processed.
The viterbi equalizer is also what is used in GSM basestation receivers to
counteract distortion. So the soundcard modem is pritty advanced compared
to the G3RUH modem design, of course at the expense of a lot of processing
power... GSM receivers use a better equalizer but this is done with DSP's
especially designed for this job. Also in GSM they use PSK with 4 bits in
every symbol (QPSK), so it is more advanced than the soundmodem, but I
think the soundmodem is the most advanced for HAM use.
In APRS this would also be ideal: high quality digis to allow your mobile
to be plain, straight forward and simple.
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