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[aprssig] 9600 baud capable radios

(SV2AGW)George Rossopoulos sv2agw at raag.org
Thu Feb 3 06:04:45 UTC 2005


Hi again

I am using 9600b here for more than 10years.
I have a modified tm-431, D-700,V-7 and a dedicated 9600 radio the TF-7.
I have replaced the TM-431 455khz filter with a digital communication filter
from Murata.
I am using it but it needs a txdelay of 17.
Since we are use 9600 mostly on TCPIP and APRS this is not a problem. TCPIP
sends very large frames ,1000bytes each so txdelay is not a problem.
On APRS my station transmits only every hour so txdelay is not a problem
here too.

TF-7 can use a txdelay of 4-5. 
D-700 and V-7 can use a txdelay of 8.
But...... Other stations cannot hear me, when using such low txdelay,
because they cannot recover so fast after transmition.
TCPIP Gateway uses a Kantronics D4-10.

Anyway in Brief
I made some tests using D-700 /V-7 sending 500 pings of 1000bytes The
results were excellent I received almost 500 responses.
In my opinion are the best off the self radios you can buy for 9600b.
The only problem is that they need very HIGH output from the 9600b modem.
Check your modem!!!

Something else, I changed also other radios 455 intermidiate filter with
digital filters, but they didn't work.

All mode radios like YAESU ft-817 work fine.

regards


micro USB TNT (A pocket TNC with Build in Tracker and APRS digi with USB and
battery for portable operation)
www.elcom.gr

(SV2AGW)George Rossopoulos
sv2agw at elcom.gr
www.elcom.gr/sv2agw
+306932465216
George Rossopoulos
Nikanoros 59
54250,Thessaloniki
Greece
 
-----Original Message-----
From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
On Behalf Of Henk de Groot
Sent: Thursday, February 03, 2005 12:33 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] 9600 baud capable radios

Ralph Milnes schreef:
> Henk seems to be saying most radios billed as 9600-capable are NOT 
> 9600-capable

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 context.

Kind regards,

Henk.

<quote>
Hello Bob,

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 the
bits.

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:

<quote>
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.
</quote>

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.

Kind regards,

Henk.
</quote>



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