[OZAPRS] 12.5Khz radio

[Chris Hill]

Hi Gavin,

You should be able to successfully interoperate this radio with _most_
other
amateur APRS users.

First, the easy bit; transmit.  If you run the two audio tones into the
modulator as per the normal voice path, they will undergo pre-emphasis
(6dB
per octave), so the 2.2kHz tone will transmitted with 5dB more deviation
than the 1.2kHz tone.  I recommend that you set the 2.2kHz tone to be
transmitted to air with 3.0kHz of deviation, meaning the 1.2kHz tone will
be
transmitted to air with 1.78kHz deviation.

Note that there are two schools of thought on this;  some people believe
that 1200bps AFSK should be fed straight onto the modulator, and taken
straight from the discriminator, as per 9600bps FSK.  I disagree;  AFSK is
just audio, and as such, needs the same "processing gain" afforded by
de-emphasis at the receiver, so that the 2.2kHz tone is received with the
same S/N ratio as the 1.2kHz tone.  If the receiver uses de-emphasis, then
the transmitter needs to employ the corresponding pre-emphasis.  (I
believe
that the Kenwood D700A does indeed employ pre-emphasis and de-emphasis of
1200bps AFSK, hence their impressive "data sensitivity").

On the receive side, you should be able to decode packet transmissions
from
other stations, as long as the other stations are not over-deviated.  The
12.5kHz channel spaced radios have filters with a -6dB bandwidth of approx
8kHz to 10kHz (varies a bit from manufacturer to manufacturer).  A 2.2Khz
tone with 3kHz deviation should have an occupied bandwidth of approx
10.4kHz, so will be mildy distorted by passing through the filter.  By
comparison, once of those LOUD'N'DISTORTED packet stations with say 5kHz
deviation will have an occupied bandwidth of approx 14.4kHz, and it is
unlikely that you will decode it successfully via the narrowband filters.


Please don't set your TX deviation to 7kHz, as that would give an occupied
bandwidth of approx 18.4kHz, which even wideband radios (with 16kHz wide
filters) will start grumbling about.

Hope this helps.



73,



Chris vk6kch





-----Original Message-----
From: ozaprs-bounces at marconi.ics.mq.edu.au
[mailto:ozaprs-bounces at marconi.ics.mq.edu.au]On Behalf Of Gavin Rogers
Sent: Thursday, 18 December 2003 11:50 AM
To: ozaprs at marconi.ics.mq.edu.au
Subject: [OZAPRS] 12.5Khz radio


Hi All.

I've recently been given a Tait T2010 12.5Khz channel spacing radio, which
I've programmed with the national APRS frequency for the permanent GPS
tracker I'm building for the car.

I'm not too sure how well the Tait will work with the "wide" deviation of
the other stations on the frequency. If a high deviation station
transmits,
the squelch cuts in, which is of course completely normal. If I tap the
pre-squelch audio, even though it will be distorted with high-dev signals,
I'm hoping that it will trip my MFJ TNC's DCD... Decoding the intelligence
of other stations isn't important, carrier detect is :-) Anyone see any
problems with that?

The other side of things is transmit deviation. If I set the transmit
level
to the maximum that this radio will allow (7Khz deviation, I think it is),
would that be high enough for standard radios and TNCs to decode?

Thanks for any help.


Gavin.

--
Amateur radio station VK6HGR
http://vk6hgr.ampr.org/

Email : grogers at vk6hgr.echidna.id.au
Packet: vk6hgr at vk6hgr.#per.#wa.aus.oc

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