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[wxsig] Problem with open squelch or carrier preventing T238 from transmitting

M PATTON k0yg at msn.com
Fri Aug 11 21:34:58 UTC 2006


Below is a discussion that I started as an email thread with Will and
Russ concerning some issues that I've been having with the T238  not
transmitting due to some carrier signals on 144.390 locally in the
Denver area.  I thought that others may be interested or may have the
same issue or ideas on how to resolve it.

Mark

==============
Hi Will,

I thought you might be interested in what I've found out.  I did some
research on the MX614 modem chip and one of the "features" is that it
is  sensitive and the "detect" signal will come on with almost any
signal it  seems.  Here's from the data sheet:

"The DET output will be set high when the level has exceeded the
threshold for a sufficient period of time. Amplitude and time hysteresis
are used to reduce chattering of the DET  output in marginal conditions.
Note that this circuit may also respond to non-FSK signals such as speech."

It turns out that open squelch or a blank carrier will trigger it.  The
"Channel Busy" LED comes on with any signal - including a carrier or
open squelch.

>From what I can tell, you use this signal to determine if the channel
is busy.   I don't know what others see, but I've verified that I have
periods of time where I get a carrier on 144.390.  I've watched to see
when the T238  doesn't transmit for a while and I check the frequency. I
can hear a low level carrier on it.  After some time, it will go away.
Other APRS traffic seems to do just fine including the digi that
is at the same location as the T238 .  I've tried tightening the
squelch  but the signal is strong enough to open it regardless - thus
preventing
it from transmitting for long periods of time.

In doing further research, this is a known problem.  This is the reason
that the KPC3  and other TNC's have gone with a software carrier
detect.   For instance if you check the recommended configuration using
a KPC3  for a digi, they suggest that you use software carrier detect
for "better collision avoidance".  I've been tracking the development of
the Tracker2 (follow-on to the Open Tracker) and Scott has implemented a
software carrier detect.

I don't know how exactly they implement this.  Scott may be willing to
give you some ideas.  I think he looks for valid data being transmitted
(how exactly he does this I have no idea).

Mark

===============

The transmit state machine waits for a clear channel for up to 25 seconds.
If it can't get 500ms of no data by that time, it gives up.

The MX-614 should only fire if there is real data.  I tried just opening
the
squelch on the radio for my T238  setup, but appear to have some kind of
problem as my Rx LED won't even light when I have real data.  I suspect a
busted cable so will go get it fixed hopefully tomorrow night and let you
know.

will

================
Hi Russ,

An update on the signal on 144.390.  It was a stuck mic.
A few guys from the local club went out and DF'd it this evening and
found it.  It was up in an
area North of town.

Mark

==============

Hi Russ,

When I got home this afternoon I checked 144.390 and I can definitely
hear a carrier.  I heard some faint audio (radio station) but I can't
make it out. The signal is about S2.  I went out and checked, and this
is opening the squelch on the radio on my weather station so it
evidently is not transmitting.  I tightened the squelch (a lot) and
we'll see if this helps until the issue with the radio station is resolved.

Interesting that my KPC3  I'm using as a digi (which will be relocated
to Franktown soon) has a software carrier detect set so it doesn't see
the interference on 144.390.  I think it is looking for an actual signal
- not just static or a carrier.

Will - What does the T238  do in this case?  Does it just sit and wait
for an opening (ie no carrier) to transmit and if it can't find an
opening then eventually give up?  Since the hardware is there on the RX
side, would it be possible to look for a a real signal (like the
KPC  does) to detect if the channel is open rather than just a
carrier?  The other reason this might be helpful is that my weather
station is going out in a field where the radio is going to get very
cold.  I've had radios that when they get cold, the squelch level
changes dramatically (ie opens).  I don't know if this radio does that
or not, but I'll probably find out this winter!

Mark

===============

Mark,
I looked at your weather data, and those drop-outs are not what I've
seen from my station over the years that I've run it.  However, in just
the past few days, an interference has appeared on 144.390 and that may
be causing problems in the Denver area.  I did check Will's N0XGA
station and it seems to be working OK and not seeing drop-outs.

This interference is apparently a spur associated with a broadcast
station, perhaps KHOW as that has been heard through the static.  I live
NE of Boulder a few miles and it appears to be either to the general NE
or SW of my location.  I'll let you know if I hear more info on it.

Russ   KB0TVJ

=============
Hi Russ,

I have my stand-alone solar weather station up and running as K0YG-6 in
my back yard.  I plan to move it out this fall to it's final destination
as I'm still wringing out the bugs.

I am puzzled by one issue that you may have an answer to.  I keep
getting gaps in the data.  Usually it's fine and reporting every 5
minutes for a while then I miss a couple of packets.  This I would
consider "normal" due to possible collisions on the frequency.
Recently I've noticed some sizable gaps. 30-45 minutes and last night I
had one that was 4 hours!

Nothing changed, it just quit and then picked up 4 hours later.

In looking at some other ham WX stations, that are close, I see that
they also have some gaps in their data.  Not at the same time as mine,
but some pretty sizable.  In others, I don't see this issue.

Do you normally see gaps like this in the data?  I'm trying to figure
out if I have an issue with my weather station or not or if it is
somehow network related.  I can't quite figure out what would cause
this.  Any ideas?

If you don't consider this "normal", I may move my WX station off on to
another frequency and let it run for a few days and copy the packets
independently to avoid the network and collision issues and try to debug
it that way.

Mark






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