[wxsig] Signal being detected with no signal

[Will Beals]

Mark:

You are correct in that the T238 does just monitor the "energy" output of
the T238.  I've looked into doing software carrier detect and it is pretty
ugly for my little micro.

I have not heard of the light being on continuously though--at least not on
the reflector.  Some good ways to see if it is oscillations in the filter or
the MX-614 itself would be to ground the audio input (low impedance to
ground should help).  If that doesn't help ground pin 5 of the MX-614.  For
a short period you shouldn't hurt the device as there is at least 100 ohms
from either of the op-amp outputs.

If neither of those turn off the energy LED, I'd lean toward a busted
MX-614.  If either fix it, then hopefully you can isolate where in the
analog circuitry there may be a problem.

As a last resort, there may be a way to bypass the op-amp filter and have
only discrete filtering, but that would take a bit more research.

will 

-----Original Message-----
From: wxsig-bounces at lists.tapr.org [mailto:wxsig-bounces at lists.tapr.org] On
Behalf Of Mark Patton
Sent: Thursday, August 24, 2006 2:20 PM
To: TAPR Weather Station SIG Mailing List
Subject: [wxsig] Signal being detected with no signal

Over the past few months my T238+ has been up and running pretty well.  
One thing that I've noticed is that for periods of time from 45 minutes to 2
hours, the T238+ will not transmit.  This is intermittent and will happen
sometimes several times per day, sometimes not for several days.

The other day when it was not transmitting, the yellow LED (showing a
received signal) was stuck on - even though there was no input signal.  
I have the radio on a switch controlled by the T238+ and the radio was not
powered at the time.  To test it further, I unplugged the radio from the
T238+ but the LED stayed on.  I touched the input plugs with my fingers
(thinking that it may induce noise) but the LED still stayed on.  Under
"normal" circumstances the LED will remain off between transmissions with
the radio off.  The T238+ won't transmit if it thinks the channel is busy
(ie yelllow LED is on).

Unfortunately, the T238+ is remotely mounted in a box outside my house so it
is difficult to get to and to monitor the LED's or to probe around when an
issue like this comes up.

The only thing I can think of is that the filter section between the audio
in and the modem chip may be going into oscillation or is locking on to some
noise from somewhere. The cables between the T238+ and the radio are short
and are shielded.

Has anyone else seen anything like this?  Does anyone have any ideas of
things I can try to eliminate the problem?

Mark




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