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PA2AGA > PACDIG 24.04.99 17:16l 162 Lines 6769 Bytes #-9884 (0) @ EU
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From: pa2aga
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Subject: PacketRadioDigest 99/91B
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>Organization: AmiKoRn
>Date: Thu, 22 Apr 1999 02:37:16 GMT
>NNTP-Posting-Host: 194.125.142.224
>X-Trace: news.indigo.ie 924748636 194.125.142.224
(Thu, 22 Apr 1999 03:37:16 BST)
>NNTP-Posting-Date: Thu, 22 Apr 1999 03:37:16 BST
> Hi, can anyone help me,
> I have a baycom modem running on an amiga1200,
> I relly need to build a digital squelch of some sort.
> Has anyone got any ideas? PLEASE
> amikorn@geocities.com
The TCM3105 has carrier detect feature available on the chip itself,
however I wouldn't recommend using it. I have experimented with its use
in the past, and had extremely dissapointing results. Basically, all it
does is measure the wide-band audio level coming into the chip. The
3105 has a CDT pin that goes high when a "carrier" is detected. It makes
no attempt to determine if this "carrier" is at the proper mark or space
tone, or if the data is at the proper baud rate. If there's loud enough
audio at the RXA (Receive Audio input) pin (open squelch or whatever)
referenced the bias voltage at the CDL (Carrier Detect Level) pin, it'll
enable the CDT pin.
You can feed the RXD (Receive Data output) and CDT into an AND gate if
you want to enable/disable the output data stream from the interface, or
you can connect CDT to your computer's carrier sense input to signal the
software of activity.
I have found the use of the 3105's "carrier detect" to be almost
useless for packet, even after careful adjustment and experimentation.
The CDT is slow to enable, often clipping packet headers, and slow to
release, creating lengthy delays before allowing the system to transmit.
When monitoring activity, it'll often stay open between the end of one
station's packet, and the beginning of the other station's packet.
Implementing it is simple enough, if you wish to experiment with its
use. A 100k pot is placed between +5 volts and ground, with the wiper
going to the CDL pin. This is pin 10 on the common 16-pin version of
the chip. The 3105 has a very rare 24-pin version, where CDL is on pin
14. CDT is at pin 3 of both versions.
Here is Texas Instrument's alignment info from the application note:
----------------------------------------
- Apply 4V to the CDL pin.
- Apply an AC-coupled, sinusoidal signal to the RXA pin at a frequency
between the mark and space frequencies for the mode selected. The
amplitude of this signal is set to the lowest signal level that is
desired for the modem to detect. A nominal signal level is -44 dBm.
- The CDT pin should be low.
- Decrease the voltage at the CDL pin until the voltage at the CDT pin
becomes high.
NOTE: The TCM3105 has a carrier detect delay of 20ms to 80ms depending
on the receive rate selected. A wait for this delay to time out
is required before the CDT pin is monitored.
- The carrier detect level adjustment is now set.
----------------------------------------
There is an alternate carrier detect circuit for the 3105 modem at the
Baycom web site. It uses an XR2211 (an FSK demodulator chip) to sense
"carrier". It is designed for the PC interface, but I'm sure you can
adapt it for your Amiga:
http://www.baycom.org/bayweb/tech/anst.htm
I haven't tried this circuit myself, so I don't know how well it will
work.
The route I chose to follow was to make a direct connection to the
tranceiver itself. I used an LM339 voltage comparator to monitor the
squelch status of the radio. One input of the comparator goes to the
wiper of a 100k pot that is connected between VCC and ground. The other
input of the comparator goes to a suitable squelch-sense or AGC point
inside the radio. Connect an NPN switching transistor so that the base
is connected through a current-limiting resistor to VCC, and the emitter
goes to ground. The collector is either left floating to switch your
device, or connected to a pull-up resistor for a logic interface. The
output of the comparator connects to the base of the switching
transistor. When the comparator output is low, the base's resistor is
pulled to ground, starving the transistor of base current and keeping it
off. Which comparator input you use for squelch sense and which one you
use to set your trip-level is determined by whether or not you want an
active-high or active-low output from the transistor. You adjust the pot
at the comparator's input so that the output responds quickly, but does
not "jitter".
If you don't wish to modify the radio and you're using the radio's
speaker output (not a discriminator tap) you could tap the audio input
line to the 3105 to drive the comparator. Tap the audio line with a
coupling capacitor, rectify the audio, and send it to the the comparator
input. You'll need a resistor between the comparator's input and ground,
and a capacitor to smooth out the pulses. You'll have to play with the
cap and resistor values. You now have a DC bias at the comparator input
whenever audio is present at the 3105's input. The success of this
methood is directly related to the response time of your radio's
squelch, though, and if you adjust the audio level of your radio you'll
probably have to re-set the trip-level pot at the comparator's other
input pin.
--
From the keyboard of Alan - VE3NNM (Kenora, ON, CANADA)
email: ve3nnm AT excite DOT com
http://www.voyageur.ca/~tokyo/
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>.
------------------------------
Date: Thu, 22 Apr 1999 13:37:53 +0200
From: "Covert" <condor_001@yahoo.com>
Subject: Emergency Comms org in S.A.
Hi all,
Come visit South Africa's elite amateur radio communications service HAMNET
website. News, real time data areas, pictures, and more. The address is
http://welcome.to/hamnetkzn
Thanks for reading...
Sean
ZR5BBL
HAMNET Official - Website Manager
>.
------------------------------
End of Packet-Radio Digest V99 #91
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