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If you wish to make up your own
IRLP board to Motorola 16 pin interface, here is what you need:
Considerations
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Receiver Audio
There is an internal
two position jumper on the logic board in the radio for
de-emphasized/gated audio or flat audio that will need to be set
for de-emphasized audio. This jumper selects the desired audio going out to pin 11 of the
accessory jack. All radios that I send out will have de-emph
audio as the default jumper. Otherwise. if you have the flat audio jumped, your
receive audio going to the IRLP board will be the raw discriminator audio which
is very thin and without any low-end
frequency response. The de-emphasized audio is also gated or muted when the
squelch is closed... that keeps it from blowing unsquelched hash into your soundcard. If you
hear those nodes running using un-processed or discriminator audio you'll know what I am
talking about. That "tin can" audio is certainly most undesirable to listen to.
The de-emphasized,
gated and PL tone filtered audio level available on pin 11 of the 16 Pin accessory is sufficient to
drive the line level in of most sound cards. You will need to adjust your AUMIX
settings to fine tune the level into your particular soundcard. In my opinion,
the receiver audio is the most important analog component in the IRLP mix,
certainly the one that everyone else on the other end will brag upon to you
most. The Radius and Maxtrac line of radios perform a good job delivering a good
audio signal to IRLP.
You may also choose to jumper 15 and 16 on the 16 pin accessory to enable your
internal speaker. This is handy for setup and gives you the ability to monitor
incoming local traffic to your node. You can safely turn down the volume in the
front without fear of adjusting any levels to IRLP, since it is totally
separate.
COS (Carrier Open or Operated
Squelch, CSQ, COR, RUS, etc. different names, same thing)
When a valid signal is received by your radio and the squelch opens, a logic
swing is noted on pin 8 of the 16 pin accessory. Most of the radios we are using
here have the logic going low or in other words- appear to be the same as
ground when a signal opens the squelch. A few of the radios with
expanded logic boards have the ability to be programmed COS high (swing up to say +5 volts) at pin 8 ...as
well as being programmed to some of the other I/O accessory pins. I will normally send a radio
with COS low, so you will need to jumper your IRLP board to active low. The M120
and some non-extended logic GM300's cleverly have an NPN open-collector switch in the circuit
that serve as the COS low logic buffer going to pin 8, so no other buffering, diodes or
limiting resistors need to be externally inserted to feed the IRLP board's COS
logic input.
The IRLP board instructions, available at http://www.irlp.net/new-install/
indicate that the green LED on the IRLP
board should light up while a signal is present on the radio, and turn off
instantly when the signal is no longer present. If it does not, then try to
switch the position of the COS jumper on the board. The jumper is in HIGH
position when the jumper is positioned closest to the "H" silkscreen
on the board. If the voltage swing is low, you may have to change the LGC
SEL jumper to the 1 volt position.
Two utilities built in the IRLP software that will help you in the setup
and fine tuning are pulsecheck and readinput. Log in as user
repeater and run readinput to check your connections for valid DTMF
tones and COS detection. You can adjust the delay recognition in your node
by using pulsecheck, to match the total delay response between the TX and RX
switch in
the software.
CTCSS
or PL Tone (Private Line is Motorola's buzz word for tone
coded squelch)
The use of CTCSS tone is an effective way to keep unwanted audio off of
the IRLP network. There is nothing worse than hearing some other area's repeater
courtesy tones, announcements, CWID's or even that occasional paging intermod
that sneaks in, on
one of our reflector channels while a large net is in progress. This also
includes the repeater hang time issue from your controller, that keeps all of the
links up on the IRLP network until your repeater drops its carrier. To keep
your system clean, use a tone coded squelch scheme on your link radio. If
tone is programmed, the PL decoder logic signal will "AND" with the COS signal to open the squelch gate logic on
the COS pin
8. This is provided that you either - have a microphone is inserted and hung up on
the grounded clip or make an RJ45 mic plug jumper
- shorting the PL line (PIN 3) to ground (PIN
4), that externally turns on the PL decoder. Without either of these two in
place, the radio is in "off hook" monitor mode and receives Carrier Squelch
mode only.
See also my note on Repeater Configurations if you are a repeater owner or are
planning to integrate your IRLP node into someone else's repeater.
Squelch Adjustment
Should you need to adjust the squelch
internally, it is a small pot with "SQ" indicated. On this photo it is
under the batwings. There is also a mod from Barry VE6SBR that will remove
squelch delay / hysteresis if you have a fixed link going to your repeater.
Most radios start adding delay to the squelch drop out time as received signal
strength becomes less. Normally this is desirable to prevent squelch chatter
when receiving weak signals. For link radio use however, as little squelch delay
as possible is desired. To remove the squelch delay feature, in the GM300 radio,
remove or lift a lead from C76 (circled in red in above photo). Here is the Cap
in circuit from Barry's reference.
Transmit
Audio
One of the common complaints I see is that the local transmit audio from
the IRLP link radio lacks full fidelity and is often low level or
non-existent... even though the node owner knows all connections are
good. The usual problem is that there is DC voltage on the mic audio
line (that powers extra stuff in the mic, like a DTMF pad and lights)
that is interfering with the soundcards op-amp driving the line level
out.
To remove the DC
voltage off of the radio mic line, add a 10uf non-polar cap (or 10uf with the +
pointing to the radio) in series with the line out of the
soundcard going to the mic audio input for a DC block to improve audio
path. For reference, the mod is indicated on the top diagram of
this page.
Then fine tune adjust the AUMIX output settings to properly adjust the
amount of audio heading out to your radio. The echo reflector is your
DVR friend.
If
you choose to leave a local mic plugged into your RJ45 jack, then note that this
will possibly load the audio level available at pin 2 of the accessory, since
they are parallel. Unplugging the mic will raise the level from the IRLP source,
so adjust keep that in mind, if you plug and unplug a local mic on the link
radio.
CW ID
To be FCC legal, you must identify your link radio. If you choose to use one
of the CWID scripts from your IRLP node computer, then you may need to adjust your
script settings for the wave or midi audio levels heading out of your soundcard.
It talks a little practice to adjust and balance that CWID or WavePlay audio to the link radio. If you choose to use a separate
outboard analog CW ID device, then you can feed its audio and PTT into RJ45 in
the front or use the spare Flat TX audio Pin 5
of the 16 pin accessory, with caution. http://www.com-spec.com/id8.htm
Ground loop Hum
Many times we see a ground potential that exhibits itself as a little 50/60
cycle AC hum on the audio path between the IRLP computer and the link radio. You
may hear it on the air or you may hear it on the node. This is commonly
due to the fact that you have several ground points i.e., the link radio power
supply ground, the computer power supply and chassis ground, the antenna ground
coming in on your coax or some other device that is on the same electrical service
ground... with one or more of them having a little voltage potential tagging
along and showing up as audio hum.
One solution is to float all service grounds except one and tie a grounding strap from
each device to that one common ground. But you first might try to run a
grounding strap of low resistance (bigger gauge wire) to all devices and see if
the hum is attenuated before lifting all the grounds. The last resort is to use
1:1 isolation audio transformers on both the line in and line out of the
soundcard going to the receive and transmit audio. Remember if you do sever the
audio lines, you still need to provide a logic ground for the COS and PTT paths.
Keep it Cool !
Use a fan to blow across your PA fins, this will help keep the transmitter
happy and extend the life of your link. Often, I recommend that you throttle
back your PA power output to keep your radio from running full tilt boogie,
especially when connected for extended QSO's. Most of the time you simply
do not need 40 watts, when 28 will do just fine. This is why a lot of
folks like the lower power radios - even the 5-10 watt versions for links... run
cooler, last longer. The transmit power out is programmed in the software,
so if it is changed, be sure to monitor the signal on a good spectrum analyzer
to avoid going spurious at lower power levels, especially of the higher power
radios. I like to advise a good linear power supply for your link radio that is rated above
your needs. The 10-25 watt radios will pull 5-9 amps depending the PA
adjustment. The 40 watt versions can load 8-12 amps.
Repeater Configurations
If you are a repeater owner or trustee you will get the most of your system by configuring your machine to
send a PL tone encode out (to your IRLP node radio) triggered only on repeater receive audio
with a valid PL decode. If your present repeater controller has that
feature built in and you already use PL access on your repeater, then this is
fairly easy to setup. It is also best to strip the tone on the receive
side and re-encode a fresh tone on the transmit side, rather than just
transmitting a users tone through.
It is fairly easy to incorporate a Com-Spec PL board ( http://www.com-spec.com/ts64.htm
) to trigger its encode off of the incoming repeater signal, if your
present controller does not do this already. It will keep your CWID's, courtesy tones, hang
time and
announcements behind PL protection... that is, sent without PL tone and off of
the IRLP network.
When using PL tones, remember that the higher the tone (192.8 Hz for instance) has
a faster response time but can sometimes be more audibly heard, the low tones (
77.0 Hz) take more time to decode ( < 200ms) but are less likely to be
noticed, so there needs to be a good balance in your choice. Good decoders will not add more than
about 150ms delay in your system - if you choose tones somewhere in the
happy medium (82.5 - 141.3 ) and based on your local coordinators plan. You can adjust the delay recognition in your node
by using pulsecheck, to match the total delay response between the TX and RX
switch in
the IRLP software.
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I
would appreciate any contributions, corrections or additions here that might
help other node owners with this series of radios as RF links. Please email me
with any suggestions. Danny KD4RAA
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