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  Programming the Transmitter Idle Frequency on the MTR2000
This page was developed by Mike Morris WA6ILQ from emails sent to Repeater-Builder by Eric Lemmon WB6FLY
   


To: Repeater-Builder
From: Eric Lemmon WB6FLY

While tuning up a UHF MTR2000 repeater, I became aware that the
"receive" LED on my monitoring handheld radio was blinking (indicating 
the channel was busy), although the repeater was idle. On a hunch, I 
shut off the repeater and the blinking stopped.

After thinking about it for a moment, I theorized that the transmitter's
VCO was running all the time, and a very low-level signal was being
emitted, but strong enough to be received in the immediate area. This
exciter leakage is often seen in MICOR repeaters, especially when the 
shields are left off by a careless technician.

While wondering about what to do with this problem, I remembered that
the RF Option page of the Channel Information screen (see the image 
below) had a "TX Idle" button and frequency window. I also remembered
that the Help file said that the transmitter idle was important for 
simplex operation (otherwise you'll block your own receiver), and could 
be set the same as the transmitter frequency for repeaters (since the 
receiver is on a different frequency).

I thought, "What if I set the transmitter idle frequency to something 
other than the repeater transmitter frequency?" So, I entered a TX Idle 
frequency that was on another channel about 50 kHz away. As soon as I 
wrote the codeplug back to the repeater, the low-level carrier moved to 
the new transmitter idle frequency. Gee, the "TX Idle" feature allows 
the user to set the frequency at which the exciter idles! Oddly, neither 
the MTR operation manual nor the service manual mentions this important 
application for a repeater.

If the exciter leakage is a problem (or merely an annoyance) with your
MTR2000, simply program a frequency in the TX Idle window that is offset 
from the normal TX frequency by at least 20 kHz or so. Be careful NOT to 
set it to a frequency that is an input of another repeater, or on a 
simplex system, remote base or link channel. There did not seem to be 
any transmitter delay introduced by offsetting the exciter idle frequency. 
This issue can occur on ANY band, not just UHF. 

The MTR2000 RSS Help file suggests these formulas for calculating the TX 
Idle frequency:

1. If the TX frequency is more than 50 kHz from the RX frequency:
     Set TX Idle frequency = TX frequency.
   Else (the TX frequency is within 50 kHz of the RX frequency):
     Set a TX Idle frequency at least 50 kHz from the RX frequency.
2. The recommended value for TX Idle = TX Freq - 50 kHz. If this 
   frequency is outside the band, use TX Idle = TX Freq + 50 kHz.
3. The TX Idle frequency must use the same VCO as the TX frequency.

If the TX frequency is within the overlapping area of high and low 
ranges shown below, the TX Idle frequency must be in the same area.

VHF high range:             152.5 MHz---------------174.0 MHz
VHF low range:  132.0 MHz---------------153.5 MHz

UHF high range:             436.0 MHz---------------470.0 MHz
UHF Low range:  403.0 MHz---------------437.0 MHz

73, Eric Lemmon WB6FLY


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Original text (the white background and the image) © Copyright 2005 Eric Lemmon WB6FLY
Hand coded HTML © Copyright 2005 by Mike Morris WA6ILQ

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.