--- In Repeater-Builder@yahoogroups.com, "vintageaudio2004"
<[EMAIL PROTECTED]> wrote:
"...I just installed a new 6-ch VHF MPT1327 system using MTR2000
Motorola repeaters (100W) with optional preselectors installed, a
Telewave M101-150-6TRM combiner, and Sinclair RM201-112S1B RX
multicoupler...
we are having a big problem (I suspect) with IM products ...
I don't want to jump into conclusions, but it seems the IM (if it is)
is being generated somewhere outside the equipment. ..."
___________________________________________________________________
There has been a lot of good discussion about your problem - offering
you a variety of thoughts to consider.
My approach would be to try to analyze the problem to determine the
nature of the mix before changing your configuration or adding more
filters.
There are three logical possibilities:
1. The mix is being created in the transmitter/combiner system and
radiated out at sufficient level to be received back into the system.
2. The mix is occuring in the receiving system.
3. The problem is external.
Analyzing the situation - with separate transmit and receive antennas
you will have some number of decibels of isolation. By measuring the
level of your transmitter carriers as received at the receive antenna
- you can determine this isolation. Then, working back from your
receiver's MDS threshold - you can determine the level at which the IM
product would be measured at in the transmit subsystem.
By placing your transmitter combiner output into a dummy load and
measuring with a spectrum analyzer connected through an isolated tap -
you can determine if the products are being generated in the transmit
branch.
My feeling is that it is highly unlikely that these products are being
generated in the combiner - given the high order mix required and the
fact that the MTR stations PA's contain a circulator plus the Telewave
combiner provides 75 dB transmitter to transmitter isolation and 90 dB
second harmonic supression. [It is a cavity-ferrite design with
nominal insertion loss of 2.8 dB for each transmitter]
Assuming that you determine that the transmit branch is not the
problem - your next step is to investigate the receive branch. The
best way to do that is to use a spectrum analyzer with sufficient
sensitivity to "see" the low level signals. Using an analyzer with
either a built in or external preamp and narrow resolution bandwidth -
you should be able to identify products that are actually several dB
below the receivers threshold.
Once you can see the products, place a switchable attenuator in the
transmission line between the antenna and the receiver system. Then
switch in a small amount of attenuation - even one or two dB - while
watching the IM level on the analyzer.
If the level on the analyzer drops by twice the amount of the
attenuation you inserted - the products are being generated in the
receiver multicoupler system. If so - that's when to start looking at
what additional filtering might be required.
If the drop is one for one - your IM products are being generated
externally.
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