Il 14/06/2019 05:09, Jeff Liebermann ha scritto:
On Thu, 13 Jun 2019 20:27:52 +0200, Eremita Analogico
wrote:
Hi, my old HT DMM is still abandoned, but...
I must repair a Vertex VRX-7000 repeater that has a noisy output:
https://i.postimg.cc/3JhhbHpC/IMG-3321-1.jpg
Argh. You're overdriving the spectrum analyzer. Most of the junk
looks like it's coming from intermod mixing between the harmonics
produced in the now non-linear spectrum analyzer input states.
I don't think so. The repeater TX has an output of 10W. Prior to enter
in the spectrum analyzer, the signal is reduced by a 20 dB in line
attenuator.
The SA shows that your input signal is 4.52dBm (about 3mw) into an
internal 10dB attenuator. What model HP spectrum analyzer are you
using and will it tolerate a +5dBm input signal level?
The SA input tolerate 1W (30 dBm)
The model is 8590B
Put some external -10dB attenuators in line and see if the junk
(noise) decreases. If it decreases more than -10dB, then it's being
generated in the SA by your overload. If it drops exactly -10DB, then
it's real.
Do you know how can generate this noise, also UNDER the trasmitting
frequency?
The receiver frequency falls in the noisy area, and when I connect a
duplexer, the sensitivity of receiver is blanked.
Duplexer tuning and cabling are somewhat of an art. For example, the
coax cables between the duplexer and the receiver input, and between
the duplexer and the transmitter output MUST be an odd multiple of 1/2
wavelength electrical long, as measured from the pickup loop in the
cavities, to the receiver front end device or transmitter RF output
transistor. Use the wrong length, or wrong connectors, and you'll
never get the rated isolation.
We have tryed with Procom and Motorola duplexers, both with 3 adjustable
cavities for RX path and another 3 adjustable cavities for TX path.
The same duplexers work perfectly with a repeater built with two
separate Simoco SRM9000, without any desensization of RX unit when the
TX is operated.
Thus, I think that the PA of Vertex VRX7000 has some issues, but I never
seen noises of that type. I wolud expect some harmonics.
The internal supply can do this noise?
I agree with Ralph Mowery. Go to:
https://groups.io/g/repeater-builder
and ask the same question, but also include what radio and duplexer
you are using. Also what test equipment you have available. Most
important, exactly what problem you are trying to solve that required
troubleshooting.
I still must begin to investigate, but if someone know something, I will
much appreciate any suggestion.
There a many articles and videos on the web on "duplexer tuning". I
would recommend one, but need to know the details of your repeater to
find something appropriate.
https://www.google.com/search?q=duplexer+tuning
Good luck.
Ok, thanks, I will try also the repeater-builder forum, but if I can
obtain some useful tips without register me I will much appreciate.
Wednesday I will resume the repeater investigation.