As I stated a few times here I have a duplexer that has been giving me
desense. To get rid of the desense I turned down the TX power until it went
away because the duplexer is rusty and hasn't been messed with in 20+ years.
I came across another duplexer in a base station cabinet we had and pulled
the duplexer out. Here are the specs of it:
TPRD-14744 (470-512 MHz)
Type: 4 cavity pass-reject
Minimum freq. spacing 3 MHz
Insertion loss 1.0 dB
Max. continuous power 250 watts
TX noise supp. at Rx freq. 90 dB
Rx isolation at TX freq. 90 dB
Temperature range -30 to +70C
I used the tracking generator and tuned and notched it on frequency.
My plan was to use this duplexer as a backup if the original one would not
tune.
When I checked desense on the original again it was around 10db. I pulled
the duplexer and found the reject was only about -75db on both sides.
I adjusted the notches and got it down under -90db but noticed it jumped
around -95db then -90db, etc... I did use double shielded cables for
tuning.
I checked the passes and they seemed ok, so I did not touch them.
Put it back in place and turned the power back up from 15 watts out of the
PA to 42 watts out of the PA.
The desense jumped big time to around 20db.
So I pulled the old duplexer and put in the spare.
Now I have no measurable desense. So that's a plus.
However, I put the service monitor on the PA output and read 42 watts.
Hooked the cabling up to the duplexer and then checked the output of the
duplexer and only read 22 watts. That's like a -2.8db loss.
I didn't think at the time to check the interconnect cable from the PA to
the duplexer, but I can't imagine that having a -1.8db loss. Since the
duplexer should only have -1.0 db insertion loss.
When I tuned the replacement I double checked all the peaks locked them down
then did the rejects. So I am pretty confident the duplexer is set right.
So, in the end the repeater went from 15 watts out of the duplexer with 10db
desense to 22 watts out with no desense.
1. Can something in the original duplexer go bad internally to cause the
desense? Or must my tuning be suspect?
2. Think I need to worry about the -2.8 db power loss? All I can think of
doing is checking the interconnect cable from the PA to the duplexer input.
Another note: When I retuned the base station duplexer, I kept the low side
low and the high side high, meaning I am now transmitting on what was
originally the RX port since it went from a base station use to repeater.
Could this be a problem if I really have -2.8db insertion loss?
-Tim
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