Just thought I would share my experience retuning a 6 cavity
DB4062 duplexer this week. Since there was someone asking
about this recently, maybe it will be of interest.
I was a bit nervous about doing this with minimal equipment
as outlined in their field tuning instructions, but it went
very smoothly. I had a signal generator, two 6 dB pads, a
50 ohm termination, and my repeater receiver with S meter
(crystalled for two channels, one on the repeater RX freq
and one on the TX freq). The pass and reject tuning is
smooth and the peaks are easy to see on the meter. There
is very little interaction between the high pass and low
pass sides of the duplexer but there is some interaction
between the 3 notch adjustments on each side. I just kept
alternately tweaking the notch trimmers on each side (1, 2,
3... 1, 2, 3...) until there was no further improvement.
Some notes from my preparation and tuning experience...
The notches are VERY deep! You need a VERY well shielded
receiver and VERY good solid or double shield cables
everywhere! Otherwise some RF will "sneak around" the
duplexer and you won't see the bottom of those notches.
That's why I ended up using my repeater receiver... every
other receiver I tried was inadequate.
The notch tuning is very sharp, particularly on the low pass
side. Tune slowly!
Some time ago I had spoken to an engineer at Decibel
Products (when it still was Decibel) and he suggested a
method for getting somewhat deeper notches. He said after
tuning all the pass adjustmets for a peak, to nudge them
away from each other (the high pass up a hair and the low
pass down a hair) until I saw a very slight additional loss
through each cavity... then lock down the nuts and tune the
notch trimmers.
Now for what it's worth...... before and after measurements:
(spec for this duplexer at 600 KHz split is 2.2 dB I.L., 100
dB notches)
Before, professionally measured with service monitor:
I.L. 1.9 dB, notch depth 95 dB / 100 dB.
Before, measured with my simple equipment:
I.L. ~2.0 dB, notch depth 95 dB / 100 dB.
After, measured with my simple equipment:
I.L. ~2.3 dB, notch depth 110 dB / 110 dB.
I'm fairly confident about my ability to measure the notch
depth but will admit the insertion loss measurements could
easily be off by a few tenths of a dB.
It was a pleasant experience and I am quite happy with the
results.
Paul, N1BUG
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
