User's Manual

TXRX Systems Inc. Manual 7-9408-1.2 07/25/05 Page 27
61-38-05 UserMan page 27 of 38
4) 50 Ohm load with at least -35 dB return loss
(1.10:1 VSWR).
5) Insulated metal blade tuning tool for adjusting
ceramic and/or piston variable capacitors.
Similar equipment from other manufacturers
should yield acceptable results.
TUNING PROCEDURE
The following general procedure assumes familiar-
ity with the use of a tracking generator.
1) Set the tracking generator to a center frequency
of the filter to be tuned. Set the tracking genera-
tor for 0 dBm output and a 10 dB/div scale.
Connect the equipment as shown in
Figure 18
but leave the load port of the bridge uncon-
nected.
2) Set a 0 dB return loss reference. For the IFR A-
7550 perform the following procedure:
a) Make sure that the unit is in "LIVE" mode
when performing step 7.
b) From the
Mode Menu
, "STORE" the above
trace.
c) Switch to the
Display Menu
and select
"REF". The trace should appear at the 0 dB
level.
3) Connect the load port of the bridge to the filter
as shown in figure 18.
4) Adjust the cavity main tuning rod for maximum
attenuation at the notch frequency.
5) Set the tracking generator for the center fre-
quency of the filter and connect the test leads
as shown in
Figure 19
but first temporarily con-
nect the leads together through a female barrel
connector and set a zero dB loss reference. On
the IFR A-7550 proceed as follows:
a) Make sure that the unit is in "LIVE" mode when
performing step 2.
b) From the
Mode Menu
, "STORE" the above
trace.
c) Switch to the
Display Menu
and select "REF".
6) Connect the leads to the cavity as shown in fig-
ure 19 and use a tuning tool or small screw
driver to engage the notch tuning capacitor and
rotate it to obtain maximum return loss at the
center frequency.
7) Tighten the cavity main tuning locking nuts.
The cavity may be put back into the repeater ampli-
fier.
LOAD
REFLECTED
SOURCE
Analyzer
Input
Generate
Output
+30
+40
+20
+10
0
-10
-20
-30
-40
RLB - 150 Bridge
2" Square VHF
Notch Filter
Tracking Generator
Tune
Figure 18:
Tuning the Notch filter for maximum
return loss.