Bird Technologies 28-88-04B - Manual (Page 10)

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 6
handle or other small tool. This will break the sur-
face tension on the probe contact fingers and allow
smoother movement of the tuning rod.
Cavity Tuning Tip
When tuning a cavity that has been in service for
some time it is not unusual to find the tuning rod
hard to more in or out. This occurs because TX RX
Systems Inc. uses construction techniques bor-
rowed from microwave technology that provide
large area contact surfaces on our tuning probes.
These silver plated surfaces will actually form pres-
sure welds which maintain excellent conductivity.
The pressure weld develops over time and must be
broken in order for the tuning rod to move. This is
easily accomplished by gently tapping the tuning
rod with a plastic screwdriver handle or small ham-
mer so it moves into the cavity. The pressure weld
will be broken with no damage to the cavity.
Once the desired response is obtained using the
tuning rod, it is "locked" into place by tightening the
1/4" shaft lock nut. Failure to lock the tuning rod
will cause a loss of temperature compensation and
detuning of the cavity.
REJECTION NOTCH
The rejection notch will track with the tuning of the
passband and therefore should be the last adjust-
ment made to the 4" Vari-Notch filter. The rejection
notch is adjusted by changing the amount of
capacitance in the loop assembly. The capacitor is
a variable tubular-piston type.
Checking the rejection notch
1. The rejection notch is checked by connecting the
tracking generator to the input of the cavity fil-
ter while the spectrum analyzer is connected to
the output, as illustrated in Figure 5.
2. Insure that the IFR A-7550 menu's are set as
follows:
DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.
Adjusting the rejection notch
The notch is adjusted by tur ning the variable
capacitor. Because of the filters sensitivity to tool
contact, an insulated tuning tool must be used to
make the adjustment. Access to the capacitor is
obtained by removing the small screw or rubber
button on the side of the loop assembly.
FINE TUNING THE CHANNELS
Once all of the individual filters have been tuned,
each of the channels as a whole must be fine
tuned. First the passband for both channels and
then the rejection notches. The following is a step
by step procedure for fine tuning the channels and
completes the re-tuning of the duplexer.
GENERATE
OUTPUT
ANALYZER
INPUT
40
30
20
10
0
-10
-20
-30
-40
dBm
40 0 10
300
dB ATT GEN dBM MSEC
KHz RES KHz/DIV
500
MHz
RLB - 150 BRIDGE
Load
So
ur
ce
R
e
fl
e
c
te
d
50 OHM LOAD
4" DIAMETER
VARI-NOTCH
FILTER
Figure 4: Checking the passband.