large area contact surfaces on our tuning probes.
These silver plated surfaces will actually form
pressure welds which maintain excellent conduc-
tivity. The pressure weld develops over time and
must be broken in order for the tuning rod to
move. This is easily accomplished by gently tap-
ping the tuning rod with a plastic screwdriver han-
dle or small hammer 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
coarse and fine tuning rods, they are "locked" into
place. The coarse rod is secured by tightening the
10-32 cap screw and the fine tuning rod is held in
place by tightening the knurled thumb nut. Failure
to lock the tuning rods will cause a loss of tem-
perature 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 6.625" Vari-Notch filter. The re-
jection notch is adjusted by changing the amount
of capacitance in the loop assembly. The capaci-
tor is variable and is either an air-plate or tubular
piston type depending upon the frequency range
of the filter. The air-plate type has a red mark
painted on the access barrel and one-half of the
adjusting screw, when the red marks line up the
maximum capacitance is achieved. On UHF mod-
els (400 MHz and over) the capacitor access bar-
rel is omitted and a 10-32 inch screw must then be
removed from the loop plate assembly to gain ac-
cess to the piston trimmer under the plate.
Checking the rejection notch
1. The rejection notch is checked by connecting
the tracking generator to the input of the cavity
filter 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 turning the variable ca-
pacitor. Because of the filters sensitivity to tool
contact, an insulated tuning tool must be used to
make the adjustment.
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.
1. Reassemble the duplexer by reinstalling the
cavities and interconnect cables in their original
locations.
2. The
passband for the channels are fine tuned
first, in a manner very similar to tuning a single
cavity.
3. A zero reference for return loss must be estab-
lished at the IFR-7550. Connect the RLB to the
analyzer / generator as shown in figure 3.
TX RX Systems Inc. Manual 7-9177-1 09/19/97 Page 6
GENERATE
OUTPUT
ANALYZER
INPUT
10
0
-10
-20
-30
-40
-50
-60
-70
dBm
40
0
10
300
dB ATT
GEN
dBM
MSEC
KHz RES
KHz/DIV
200
MHz
VARI-NOTCH
FILTER
0
15
20
5
0
Figure 5: Checking the rejection notch.
