Bird Technologies 15-88-01 - Manual (Page 6)

TX RX Systems Inc. Manual 7-9150-2 07/19/07 Page 2
Tuning Procedure
Tuning of the filter requires adjustment of the pass-
band and the rejection notch. The passband is
adjusted while observing the return loss response
and the rejection notch is adjusted by monitoring
the output of a tracking generator after it passes
through the filter. To insure proper tuning of the 4"
Vari-notch filter all adjustments should be per-
formed in the following order:
1. Rough tune the passband.
2. Rough tune the notch.
3. Final tune the passband.
4. Final tune the notch, always the last adjustment
made.
PASSBAND
The peak of the passband will correspond very
closely to the point of minimum reflected energy
from the filter and maximum forward power through
it. A transmitter connected to the filter will operate
best when the reflected energy is lowest, therefore
the return loss response will be used to set the
passband. The passband can be checked and
adjusted using the following procedure.
Checking the passband
1. A zero reference for return loss must be estab-
lished at the IFR A-7550 prior to checking the
passband frequency, this is done by connecting
the return loss bridge to the analyzer / genera-
tor as shown in Figure 3.
2. Set-up the analyzer / generator for the desired
frequency (center of display) and for a vertical
scale of 10 dB/div.
3. Do not connect the return loss bridge to the
cavity, leave the "load" port on the bridge open.
This will supply the maximum amount of
reflected energy to the analyzer input.
4. Insure that the IFR A-7550 menu's are set as
follows: DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.
5. The flat line across the screen is the return loss
curve. Select the "Mode" main menu item and
then choose the "Store " command.
6 Next select the "Display" main menu item and
choose the "Reference" command. This will
cause the stored value to be displayed at the
center of the screen as the 0 dB reference
value.
Cavity Resonator
Tuning Rod
1/4" Shaft Locking Nut
Loop Assembly
Variable Capacitor
Input/Output Ports
Schematic Symbol
Figure 2: The 4” Vari-Notch filter.