Bird Technologies 28-88-04B - Manual (Page 9)

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 5
Tuning while under transmit power can result in
damage to the duplexer.
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 connect-
ing the return loss bridge to the analyzer / gen-
erator 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 (RLB) 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.
7. Connect the "load" port on the RLB to the input
of the loop assembly, make sure the output of
the loop assembly is connected to a 50 ohm
load, refer to figure 4. The display will now
present the return loss curve for the 4" Vari-
Notch filter being measured. The passband is
that frequency range over which the return
loss is 15 dB or greater.
Adjusting the Passband
Adjust the passband by setting the peak (maximum
negative value) of the return loss curve at the
desired passband frequency (should be the center-
vertical graticule line on the IFR A-7550's display).
Refer to Figure 4.
The resonant frequency is adjusted by using the
tuning rod, which is a sliding adjustment (invar rod)
that rapidly tunes the response curve across the
frequency range of the filter. Resonant frequency is
increased by pulling the rod out of the cavity and is
decreased by pushing the rod into the cavity. For
ease in making adjustments, rotate and slide the
rod while gently tapping on it with a screwdriver
WARNING
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
200
MHz
RLB - 150 BRIDGE
Load
So
ur
ce
R
e
fl
e
c
te
d
Figure 3: Setting the return loss reference.