Bird Technologies 28-69-04A - Manual (Page 9)

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 refer-
ence value.
7. Connect the "load" port on the RLB to the input
of the loop plate, make sure the output of the
loop plate is connected to a 50 ohm load, refer
to figure 4. The display will now present the re-
turn loss curve for the 6.625" Vari-Notch filter
being measured. The passband is that fre-
quency range over which the return loss is
15 dB or greater.
Adjusting the passband
Set the fine tuning rod at it's mid-point. Adjust the
passband by setting the peak (maximum negative
value) of the return loss curve at the desired pass-
band 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
coarse 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. Additionally, the fine tuning rod, also a
sliding adjustment (silver-plated-brass rod) allows
a more precise setting of the frequency after the
course adjustment is made. The frequency is in-
creased by pushing the fine tuning rod in and is
decreased by pulling it out; the exact opposite of
the coarse tuning rod. For ease in making adjust-
ments, rotate and slide the rods while gently tap-
ping on them with a screwdriver handle or other
small tool. This will break the surface tension on
the probe contact fingers and allow smoother
movement of the tuning rods.
Cavity Tuning Tip
When tuning a cavity that has been in service for
some time it is not unusual to find the main tuning
rod hard to move in or out. This occurs because
TX RX Systems Inc. uses construction techniques
borrowed from microwave technology that provide
TX RX Systems Inc. Manual 7-9177-1 09/19/97 Page 5
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
LOAD
SOURC
E
REFL
ECTED
RLB - 150 BRIDGE
0
15
20
5
0
50 OHM LOAD
VARI-NOTCH
FILTER
Figure 4: Checking the passband.