Bird Technologies 20-70-26 - Manual (Page 7)

TUNING
Required Equipment
The following equipment or it's equivalent is
recommended in order to properly perform the
tuning adjustments for the Series-Notch filter.
1. IFR A-7550 Spectrum Analyzer with optional
Tracking generator installed.
2. Eagle Return Loss Bridge model RLB150N3A.
3. Double shielded coaxial cable test leads
(RG142 B/U or RG223/U).
4. 50 Ohm load, with at least -35 dB return loss
(1.10:1 VSWR).
5. Connector - female union (UG 29-N or UG
914-BNC).
6. Insulated tuning tool (TX RX Systems Inc. part#
95-00-01).
7. 5/32" hex wrench.
Tuning Procedure
Tuning of the filter requires adjustment of the
notch
frequency and the passband frequency .
Adjustment of the notch depth is optional because
it is usually factory set in most cases. The
passband frequency is adjusted using a return loss
curve which is generated using a tracking
generator and return loss bridge. The notch
frequency and notch depth are adjusted by
monitoring the output of a tracking generator after
it passes through the filter. To insure proper tuning
of the Series-Notch filter, all adjustments should be
performed in the following order:
1. Adjust the notch depth (optional).
2. Rough tune the notch frequency.
3. Rough tune the passband frequency.
4. Fine tune the notch frequency.
5. Fine tune the passband frequency.
NOTCH DEPTH AND FREQUENCY
Notch depth is not normally adjusted when
re-tuning the Series-Notch filter in the field. Notch
depth is factory set, at which time a relative index
label is attached to the top of the cavity next to the
loop plate and a calibration mark is stamped into
the loop plate itself. The relative index label is used
to log specific filter performance. Changes in the
notch depth will cause a shift in both the passband
frequency and notch frequency. Smaller notch
depths allow closer spacing of the notch and
passband. Notch depth and notch frequency can
both be checked using the procedure listed below.
Checking the notch depth or frequency
1. A zero reference must first be established at the
IFR A-7550 before the notch depth or frequency
can be measured. This is accomplished by tem-
porarily placing a "female union" between the
generator output and analyzer input, refer to fig-
ure 6.
2. Setup the analyzer / generator for the desired
frequency and bandwidth (center of display) and
also a vertical scale of 10 dB/div.
3. Insure that the IFR A-7550 menu's are set as
follows; DISPLAY - line
MODE - live
FILTER - none
SETUP - 50 ohm/dBm/gen1.
4. The flat line across the screen is the generator's
output with no attenuation, this value will be-
come our reference by selecting the "Mode"
main menu item and choosing the "Store"
command.
5. 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.
6. Connect the generator output and analyzer in-
put to the input/output ports of the loop plate as-
sembly. The notch depth and notch frequency
can be read from the display on the IFR
A-7550's screen, see figure 6.
Adjusting the notch depth
Adjustments are made by loosening the three
10-32 screws that hold the loop plate into position
and then rotating the plate itself. When the
calibration mark is pointed at the relative index
setting of 0 the notch depth will be 15 dB
TX RX Systems Inc. Manual 7-9146-1 07/25/96 Page 3