Bird Technologies 28-88-04B - Manual (Page 8)

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 4
require no further adjustment or maintenance.
These devices will stay properly tuned unless they
have been physically damaged or are tampered
with. Check for loose or corroded connectors on
the interconnect cables whenever an inspection is
performed on other station equipment.
Because duplexers are passive devices, field
repairs are rarely required. Field repair of duplexers
is limited to the replacement or repair of damaged
cables. Cavity damage, when it occurs, is usually
due to catastrophic failure from lightning or power
far in excess of the duplexers rating. If cavity prob-
lems are suspected, the unit should be returned to
the factory for repair. Due to the critical alignment
of parts inside of the cavity resonators, field repair
is not recommended.
TUNING
Vari-Notch duplexers are originally pre-tuned at the
factory to the customers specification. To retune
the duplexer, each resonant cavity must be sepa-
rated from the group and adjusted individually.
Then the individual cavities are re-connected and
each channel is fine tuned to peak it's overall
response. When reconnecting the assembly, it is
mandatory that each filter and cable be replaced in
it's original position.
There are two adjustable parameters in a Vari-
Notch filter; the pass frequency and the rejection
notch. Adjustment of the tuning rod will allow the fil-
ters passband to be centered at the desired fre-
quency. The rejection notch frequency is adjusted
by turning the variable capacitor located on the
loop assembly.
The insertion loss of each cavity is determined by
the design of the loop assembly and is not field
adjustable. The loop assembly on a 4" resonant
cavity should never be loosened or moved from
it's factory preset position.
It is also important to note that the insertion loss
specification in table 1 for each of the different
models, is the total insertion loss for each channel
of that model. For instance, the specification for
model 28-38-03A/D is 1.9 dB, this means both the
high and low frequency channels will each have a
total of 1.9 dB of insertion loss. The total insertion
loss is the sum of losses from each cavity in the
channel as well as the interconnecting cables
between the cavities.
Required Equipment
Due to the sensitivity of the adjustments, it is
strongly recommended that the proper equipment
be used when tuning the individual filters, other-
wise the filter should be sent to the factory or an
authorized representative for retuning. The follow-
ing equipment or it's equivalent is recommended in
order to properly perform the tuning adjustments
for the Vari-Notch duplexer.
1. IFR A-7550 spectrum analyzer with optional
tracking generator installed.
2. Return Loss Bridge (Eagle 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). The JFW Industries model
50T-007 or equivalent.
5. Female union (UG29-N or UG914-BNC).
6. Insulated tuning tool (TX RX Systems Inc.
part# 95-00-01)
7. 1/4" open-ended wrench.
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.
All Vari-Notch filters should be temporarily removed
from the system and tuned on the bench using test
instrumentation only. Do not adjust the filters while
they are under transmit power. To insure proper
tuning of the 4" Vari-Notch filter, all adjustments
should be performed in the following order:
1. Rough tune the passband.
2. Rough tune the rejection notch.
3. Final tune the passband.
4. Final tune the rejection notch, always the last
adjustment made.