Bird Technologies 11-70 Series - Manual (Page 9)

The filter's loop plate assembly may be changed in
order to convert the cavity from one type of filter to
another. Conversion kits can be ordered which
contain all required parts for the conversion. The
available conversion kits are listed by part number
in table 2. Refer to the appropriate TX RX Systems
Inc. manual for the specific filter type once the kit is
installed.
Converting to Bandpass
When converting a Series-Notch or Vari-Notch filter
into a Bandpass filter an additional index label must
be applied to the cavity. Follow the procedure listed
below for correct placement.
1. Install the bandpass loop plates into the loop
plates holes of the cavity.
2. The bandpass loop plate installed at the posi-
tion of the existing index label should be rotated
until it's calibration mark aligns with the index
value of 10 and then tightened down into place
3. Measure its rejection notch as shown in figure 4
and record the value for comparison with the
unlabeled port. The amplitude of the rejection
notch is directly proportional to the insertion
loss .
4. Connect the tee to the remaining loop plate and
rotate it until its rejection notch is equal in value,
then tighten it down into place.
5. Apply the second index label so that the value
of 10 lines up with the calibration mark.
6. Tighten all loop hold down screws.
When converting from a T-Pass filter into a
Bandpass the six steps listed above will not be
necessary. The T-Pass filter already has two
properly affixed index labels.
MULTIPLE CAVITY BANDPASS FILTERS
Bandpass filters can be ordered in multiple cavity
arrangements of either two or three combined
cavities. The filters are connected in a cascaded
fashion with the output of each filter fed to the input
port of the succeeding filter. The advantage of this
is that the amount of attenuation provided by each
of the filters is additive.
The interconnecting cable between the two filters,
when cut to the correct length (odd multiple of 1/4
λ
), will provide up to 6 dB of additional attenuation
due to a mismatch of impedance between the
cable and the filters. The 6 dB of mismatch
attenuation does not occur at the filters passband
but, only at frequencies where moderate to high
attenuation occurs.
Because each of the filters in the multi-cavity
arrangement are identical, the passband for the
entire arrangement is generally the same as the
passband for the individual filters. However, each
filters individual insertion loss is also additive.
When tuning a multi-cavity arrangement, each filter
is tuned individually prior to interconnecting them.
Then each is fine tuned to peak the overall
response of the multi-cavity arrangement.
TX RX Systems Inc. Manual 7-9145-1 08/05/96 Page 5
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MHZ / DIV MHZ
98.00 300
KHZ RES
dB
40 dB ATT GEN dBM 0 10 MSEC
ANALYZER GENERATE
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BANDPASS
FILTER
TEE CONNECTOR
Attach "UG-107 tee connector"
directly to the input / output port
Figure 4: Measuring the rejection notch.