Bird Technologies 15-88-01 - Manual (Page 8)

TX RX Systems Inc. Manual 7-9150-2 07/19/07 Page 4
Adjusting the rejection notch
The notch is adjusted by turning the variable
capacitor. Because of the filters sensitivity to tool
contact, an insulated tuning tool must be used to
make the adjustment. Access to the capacitor is
obtained by removing the silver dot, small screw or
rubber button on the side of the loop assembly.
MULTIPLE CAVITY VARI-NOTCH FILTERS
Vari-Notch filters can be ordered in multiple cavity
arrangements of either two or three combined cavi-
ties. In these arrangements, identical filters are
connected in a cascaded fashion with the output of
each filter fed to the input port of the succeeding fil-
ter. The advantage to this arrangement is that the
amount of attenuation provided by each of the fil-
ters is additive. In the case of the rejection notch
frequency, the dual cavity can provide attenuation
of over 60 dB (30 dB for each filter).
Also, the interconnecting cable between the two fil-
ters, when cut to the correct length (odd multiple of
a 1/4
λ), will provide up to 6 dB of additional attenu-
ation due to a mismatch of impedance between the
cable and the filters. The 6 dB of mismatch attenu-
ation does not occur at the filters passband but,
only at frequencies where moderate to high attenu-
ation occurs, such as at the rejection notch fre-
quency. 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.
Analyzer
Input Generate
Output
0
-10
+10
-20
-30
-40
-50
-60
-70
4" Diameter
Vari-Notch
Filter
Figure 5: Checking the