Bird Technologies 20-70-26 - Manual (Page 5)

GENERAL DESCRIPTION
The Series-Notch® cavity filter passes a relatively
wide band of frequencies (
passband ) while
simultaneously rejecting a very narrow band of
frequencies (
notch frequency ). Minimum
separation between passband and notch frequency
is 50 KHz. The notch depth is variable from 15 to
25 dB. A variety of models are available that cover
the range of frequencies from 30 to 960 MHz. The
frequency range that each model will tune across is
determined by the cavity's physical length.
Either 6-5/8" or 10" diameter resonator shells may
be used to construct the filters. The diameter
difference between the two determines the filters
selectivity and it's maximum power dissipation. The
10" diameter filters have a slightly higher selectivity
(more attenuation at the notch frequency)
compared to the 6-5/8" models. Additionally, the
10" filters can safely dissipate up to 40 Watts of RF
Power, while the 6-5/8" filters can dissipate up to
30 Watts. Maximum input power for the 6" and 10"
diameter filter's is listed in table 1.
There are three adjustable parameters found in a
Series-Notch filter including the passband
frequency, the notch frequency, and notch
depth. Each of these parameters is labeled on the
response curve shown in figure 1.
Two types of Series-Notch filters are available,
lowpass and highpass. Lowpass filters permit a
very narrow separation between the notch and the
low frequency portion of the passband. Likewise,
highpass filters will permit a very narrow separation
between the notch and the high frequency
passband. The lowpass filter, unlike the highpass
filter, can be tuned for a symmetrical response.
The difference between the two types of filters is
determined by the loop plate assembly used. The
cavity itself remains identical for both types. The
part number is stamped on the loop.
Figure 1 shows the response curve of a lowpass
filter while figure 2 shows the same filter's return
loss curve. A symmetrical response can be seen
in figure 3 where the notch is centered between the
low frequency passband and the high frequency. A
symmetrical response can only be obtained with
relatively large separations between pass and
notch frequencies. Figure 4 shows the resulting
return loss curve.
All of the physical components of the filter are
labeled in figure 5, with the adjustable parts shown
in emboldened italics. Coarse and fine tuning rods
are used to adjust the notch (resonant) frequency.
TX RX Systems Inc. Manual 7-9146-1 07/25/96 Page 1
dBm
0
10
-10
-20
-30
-40
-50
-60
1
MHZ/DIV
98.00
MHZ
300
KHZ/RES
10 MSEC GEN 0 dBM 50 dB ATT
20
Notch Frequency Notch Depth
Passband
Figure 1:
Spectrum Analyzer / Tracking Generator display of
the Series-Notch filter tuned lowpass. Response
curve above is for model # 20-29-01 ( 88 - 108 MHz).
dB
0
10
-10
-20
-30
-40
1
MHZ/DIV
98.00
MHZ
300
KHZ/RES
10 MSEC GEN 0 dBM 50 dB ATT
20
Passband
Notch Frequency
40
30
Figure 2:
Return loss response curve for the "lowpass"
Series-Notch filter shown in figure 1. Response curve
above is for model # 20-29-01 ( 88 - 108 MHz).
Insertion loss 6" diameter
power rating
10" diameter
power rating
0.3 dB 449 Watts 599 Watts
0.6 dB 230 Watts 308 Watts
Table 1: Input power ratings