LDI DIGISONDE 4D - Tracking Bandpass Filters

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Not for secondary distribution or replication, in part or entirety.
DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 4 – HARDWARE DESCRIPTION 4-15
TRACKING BANDPASS FILTERS
4:26. Analog bandpass filters at the receiver front end efficiently suppress the out-of-band interference
since the filters are synchronously tracking the transmitter frequency. Figure 4-11 shows the block diagram
of the Tracking Bandpass Filter. The input transformer splits the entire frequency range into five bands with
switch points at 1.2, 2.9, 6.5, and 15.9 MHz, plus one low-pass filter for f < 0.5 MHz and one high-pass filter
for f > 32 MHz (for future applications). The main filtering in each band is achieved with serial circuits in
the input that are tuned with voltage-sensitive varicaps. For the high frequency band (f  16 MHz), an addi-
tional tuned parallel circuit at the output (top of Figure 4-11) further attenuates all signals with frequencies
different from the tuning frequency.
Figure 4-11: Tracking Bandpass Filters Suppress Out-of-Band Interferers
4:27. The bandwidth of a serial filter is Δf
s
= R
s
/ 2π L
s,
where R
s
and L
s
are the serial resistance and in-
ductance of the tuned circuit, respectively. Using varicaps for the tuning keeps L
s
constant, i.e., Δf
s
is con-
stant in each band. The L
s
values for each band were selected so as to make the relative bandwidth for each
band ~10%. To make Δf
s
sufficiently small requires R
s
to be small, which is accomplished by a step-down at
the input and a step-up at the output of the serial filters. The bandwidth of a parallel resonance circuit, Δf
p
, is
proportional to 1/C
p
, and therefore increases with f
2
since 1/C
p
= L
p
(2πf)
2
. Figure 4-12 shows one of the
four Tracking Bandpass Filter PCBs.