RF BAND PASS FILTER
The band pass filter unit is one of the most important parts of of the transceiver. The PCB has four filters
and space for a fifth, in case I decide to add an extra band later. There are three parallel tuned circuits in each
filter. Each tuned circuit is coupled to the next by a capacitor (C2, C3). Diodes are used for input/output switching.
The filters have an input and output impedance of 50 Ohms.
BAND PASS FILTER
The schematic shows the arrangement of the band pass filters and i/o switching. To keep things clear
and simple, only two bands are shown. You can have as many as you like. When using ordinary Silicon
diodes instead of proper PIN diodes, it is very important to ensure that the DC
current (more than 20mA in this circuit) is greater
than the RF signal current. The diodes in the filters that are
switched out, have several volts of reverse bias applied.
BPF TOP VIEW
Hand drawn PCB layout for the BPF circuit board. I drilled the holes for the inductors first, then drew the
PCB tracks with an etch resist pen. It looks ugly but it works quite well. I used a simple template to help
drill the holes for the Toko inductors.
TOKO 10K Coil
All of the inductors were wound on Toko 10mm formers that were recovered from scrap equipment. These
transformers are used in television receivers, car radios, CB radios etc. Most of the Toko
formers in the BPF were stripped from an old VHF two way radio. The metal screening can on
the Toko coils is secured to the plastic base of the coil former by four dimples. They can
be flattened by using a carpet knife. It takes a bit of pressure so BE CAREFUL. Once
the metal can is out of the way, it is quite easy to remove the copper wire from the former.
Use a jewellers screwdriver to break the wire away from the pins, then unwind the wire from the
former. If there is a capacitor in the base of the coil, cut the leads off and remove it.
Use enamelled copper wire for the new windings. See table below for details.
The inductance of a Toko 10K coil is approximately: L in microHenries = (16 * turns squared)/1000. Despite the
fact that there are many different types of 10mm inductors, this formula usually gives reasonably accurate results.
This type of coil has a tuning range of about 30%. I tested the BPF unit by connecting it to the input of my
nine band HF rig. The 40M, 17M and 10M filters worked perfectly, the 80M filter was very lossy. I had to
change the original 220pf capacitors to 270pF to get it working properly. I used ceramic disc capacitors for
everything except the 0.1mFd de-coupling capacitors. They are surface mount types, mounted on the solder side
of the board.
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