Band pass filter

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

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 DETAIL

BPF TOP VIEW

Bpf-PCB

PCB
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.

DRILLING TEMPLATE

Toko coil

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.

Band	T1	T2	T3	C1	C2	C3	C4	C5
80M	21T,4T	21T	21T, 4T	270p	22p	22p	270p	270p
40M	18T, 3T	18T	18T, 3T	100p	7p	7p	100p	100p
17M	10T, 2T	10T	10T, 2T	47p	5p	5p	47p	47p
10M	8T, 2T	8T	8T, 2T	33p	3p	3p	33p	33p

COMPONENT VALUES
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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