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.

Band pass filter 2
BPF DETAIL




BPF TOP VIEW
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.


TEMPLATE
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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