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Wednesday, 13 December 2017

Homebrew SDR HF Transceiver - Band pass filter

The band pass filter (BPF) is used to filter out strong signals from outside of the frequency band of interest. I have built a 7MHz BPF so the receiver will initially be limited to the 40m band. However, to cover other bands a BPF will be required per band and switched in and out of circuit as the receiver is tuned to the different bands.

I used a circuit that I found on VK4FFAB's site, I initially built this as input to a NE602 mixer where the impedance needs to be 1500 ohms but for this project replaced the 270 & 18 pF capacitors with 220 & 56 pF ones to create 50 ohm output.

The circuit schematic is below, the output is chopped off but is just to the right of the 18pF capacitor with the output of the 18pF capacitor carrying the signal to the next stage.

Credit: VK4FFAB

The Elsie (filter design tool) plot for this is:

My built version looks like this:

Having built the circuit I connected my Wideband RF Noise Generator to the input of the RF amp and the output of the RF amp to the input of the BPF. I then connected my oscilloscope to the output of the BPF and used the FFT function to plot the frequency that the filter passes through. 

The result is shown below. The yellow line is the signal from the wideband noise generator, the purple line shows the response of the filter centred nicely on 7.1 MHz. 

The variable capacitors in the circuit can be used to fine tune the centre of the filter. If you don't have an oscilloscope then a cheap'ish device like the Yaege FC-1 can also be used. I have one of these and it can be used to measure very low power. Using a short piece of coax connected to the BNC connector with two crocodile clips or similar connected to the centre and braid at the other end of the coax you can connect to the output of the BPF and ground. If you input a very low power at 7.1 MHz into the BPF you can then tune the capacitors in the BPF to give maximum power output meaning that the circuit should be tuned to pass most power at 7.1 MHz.

You can also use the FC-1 to check that the RF amp is amplifying an input signal.

Next post: RF Splitter.

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