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M0LTE / pi-transceiver

An attempt to build a complete ham radio transceiver around a Raspberry Pi
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RF Bandpass Filter #22

Open G4CDF opened 2 years ago

G4CDF commented 2 years ago

The issue here is what specification and design for the 144MHz bandpass filter. The aim is to use the same filter for tx and rx with PIN diode switching.
Specification has been taken from https://www.sotabeams.co.uk/bandpass-filter-for-2m/ (and modified as required) Filter bandwidth 144 - 146 MHz (-1dB) ie centre frequency 145 MHz Attenuation: In-band typically < 3dB 433 MHz > 70dB 155 MHz approx. 25dB FM broadcast band (88 to 108 MHz) >70dB Air band (108 to 137 MHz) > 50 dB

On the basis of above assuming a symmetric filter, the -25 dB bandwidth is <20MHz, -50dB bandwidth is < 15MHz and 70dB bandwidth < 94MHz It may be that a bandstop section would be best to reject the air band

From Zverev p99, a 4 pole symmetric filter (Chebychev 0.5dB ripple) will provide >55dB attenuation at 15MHz bandwidth, so should be suitable.
It is intended to design one and simulate using QUCS (Quite Universal Circuit Simulator) before prototyping

G4CDF commented 2 years ago

Decided to extend 1dB bandwidth to 144 to 147MHz to allow use in the NOV band. Need to check that 1dB slope is ok for the reduced bandwidth TV signals etc.

G4CDF commented 2 years ago

Various design files are being committed to G4CDF branch please ignore at present as they are not complete.

G4CDF commented 2 years ago

Currently the BPF will occupy 35 mm x 35 mm of PCB area and will be about 20 mm high. Whether a screening can will be required remains to be seen but in any case it wil be 5 sided and soldered to the PCB. Alignment requires adjustment of 4 capacitors only I hope. This can be done with spectrum analyser and tracking generator or VNA. I intend to build the first few on a PCB about 50 mm x 35 mm.

G4CDF commented 2 years ago

After much work I have decided to use a 4 pole butterworth filter. The -3dB bandwith will be 4MHz. The inductors will be mounted in 22mm pipe caps and have a predicted Qu of 520. This has been checked by measurement.