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Old 20th Mar 2019, 10:04 pm   #19
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Homebrew SSB Transceiver

Quote:
I have made quite a few different filters based on designs from the interweb... mostly the Butterworth configuration
I think the default Fc for a Butterworth will be at the -3dB point so if you design for Fc= 15MHz the filter will already have 3dB loss by 15MHz. I can't explain why you are seeing >13dB loss unless you have a component or circuit issue. The PCB layout is a bit odd with inputs crossing outputs and lots of long link wires but I don't think this will cause the 10dB extra loss.

A Butterworth filter won't give a steep rolloff rate and I would have gone for a classic cheby LPF and you could expect over 50dB rejection by 30MHz. For a 100W+ PA I would have gone for T80-6 toroids but for low duty cycle SSB you will probably be OK with T50-6 toroids.

I'd expect a 7th order LPF to have about 0.25dB insertion loss at 14.3MHz and 50-55dB rejection by 30MHz if built with good components.
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Regards, Jeremy G0HZU
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