UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Improving Selectivity In Vintage Receivers

G3PIJpeter · 26th Oct 2022, 3:13 pm

I have found a couple of circuits for impedance matching of filters with centre frequency at the receiver IF - see attached.

The single emitter-follower circuit was given in an article about fitting a narrow CW crystal filter to Collins receivers. I imagine it would work OK with a ceramic filter.

The two-transistor circuit is from the RSGB digest of Pat Hawker's 'Technical Topics' articles. The collector resistance R1 is made equal to the filter input impedance; the two R2 resistances are each twice the filter output impedance. I have yet to come across a filter where input and output impedances are not essentially the same.

Can anyone find a circuit for the up / down IF converter? G3JIR wrote a comprehensive set of articles on the design of ladder filters in Radcom - something else to dig out.

Peter

26th Oct 2022, 3:13 pm	#7
G3PIJpeter Hexode Join Date: Nov 2011 Location: Chippenham, Wiltshire, UK. Posts: 323	Re: Improving Selectivity In Vintage Receivers I have found a couple of circuits for impedance matching of filters with centre frequency at the receiver IF - see attached. The single emitter-follower circuit was given in an article about fitting a narrow CW crystal filter to Collins receivers. I imagine it would work OK with a ceramic filter. The two-transistor circuit is from the RSGB digest of Pat Hawker's 'Technical Topics' articles. The collector resistance R1 is made equal to the filter input impedance; the two R2 resistances are each twice the filter output impedance. I have yet to come across a filter where input and output impedances are not essentially the same. Can anyone find a circuit for the up / down IF converter? G3JIR wrote a comprehensive set of articles on the design of ladder filters in Radcom - something else to dig out. Peter Attached Thumbnails Last edited by G3PIJpeter; 26th Oct 2022 at 3:23 pm.