[Jon_G4MDC]

After Bazz started the thread about Hammarlund it reminded me that before the Christmas break I had a project in mind to fit a Murata CFJ455K ceramic filter to an HRO.

It is centred on 455kHz with a bandwidth of 2.7kHz. The shape should be reasonably symmetrical so BFO can be tuned high or low of it for USB/LSB reception. The in/out impedance is 2000 Ohms. I plan to abandon the HRO single crystal filter and use the LC components instead in a matching network to suit the new filter.

I found in my folder concerning the HRO the following measurements on the coil which feeds the grid of the first IF stage.
I forget now where they came from - it deserves proper credit if anyone knows. I have modelled this using the spice statement MUI 1 2 4 0 L1=1.13mH L2=0.15mH M=0.28mH where nodes 2&4 are shorted together as the tap. It tunes to 455kHz with about 67pF across it which ties up with other measurements fairly well. I have added 500k in parallel to represent the grid bias resistor.

The tap shows ~27k resistive at resonance. (Reversing L1 & L2 the result is 300k as would be expected with the tap at the hot end instead)

Of course so far I have assumed the inductor is perfect. In reality it has a Q of probably about 100. Is it fair to add a further 530k of swamp resistor to account for that? (XL is about 5.3k at 455kHz)