[1000mp] IF filter circuitry design

[Pitman, Zoltan, VF-NL]

Dear all, 

During contests I heavily use the 2nd VFO of my MP and I was always unhappy with its performance in crowded band conditions especially on 80/40m SSB. 

I started to look for options to improve selectivity and big signal handling of this second RX. I summerized and illustrated my ideas on my webpage at www.hg6n.hu/ha1ag under FT1000MP sub-vfo options. However when analyzing Yaesu's implementation of 455 kHz IF filtering I've got a few questions I can't answer. I would be very happy if someone could answer them or give a hint about where to look further. 

The 455 kHz IF filter blocks are placed between two transformers and the actual one is selected by diodes. There are auxuliary components added for impedance matching and diode biasing. The T-circuit of resistors is placed _behind_ the IF filter. I think it is crucial to provide constant resistive load to the mixer. Thus I should expect them to go in front of the filter, shouldn't I?  Is there any reason of it being there?

I still have to measure the S-parameters of the filters but I expect them to have a low reflection coeffiecent off the nominal frequency. It would also imply to put the attenuation between the mixer and the filter. 

Is there any literature where I can read about designing IF stages?

Any comments would be gratefully appreciated, 

zoli ha1ag

www.hg6n.hu/ha1ag