[Boatanchors] SP600 / HP 5245L as digital display?

[Eugene Hertz]

Hello all,

Hoping for some electronics circuit design assistance.

I have a project to interface the HP 5245L to my hammarlund sp600 (boatanchor) to make it a frequency read out. I have a modern frequency readout but I think the nixie tube counter would be SO much better! As you know, the receiver has two conversions, one to 455Khz and one at 3.955mhz. 

What I need to to is somehow figure out a ckt that will take the local oscillator (HFO) and subtract 455khz or 3.955mhz depending on the band I'm on (could possibly be determined by the HFO freq itself). For purposes of this discussion, I'd be happy if someone could help me figure out just how to subtract 455khz. From there I could figure out the rest. 

Although I haven't measured, I believe the HFO goes from about 995khz (for 540khz) to about 57.955Mhz (for 54Mhz, 6 meter band). I thought that I might use a double balanced mixer with an LO of 455khz, but then I didnt know how I could eliminate the upper image. I also know that the LO to the mixer would have to be exceptionally well shielded. I am pretty new to ckt stuff like this. 

I know there are several other possible approaches such as:
1. use a pic or something do it all digitally, then output a freq using some DDS circuit to drive the nixie counter
2. Use a F-V converter then adjust the V and then use a V-F converter
3. some kind of mixer (dunno how)
4. The counter has an input for external 1Mhz standard. I've been playing around with putting in other freq into this, but I dont think it works right. I can get it to display the right freq but only that one freq.


Any way, since this is slightly OT, please contact me directly if you have any ideas! 
Thanks for the bandwidth 
Eugene