[Premium-Rx] bandscope frequency conversion question

[H. L.]

Just discovered the premium-RX group and am enjoying the archive 
very, very much.  Quick question. Has a circuit been published for an 
outboard mixer/oscillator that will convert a premium receiver's 455 
kHz IF output to 21.4 MHz for connecting to a WJ (or other) panoramic 
bandscope spectrum analyzer. I understand that the display frequency 
range will be highly limited (or compressed) and that a lower quality 
Heathkit SB620 bandscope will give a better, higher resolution 
display than an expensive WJ scope because it is designed for a 455 
kHz input.  I don't understand the mathematics of this issue and 
would appreciate an article reference if available.  Regards, Hal, 
KK6HY (main premium receiver: Stoddart NM-26T -- with a 455 kHz IF 
output jack). My Stoddart radio history website: 
http://online.sfsu.edu/~hl/src.html