For this job I used an microprocessor board I had build with a MC68008 8/32 bits processor, MC68230 IO, 32k RAM and some ROM, and a ADC0820 ADC. Also included is some logic to speed up the transfer of data to the LCD. The 68008 runs on 12.960 MHz, and the refreshrate of the screen is some 30ms, which is just enough for a flicker-free image.
In the picture a preliminary setup is shown. (In this picture you see the spectrum and ghostimage of it, due to the flyback...)
The video is sampled under interrupt at a smplerate of 158 microsecs, so a full image is sampled 63 microseconds. To increase the horizontal resolution the signal is averaged in factors of 2. The software searches for the maximum ignal in the spectrum and displays a marker on top, and the magnitude and frequency are shown. It is also possible to put the marker manually elsewhere.
The schematics of the processorboard was drawn in Protel and uses a GAL type 20V8 for address decoding and control. The 68008 source was written in assembler.