25 April 2010

A modification to obtain a DRM receiver?

Back to an old friend, DRM (Digital Radio Mondiale) ?! Perhaps.

I have been given a working Roadstar TRA-220, which includes few short wave bands in AM. With PLL tuning at 5 kHz steps it is not suitable for SSB/CW reception, but it can be adapted for DRM. Moreover that radio has always suffered from some design mistakes, like:
  • volume control slider too sensitive
  • audio volume too loud
  • strong hum on audio
  • loss of presets and clock unless 2xAA cells are fitted in

The radio has plenty of room inside and all components are through-hole. At a first glance the probable cause of hum is evident: 15VAC and audio line to speaker run together to/from the main board.

I have to build a (circa) 455 kHz to 12 kHz downconverter.

The IF system chip is BA4237L; the AM IF filter is marked SFU450B (thus 10 kHz BW at 450 kHz). First decision to be taken: should I pick the IF before or after the filter?

If I pick it after the filter I need a L.O. at + or - 12 kHz, so 462 or 438 kHz: how to make a stable one? (I will use my xtalfind tool to see if any of my surplus XTALs fall close to those values)

If I pick it before the filter I can make a stable 455 kHz L.O. and retune the receiver L.O. so that the resulting difference is 12 kHz. Would the incoming signal be too wide?

Let experiments begin!