VK3SMB

Using Phil's idea, A PRM80/FM828 Hybrid VHF high bandreceiver has been made.

The idea being, use the reliable crystal controlled oscillator of the last 40ish years with a current day amplified front end, shoved into a double balanced passive mixer at a high level then use a 21.4Mhz IF amp at 10.7Mhz meaning more gain.

The schematic shows, from the PRM80 I have used:

• antenna filter
• front end
• mixer
• buffer amp, and
• IF amp.

From the 828, I have used

• Crystal oscillator
• tripler,
• IF preamp and
• the rest of the FM828.

The Power supply is a simple voltage reg.

This whole receiver draws 20 mA. so far.

=Dissecting the boards=

Pictures speak louder than words, but basically the two receivers have been stripped of any component not needed to stop any chance of intermod from something not connected.

Upon hooking it up (and forgetting something - thanks Phil), I tested it at 163Mhz as thats where I had crystals for. My service monitor generates signals down to -140dBm. I could still hear the tone down at that. So plus 1 for sensitivity.

After the sensitivity test was good. Time to tune it for 2M. On the IFR2398, I set the marker at 147.800Mhz and shoved the tracking generator in.

Shown in the picture, there is 10dB roll-off at +- 3Mhz from centre and that the whole plot is only 20Mhz wide to the noise floor. The red dot is showing a blip on the rolloff curve. that is the IF frequency from the crystal I was using. (152Mhz odd).

Measuring the signal from the tripler, and then after the buffer amp, the buffer amp is an increase of 30dB. Very high level mixing!

Now need a correct crystal!

It has been decided to use a +-7.5khz bandwidth filter on the front to narrow down the bandpass of the IF. This will help with lower level signals (by decreasing the BW we are also decreasing the noise floor, hence, adding more to the sensitivity and lessening the chance of desense).