3.FM-DE/Modulator May 1980

I cannot remember the exact date when I got this idea of looking at modifying a AM CB radio to FM modulation..

In the 1980 to 1990 when Citizen band radio was very active and the all the big boys where moving to SSB CB 

and the Children Where talking on AM (Thats why it was called a CB (childrens Band) was like social Media platform..

There was a artical in the May 1980 practical wireless about a Narrow band FM demodulator to use in a AM reciever..

Now with a little knowledge we had from the Radio repairs and basic radio theory the AM demodulation was very simple

and did not require any special IF chips for recovering the audio from a AM signal.With the added atvantage of a 

cleaner and better signal on FM nobody knows why 10m AM Radios where never fitted with FM mode...Although some HF 

Amateur radios where available with FM or extra FM modules fitted extra device...The reason for the FCC and other 

autorities not type approving the radios for FM was to do with the bandwidth it was taking up and maybe on the

manufactures side the availabilty of the IF chips..Motorola made a nice Chip range MC3350 and later MC3363 for

the 2m or VHF radios and philips made a wide band Chip like a TDA7000 etc...Then there was some others made by 

Texus Instruments the SN76660 that was cheaper and available from electronic outlets...And that is the one we used 

in our project...

When we looked at the FM modulation side it was obvious that its was easy to do when you had a VCO block and Reference 

Xtal and a Varicap Diode to create FM modulation...But still needed some amplication to get the microphone audio to a 

acceptable voltage to give you 5Khz FM modulation..

We made three boards for this project, The Demodulator ,The Varicap Mod board and then the Audio Preamp board..It was

fitted in my General Electric AM CB and worked great with one of my electronic mates when we tested it..But that was end 

of that experiment..Not everybody had a GE and different Radios like the Royce and Cobras that where out there had different

circuits and mods had to be adjusted according to the makes and operators did not see the need for FM modes..

Later in the european manufactures started introducing FM and AM in their little radios and it was approved by the FCC..

ICASA or department of P&T did not type approve the CB radios if it had FM mode..

In 2020 we made a new design on one small board with SMD to fit into all these useless CB's to give them a new clearer 

way of communications but not sure if the local radio Authorities will allow it...to be continued.. But a nice project to build

only some homework needs to be done to fit it into different models.

The Old 1980 project made with homemade PCB's and Through hole components .

The new Schematic drawn in Eagle cad.

The new updated PCB version 1.0 with a few mistakes .