Simple Amplitude Modulation Circuit

[bruzzac]

Hi guys,

I'm not sure if you have heard of DCF77, but I need to create a simple DCF77 generator which is basically a 77.5kHz carrier that has 1 bit a second encoded into the carrier. It is lowered to 20% of the amplitude for either 0.1 or 0.2 seconds at the start of each second depending on whether the bit for that second is a 0 or a 1.

I have tried using a simple voltage divider network such as the one below, but with a transistor and resister in parallel with the second resistor which would control the amplitude. However I am getting a weird waveform.. the negative side of the sine wave does not have the same amplitude as the positive side.

---/\/\/\/\-------/\/\/\/\-------

I would like to know if anyone has had any similar experiences with this kind of modulation scheme and see if anyone has any suggestions.
Thanks in advance

Bruzzac

[Bill_Marsden]

Sounds like clipping, but without the whole schematic it is impossible to be sure. Need input.

Are you switching between the two points with a transistor or what?

[bruzzac]

Hi,

this is basically what i have come up with.. the resistors have been setup accordingly so that it should bring the 77.5kHz carrier down to be about 0.2%-0.25% however, with this setup I am getting a waveform like the one attached. So I was wondering if there was another way of implementing such an switching attenuation network?

Also, I am not really able to use an amplifier, as I will only have a positive 5V available. Although, I maybe able to use an amp with biasing the signal 2.5V.

[thingmaker3]

Since the transistor conducts in only one direction, the divider attenuates only the negative going peaks. If you had used a PNP instead of an NPN, only the positive going peaks would have been attenuated.

Perhaps a voltage-controlled amplifier would better suit your needs.

I suppose a clever person might be able to use a triac instead - don't know for sure.