# Building a Modulator

Discussion in 'Homework Help' started by justinf951, Nov 30, 2008.

1. ### justinf951 Thread Starter New Member

Nov 30, 2008
2
0
Hello and Thank you for your help,

Alright I have a group project for my physical layers of communication class in which it appears I will be forced to do all the work. Basically we have to build a simple modulator. Basically we have a digital signal that delivers a bit every 60 msec and a carrier frequency we are going to put through a voltage divider to get frequencies to represent the 1's and 0's. What I am trying to find is something that can be used to look at the digital signal and based on if it's a 1 or 0 switch between the low or high frequency inputs we have. Our professor suggested we look into double pull single throw relays or digital gates, but I have had little luck on that front. The problem with school is that all they teach us is theory then expect us to actually complete a real world assignment so any help would be greatly appreciated thank you for your time.

Justin

2. ### leftyretro Active Member

Nov 25, 2008
394
2
" through a voltage divider "

Did you mean frequency divider there?

What you are describing is frequency shift keying, a standard modulation method used in the old analog modem days, See Bell 103 and Bell 201, etc.

Relays would be much to slow to reliable perform the logic switching. A couple of logic gates and a flip flop for the divider should be all that is needed in theory. In reality to actually build something functional one needs to know the voltage and impedance values of the input and outputs for the circuit.

Lefty

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
A simple VCO could do FSK. There are many ways to do this, another would be to use gates to select between two freqency streams.

4. ### justinf951 Thread Starter New Member

Nov 30, 2008
2
0
Upon further inspection of the directions we were hastily given I think what our teacher is looking for us to build is an ASK modulator. Because it looks like he wants the analog output to vary in amplitude based on the digital input. Sorry for the confusion this is not really my strength, it is just a required course. Basically what we are given is a bit stream, a frame clock, a bit clock, and a carrier frequency and we have to turn the digital bit stream into an analog signal that another team is then going to convert back from analog to digital. Once again thanks for your help.