# Summing two frequencies

Discussion in 'Homework Help' started by ChuNya, Jun 15, 2010.

1. ### ChuNya Thread Starter New Member

Jun 15, 2010
4
0
Good evening,
I would really appreciate a suggestion to the next problem.
I need to build a circuit that summing the frequencies of two square waves, for example if the input is 50Hz and 30Hz the output will be 80Hz.
The problem is that I have to use specific component (LM741, DAC, ADC, counters).
P.S the phase and the amplitude of the new wave is not important only the frequency.
any suggestions?

2. ### Wendy Moderator

Mar 24, 2008
20,765
2,536
What you need is heterodyning. This is usually done with diodes or some other non-linear system.

Feeding two waveforms into such a circuit will produce the sum and the difference of the input frequencies, as well as both the input frequencies, so filtering is required.

There are many ways to make a mixer, which is one circuit that does this (note, mixer is a generic term, not all mixers heterodyne). Balanced mixer or an XOR gate are also examples of a heterodyne circuit.

3. ### steveb Senior Member

Jul 3, 2008
2,433
469
Another method is to use frequency to voltage converters on each signal, add the voltages and then use a voltage to frequency converter.

4. ### ChuNya Thread Starter New Member

Jun 15, 2010
4
0
I think that the heterodyne mixer will not work here because I have to use digital components thus am not sure I can use it (because if I remember correctly one of the ways to make heterodyne mixer is using the diode bridge).
About the F to V converter and then V to F converter way, I thought about it from the beginning but am not sure how to implement it using the components I've listed.
If anyone can give me a hint I'll appreciate it.

5. ### steveb Senior Member

Jul 3, 2008
2,433
469
You should start assignments like this by brainstorming as much as possible. Break the problem into pieces and think of as many ways as possible to achieve the sub-functions.

For example, a voltage to frequency converter is available in various chip solutions. If you look at the theory of operation of a given chip, you can get hints. For example, I've attached a spec sheet.

For frequency to voltage conversion, think about how to measure frequency. For example, frequency is cycles per second. Therefore, if you activate a counter with an enable bit that is active for a set period of time, and use the signal as a clock, you will count cycles in a given time period.

For conversion of the counter output into a voltage, maybe a DAC does that for you automatically?

Keep thinking along these lines. Sketch out various ideas and circuits and eventually the solution will be clear. Don't be surprised if the answers come while you are sleeping. Keep a notepad and pen by your bed.