summing 2 frequencies

[zesla]

Hi,

I want to sum 2 frequencies say A and B. For instance say the A is equal to 100kHz
and B is equal to 1kHz (B frequncy changes till 8kHz for sometimes) and get C=100kHz + 1kHz =101kHz. How can I do so?
Thanks.

 

[Nigel Goodwin]

You need a double balanced mixer, followed by a VERY sharp filter to remove the 99KHz signal (you get sum and difference out).

 

[crutschow]

Technically, you want to multiply the signals (using a mixer a Nigel stated) to get the sum (and difference) frequencies.

 

[zesla]

Oh, I just thought maybe just a summer based upon resistorsa and an op-amp will do so?

 

[zesla]

By the way as I know it is not so easy to design a sharp filter while the diference is just 1 to 2kHz?

 

[Nigel Goodwin]

Oh, I just thought maybe just a summer based upon resistorsa and an op-amp will do so?

No, a 'summer' wouldn't do it, as suggested a 'multiplier' is what's required, however it's not as complicated as it sounds. If you didn't want double balancing then almost anything will do, a transistor, a diode etc.

But a standard mixer isn't balanced, so you get out of it sum, difference, and BOTH inputs - so a double banacned mixer cancels out the original, something like an MC1496 would be fine.

 

[crutschow]

Oh, I just thought maybe just a summer based upon resistorsa and an op-amp will do so?

A summer adds the two signals together, but not in the way you want. If you summed them as you proposed, you would end up with a 100KHz signal riding on a 1kHz signal. But you would not have a 101kHz signal. To get that requires a non-linear multiplier (mixer) circuit.

 

[zesla]

No, a 'summer' wouldn't do it, as suggested a 'multiplier' is what's required, however it's not as complicated as it sounds. If you didn't want double balancing then almost anything will do, a transistor, a diode etc.

But a standard mixer isn't balanced, so you get out of it sum, difference, and BOTH inputs - so a double banacned mixer cancels out the original, something like an MC1496 would be fine.

Thanks for your help Nigel

What is a DOUBLE BALANCING MODULATOR really and why they are called so?
I just want ONE output (101kHz for instance). So is it easy to get just one output while there is no ideal filter and it is not so easy to get a very sharp filter?

 

[zesla]

A summer adds the two signals together, but not in the way you want. If you summed them as you proposed, you would end up with a 100KHz signal riding on a 1kHz signal. But you would not have a 101kHz signal. To get that requires a non-linear multiplier (mixer) circuit.

Ok thanks, but the main problem is how to get it by a multiplier/ modulator? I just want one output, so the difference freq and the so called carrier (i.e 100kHz) would be problematical.

 

[zesla]

Any circuit/chip to get just one output as I want?

 

[Mikebits]

Any circuit/chip to get just one output as I want?

Nigel already told you a chip to use.
https://www.electro-tech-online.com/custompdfs/2010/09/mc1496.pdf
https://www.electro-tech-online.com/custompdfs/2010/09/mc1496apnote.pdf

 

[zesla]

Nigel already told you a chip to use.
https://www.electro-tech-online.com/custompdfs/2010/09/mc1496-1.pdf
https://www.electro-tech-online.com/custompdfs/2010/09/mc1496apnote-1.pdf

Yes I took A look at it when Nigel introduced it, But there is a bad problem, and it is removing the lower freq, (I think it is called the lower sideband) and just get the upper one on the output.
By the way I have found that a chip like AD633 does as a multiplier too, but as I told I have the problem of removing the lower sideband.

P.S why we ended up with an AM modulator for my job? is it just the AM modulation that act as a summer like what I want?

 

[dougy83]

Maybe 2 frequency-voltage converters, outputs summed & fed into a voltage-frequency converter.

 

[crutschow]

Producing a signal with a single-sideband and no carrier is not trivial. You can use a modulator circuit designed for that purpose such as a Hartley (phase shifting the modulating signal) or Weaver circuit (quadrature modulation). A third way is to use a balanced modulator which suppresses the carrier but produces the upper and lower sidebands, so you need a sharp filter to remove the undesired sideband.

 

[Diver300]

You could just measure both frequencies with a microcontroller or two, add the resulting numbers, and send the sum to a DDS IC.