amplitude modulator for low frequencies (1 to 5000 Hz)

Thread Starter

LarryWalker

Joined Aug 18, 2022
2
Hi all,

For my next project, I need to mix 2 sinusoidal frequencies.

I've found some modulator IC (Ex. MC1496) and some modules on Aliexpress, but the problem with those is that they work with high frequencies (Ex: 85 to 108 kHz).
The frequencies I need to mix are a lot lower AND they have (more or less) the same order of magnitude.
An example of frequencies I want to mix is:
- carrier frequency : 311 Hz.
- modulating frequency : 7 Hz.


The amplitudes of those frequencies are not equal (one of the signals is stronger than the other) thus I need to equalize the amplitudes, before mixing them.

Can somebody point me to the right direction?
Ideally, I would like to find a module already made, but if there is none, I know how to conceive a PCB an solder the components on it.

Thank you all for the time you consumed in reading this message.
Larry Walker.
 

ericgibbs

Joined Jan 29, 2010
16,797
The amplitudes of those frequencies are not equal (one of the signals is stronger than the other) thus I need to equalize the amplitudes, before mixing them.
Hi Larry,
Welcome to AAC.
Do have a figure for the amplitudes of the two Sine wave frequencies.?
E
 

Ian0

Joined Aug 7, 2020
6,711
An MC1496 is just a Gilbert cell - it will work down to DC.
If it’s part of a module, it will be the coupling capacitors that set the frequency response.
Where do these signals come from?
 

DickCappels

Joined Aug 21, 2008
9,513
@Ian0 It is actually two Gilbert cells.

@LarryWalker Why do you think the MC1496 cannot be used at low frequencies?

This is the only reference to low frequency operation I have found.
1660825865460.png

This might be related to the fact that if the transistors have very small geometries they may heat slightly under the influence of the signal currents being handled and that can result in some signal distortion. You might find the MC1496 works well in your application.

You can make your own multiplier with a few transistors. I did this many times about 40 years ago.
1660825584713.png

(above) source is http://techlib.com/electronics/amxmit.htm
This circuit is an AM transmitter featuring a single Gilbert cell (the two 2N4401 transistors near the center of the schematic.
For your purpose it might be wise to bias the right-hand transistor with a bypassed Zener diode rather than a resistive voltage divider.

Below is my favorite balanced modulator for low frequency use when the carrier is a square wave or a sine wave that has amplitude sufficient sufficient to fully switch the bipolar transistor on and off. To be balanced the ratios among the 5 resistors (marked R and 3.6R) must close to idea and the output must drive a high impedance load. The opamp runs with a gain of 11 db. Output signal is 6db down from the baseband modulation signal input.

1660826567838.png
(above) Schematic of balanced modulator. No the transistor was not accidentally drawn upside down - the emitter presents a very low impedance when the base-collector junction is forward biased. Note that this is a small signal circuit because large negative signals can cause the base-emitter junction to avalanche, causing clipping of the modulated signal.

1660826738724.png
(above) The result after low pass filtering in simulation.

Edit: Removed confusing simulation model schematic.
 

crutschow

Joined Mar 14, 2008
31,126
You can also use an analog multiplier, such as the AD533, which operates down to DC.
LTspice simulations below for standard AM modulation, and AM suppressed-carrier double-sideband modulation.

1660829722811.png
1660838262142.png
 
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