MOTM-890
Frequency Mixer Module

This is an implementation of the Electronic Design article #11935 on implementing a frequency mixer with op amps using the Tellun MUUB-4 pcb.  This design requires that the LO input be greater than the RF input for correct operation.  I used the fourth available op amp section to add a bit of gain (1.2X) to the LO input. 

This is a photo of the assembled MUUB-4 which will mount as a daughterboard on a MOTM-890 uMixer.

 

Frequency Mixer  schematics using the MUUB-4

 

Here are some scope images of this mixer with a sine and square wave inputs.

 

Audio Samples

  FM  Demo 1 
This is a sawtooth waveform into the FM input and a triangle waveform into the LO input.  The triangle waveform is frequency modulated by a slow LFO

  RM Demo 1
This is a same setup as above but into a two transformer / four diode ring modulator.  It sounds virtually identical to the frequency mixer.

  FM Demo 2
This is a triangle waveform into the FM input and a triangle waveform into the LO input.  You can hear the frequency double as the LO goes through 0 volts.  This sounds very much different from a ring modulator.

  RM Demo 2
This is a same setup as above but into a two transformer / four diode ring modulator. It sounds much different from the frequency mixer since there is no audible frequency shift.

  Krell
This is a demo of the frequency mixer.  One VCO is driving a Blacet Mini-wave  which is the FM input.  Two LFOs are driving the bank and wave inputs on the Mini-wave.  Another VCO is driving the LO input.  This VCO is frequency modulated by a sawtooth LFO.  The LFO is frequency modulated by a sample & hold through slew.  Bits of it sound like the Krell machine from the Fobidden Planet.

 

Construction

I decided to use a MOTM-890 as a base module for the frequency mixer.  It was inexpensive and easy to make modifications.  I eliminated the IN-2 and IN-4 inputs and replaced them with the FM and LO inputs.  I normalled the frequency mixer output to IN-5 jack.

The FM input is normal when LO = +5V, inverted when the LO = -5V, and is full wave rectified when LO = 0V.  With no input to LO, this unit can function as a 3 input mixer with full wave rectification of the FM input.  Neat.

I also piggybacked an extra op amp buffer on the MUUB to raise the input impedance of the FM input.

 

I used black printed labels to cover -2 and -4 on the controls, the stripes between the jacks, and to add the FM and LO labels on the jacks.

 

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