Ring Modulator : mods for REV1 and REV2

The Ring Modulator of the Wildcat is based on an old design by Tom Gamble tusing a CA3046 transistor array to form a Gilbert balanced modulator. Instead of the CA3046, the Wildcat uses a very efficient and classical balanced modulator IC (MC1496). However, the result is far from being convincing : high amount of distorsion and poor carrier and modulation nulling. In order to improve the circuit, I studied the datasheet of the MC1496. There, it is stated that the input levels must be limited to 300 mVpp to reduce the amount of distorsion. Thus my first modification was to  add 100k resitors in series with the input capacitors (as a matter of fact these two resistor cannot be inserted on the PCB, these will have to be directly soldered to the input plugs. This way, if you feed the Ring modulator with the sine signals of the Wildcat's VCOs, the level is limited to 130 mVrms at pins 8 and 4. However, with such a limitation the output level of the module remains really low. To increase the output level, I changed the value of R8. This resistor sets the gain in the MC1496. I selected a value of 100 ohms that provides the highest gain while limiting the distorsion amount. A smaller value introduces too much distorsion.
Next is the poor rejection of the carrier and modulation signals. As a matter of fact, both the positive and negative inputs (pins 1 & 4, pins 8 & 10) must be polarized. In Tom's design the signal inputs (pins 4 & 8) are missing such a polarization. Therefore, I have added some polarization resistors to pins 4 and 8. A 120 ohm resistor connects pin 8 to the wiper of T1, and a 10k resistor connects pin 4 to the wiper of T2. I also inserted a 120 resistor between pin 10 and the wiper of T2.

original schematic (copyright EFM)
modified schematic
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view mods track side

Next are some clues on how to set up the two trimmers T1 and T2. You need an oscilloscope and spectrum analyzer. Basically you may connect the output of the ring modulator to the signal input of a PC and use a dual channel oscilloscope + spectrum analyzer software (there are some freewares and sharewares offering these functions).
Here is how to proceed :
  1. Feed input A with a 1kHz sinewave with a 7Vpp level (signal from VCO1). Adjust T1 & T2 , alternatively in order to minimize the output signal  (few mV). You can do it also by ear, adjust T1 & T2 to  mute the output signal.
  2. Disconnect the sine wave signal from input A and connect  it to input B.  Re-adjust T1 & T2 in order to minimize the output signal.
  3. Repeat points 1 and 2, until achieving the lowest bleedthrough...
Then connect a 1kHz sinewave from VCO1 to input A and a 1.55kHz sinewave from VCO2 to input B. Connect the output to the audio signal input of a PC, and observe the obtained spectrum. You should observe the spectra below depending of the trimmers settings.

Once you have obtained the second spectrum, your ring modulator is nearly trimmed !

A last trimming will complete the procedure. For this you need a good sinewave generator with a low harmonic distorsion. This means that, definitely, you cannot use the sinewave signal from one the Wildcat's VCOs.  Once you have found this pure sinwave generator, set its frequency to 1kHz and its amplitude to 2Vpp. Now, connect this sine signal to both inputs of the Ring Modulator. Then connect the output of the Ring Modulator to the first channel of your oscilloscope. Connect the output of the sinewave generator to the second channel of your oscilloscope. There you must see a 1kHz sinewave on channel two and a somehow dissymmetrical 2 kHz sinewave.  Now, you must  slightly adjust T1 and T2 in order to obtain a perfectly symmetric 2kHz  sinewave.  That's it !

Unbalanced modulation spectrum
The spectrum on the left shows a typical unbalanced setting. The carrier and modulation signals (1kHz and 1.5kHz) are not correctly rejected. They are 20dB above the generated sum and differences signals also called sidebands (C-M : 1500-1000->500Hz; C+M : 1500+1000->2500Hz). The other peaks are overtones created by the harmonic distorsion. If you see this, adjust T1 & T2, in order to decrease M and C, and increase (C-M) and (C+M).
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Balanced modulation spectrum
The spectrum on the left shows a typical balanced setting. The carrier and modulation signals (1kHz and 1.50kHz) highly reduced. They are 35 to 45dB below the generated sum and differences signals (C-M : 1500-1000->500Hz; C+M : 1500+1000->2500Hz). The other peaks are overtones created by the harmonic distorsion. If you see this, that's it you're done with the settings !
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When optimal carrier and modulation nulling is achieved, the output of the module looks like shown on the left with a 1kHz and a 1.55kHz input signals.
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The same with a 150Hz and a 1.55kHz input signals