Dual Analogue Frequency Shifter

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Introduction

History

Circuit Descriptions

PDF of Input Filter and Phase Sequence Network freq_shift03-01.sch



This sheet starts with an input filter network. This is two three pole bandpass filters with the HPF Fo about 50Hz and the LPF Fo about 15kHz. This limits the input bandwidth to the phase sequence array, which can produce 90° phase shifts only over a limited bandwidth. The array is driven with a true and inverted signal, and the array outputs are buffered to feed the following circuitry. The phase sequence array itself has a gradual -4dB dip mid-band and the input filters partly compensate for this. The exact values of the capacitors are not critical but each value set should be matched to 1%.The S_0 and S_90 output buffers provide convenient input points for the carrier feedthrough trims. +/-5V 100mA regulators provide local voltage references.

PDF of Through Zero Quadrature Output VCO, CV Exponentiator and Current Sources freq_shift03-02.sch



This sheet starts with a control voltage exponentiator which is a standard 1V/octave type as seen on other designs. This is followed by a quadrature output VCO which provides both positive and negative output frequencies depending on the polarity of the control voltage. It provides a function that has traditionally been implemented in the Bode/Moog frequency shifter by using a fixed BFO at 20kHz and a variable sine oscillator going above and below 20kHz, both feeding into mixers. This new circuit reduces the number of oscillators to one, and uses no mixers. As a result the output has fewer spurious harmonics and no unharmonically related harmonics, resulting in a cleaner output overall.

Circuit Selling Features For The TZQVCO

TZQVCO Circuit Operation

The two lower transconductance amplifiers form a standard double integrator bandpass-lowpass filter as shown in the Nat Semi data sheet for the LM13700. These lower TAs are active for positive control voltages, and the upper TAs are effectively turned off by being fed no bias current; These play no part when the CV is positive. The TL074 op-amps are merely buffers to enable easier choice of resistor values on the TA inputs and to make sure that the integrator capacitors are not loaded excessively. The resistor values set Q to be >1. Feedback from the bandpass output to the non-inverting input via the diode limiter provides feedback to make the filter oscillate. Adjusting the trimmer gives output amplitude trimming and allows the drive to the diodes to be set to a level that ensures oscillation but minimises distortion. This fairly standard configuration gives quadrature outputs from the two buffers with the highest distortion product about 40dB down on the fundamental.

For negative control voltages the upper TAs are fed bias current and the lower TAs are shut off. The upper TAs work in exactly the same way as the lower ones but they are cross connected to the inputs and integrator caps. In this way the in-phase and quadrature outputs are reversed for negative CV, thus creating an inherently smooth transition to what can be considered to be a negative frequency. In operation, viewing the outputs on an X-Y trace the circular rotation of the dot becomes slower as the CV is adjusted to near zero and then reverses direction perfectly as the CV goes negative, without any unwanted crossing of the circle or drifting off beyond the circle. This is true provided that the transition to -ve CV does not take many seconds.

The two current sources are fairly self explanatory, with the upper source operating for -ve CV and vice versa. The gain trimmer on the upper source allows the oscillator to be adjusted such that a given -ve CV gives the same frequency as the equal magnitude +ve CV. This compensates for differences in Gm of the two separate dual TA packages. The diodes across the op-amps avoid heavy output saturation when that source is turned off. Low Vf Schottky diodes are better in this application.

Both oscillation modes need to be trimmed to give equal output amplitude using the diode limiter drive trimmers.

A front panel switch selects LOG or LIN CV and a range switch reduces the oscillator frequency when closed. For +/-10V linear CV the oscillator should give about +/-10kHz.

This was published in Electronic Design News in January 2004. I received a cheque for $100 USD. Fab!



PDF of Balanced Modulators, Output Sum/Difference and Duo-Trio Summing freq_shift03-03.sch



Here we have the modulators fed with the audio and carrier signals. The output of the oscillator is so-called carrier because this circuit is identical to that used to create RF sideband signals, only the RF carrier frequency is actually in the audio band in this case. The carrier signal is potted down from 2Vpp to about 20mVpp which is a suitable level for driving these modulators in their linear mode. This circuit can be thought of either as a sideband generator running at AF as described or as a continually sine-weight panning between the 0, 90,180 270, 0 …. outputs of the phase sequence array. For a given sine input to the array, this scanning creates a situation analogous to doppler shift where the listener is running towards or away from a sound source and therefore intercepting more or fewer wavefronts.

Readers may note the similarity here between this pitch shifter and the original Hammond organ scanning vibrato, though the Hammond LC line is closer to a true delay than this, wholly phase shift oriented design. For this reason the Hammond vibrato preserves the harmonic relations of the signal better than this design, though far from perfectly. The phase-shift only nature of the pitch shifter allows us to "scan" and thus frequency shift, in one direction continously, while the Hammond LC line has to be scanned up and down due to the time discontinuity that would be experienced when trying to flyback to the start of an LC line.

The output from the second board is taken back to two summing amplifiers on this board. One sums the output of the two boards to form a duo chorus output. The other sums the output of both boards plus the input to board 1 to form a trio chorus output. The signal switching of the front panel jacks is arranged so that the input to board 1 is fed to both board 1 and board 2 when there is no jack connected to the board 2 input.

PDF of Board 2, a repeat of freq_shift03-01 freq_shift04-01



PDF of Board 2, a repeat of freq_shift03-02 freq_shift04-02



PDF of Board 2, a repeat of freq_shift03-03 freq_shift04-03



This repeats sheet 3 of board 1 but has an extra audio output connector and no duo/trio summing.

Other Notes

Pictures

Frequency Shifter Insides Looking Down



Frequency Shifter Insides From The Front



Frequency Shifter Insides From The Rear



Frequency Shifter Circuit Boards Close Up

Sounds

Later Musings

Antiques Roadshow

Is it just my imperfect memory, or is it really the case that this electronic version was used as the theme tune in the very early days of BBC Television's Antiques Roadshow programme? Answers on a postcard or the back of a sealed-down envelope to... ... W12 8QT.

And the answer is, after a recent 20th birthday anniversary special of Antiques Roadshow, Yes. The original opening sequence was shown, using this tune complete with pencil-drawn animation and the hilarious comedy bit with Arthur Negus opening up the doors on a large item of furniture to reveal the TV set within, cue zoom to the screen and wibbly wobbly static followed by the Antiques Roadshow logo. Definitely a four star rating TV Cream nostalgia moment. Hmmm, smell the gravy!

Radio Use; G5TMA Calling CQ Top Band

Could you use a similar circuit as a USB or LSB sideband generator for use in amateur radio? I don't see why not. You'd want to filter the input audio properly from 300Hz. to 3kHz, use the better op-amp based phase shifter circuit with a lower bandwidth but better accuracy, and the quadrature VCO would be fixed frequency with a tweak for accurate quadrature. The oscillator would run at your IF, probably 455 kHz, then your transceiver VCO would be mixed with that to create the SSB output to be fed to the TX chain. Or something like that. Such circuits already exist. It would be interesting to do the calculations to determine the required accuracy of the phase shifters in order to meet the generally accepted unwanted sideband suppression level.

Later Additions

It looks like I added output sockets for both quadrature oscillators. They can be handy if you want to create fancy oscilloscope traces like those seen on the ambisonic panner page.

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