scanned the frequency shifter diagrams

Haible Juergen Juergen.Haible at
Mon Nov 17 18:42:14 CET 1997


Over the weekend I had time to redraw the schematics of my frequency
and I have scanned versions now.

I have posted some bits about this project to DIY every now and then,
and I have
discussed some aspects of the circuits with several people privately. It
is still not
built into an enclosure, and it still lacks interfacing such as a mic
preamp, but
the core is finished, and it works to my full content.

I had started with a BFO design like the Moog / Bode and Electronotes
but I ran into severe problems when I tried to get very low shift
(fractions of one Hz).
Now, thru-zero operation was the reason why I tried the BFO method, but
people convinced me that the slightest jitter in either of the HF
oscillators would result
in severe phase errors near zero Hz Beat frequency.

So I designed a base band Sin / Cos VCO that can go thru zero. This is
achieved by
full wave rectifying the frequency control voltage, and changing the
"spinning direction"
of the sin / cos oscillator with the Sign of the same control voltage.
It took me some time to figure a practical circuit out, but in the end
it was simply
combining an idea from Electronotes (for a "simple", i.e. not
quadrature, VCO), with
a tri-shaped quadrature oscillator from Tietze/Schenk,
The VCO runs from -20kHz to +20kHz, and it is no problem to set it to
like 0.1 Hz as well. There are linear (thru-zero) inputs and exponential
(V/Oct) Inputs.
FS_1.JPG shows the control circuit, FS_2.JPG shows the quadrature vco

Another circuit detail is the use of one cheap chip, the MC1496, to get
both, low-distortion
tri-to-sine-shaping (degenerate emitter method), _and_ multiplying audio
signal and carrier,
at the same time. (FS_3.JPG)

Carrier suppression of this system wasn't that bad (never measured it,
but I think it's way
below -60dB), but I thought a compander system would be nice to reduce
the noise of the
multipliers and to quiet the carrier completly when there is no input
signal. (I don't like the
idea of noise gate thresholds, and there's a hint in the Serge catalog
that they do it
in a similar way.) So I took the compander circuit of the Roland VP-330,
adjusted time
constants for an application where input- and output waveforms aren't
similar anymore,
and the resulting circuit is shown in FS_4.JPG. 

The Hilbert Transform approximation (or "dome filter" in Moog speak) is
a direct copy of
the Electronotes 12-pole design, and so I have not redrawn this part of
the circuit. Simply
a pair of 6 phase shifter stages, with high tolerance capacitors from
the box, but then
measured exactly with a DMM, the resistor value calculated for an exact
RC product,
and then implemented in the form of 2 resistors in series. (One slightly
smaller than the 
required - odd - value, taken from E-24 1% box, measured, and
complemented by the right
small resistor to fill the difference.)

If you are interested in the circuits (and surely I am interested in
feedback / discussion),
I can send the 4 JPGs (approx. 200k each) to someone who could put them
on a web site.
I can also send them to the list, if you want.


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