Frequency Halver

Sine or square wave?

For square waves, you could use a synchronised multivibrator. This technique was widely used before the advent of TTL. Make a 10 KHz multivibrator and feed-in a small amount of the 20 KHz signal. This works by "pushing" it over the threshold at exactly the right moment.

For sine waves, there's no "simple" circuit.

Sorry John, but I think you have just described a frequency doubler, rather than a frequency halver.

For what it is worth, I'm pretty sure that there is no analog technique that will halve the frequency of a sine wave. It is easy enough to use a largely digital circuit to produce a sine wave which follows half the average frequency of of an analogue sine wave, and if you got really cute, you could track the phase of the input sine wave and produce an output whose phase varied at half the rate - though since the maxima and minima of a sine wave don't tell you much about the instantaneous phase, this isn't going to be prefect either.

--------- Bill Sloman, Nijmegen

Injection-locked oscillator, maybe, but not all that simple.

John

I've done this for boom-box designs:

Square signal

Divide by 2 (digitally)

Filter to get back a sine wave

Use ENVELOPE of original signal as envelope for Div2

Actually sounds good even on classical music ;-)

...Jim Thompson

A 20kHz sine wave repeats every 50usec. A half-wave rectifier without a filter capacitor produces a series of half-sine pulses repeating every

A suitable monostable can be used to ignore an arbitrary period after an active pulse, which, for a limited range of input requencies, can give you a half rate output. Going from there to a half frequency sine wave is a bit more of a stretch.

------------ Bill Sloman, Nijmegen

Or a parametric oscillator: one L, one C, one varicap.

John

Accurate 'halving', not really. This is easiest to do digitally. Simple counter, gives you any division wanted. For a sinusoidal output, you could use a PLL, for a given range of frequencies. For a single frequency, you can use a mixer, but this will only give 'half' for one input value.

Best Wishes

yeah, they call it a flipflop, of course the signal will be square afterwards. if you want to really down convert it to keep it in its sine wave form, then try something like a Double Balanced modulator! something like the old LM1496 rings a bell. inject 30Khz and the 20Khz into the other input and you get 10KHz and 50Khz., you simply filter the upper side out. how ever, one should note that this will scramble your audio if you were to pass compound/complex waveforms through it. :)

I read in sci.electronics.design that Andrew Holme wrote (in ) about 'Frequency Halver', on Fri, 18 Mar 2005:

Half-wave rectify without a filter cap. Use the unidirectional pulses to trigger the multi or, better, a triangle wave generator. Low-pass filter the resulting waveform.

--- Not stupid at all, it's been the holy grail for pitch-shifters for quite some time. I don't believe there's a simple analog solution and even if you do it digitally in real time by sampling the input with one clock, doing an A to D conversion on the slices and then doing a D to A and outputting that analog signal at half the input sample clock frequency implies sufficient storage to keep the buffer from overloading.

Think about it like this: You've got a bathtub filling up at one gallon per second and, at the same time, being drained at two quarts per second, so eventually it'll overflow.

-- John Fields

I read in sci.electronics.design that snipped-for-privacy@ieee.org wrote (in ) about 'Frequency Halver', on Fri, 18 Mar 2005:

If you mean me, the HW rectifier produces one pulse every 200 us for 20 kHz input. Triggering the multi or TWG in the correct way to get one cycle per pulse is no different from what the sync circuit of a scope does.

I read in sci.electronics.design that John Fields wrote (in ) about 'Frequency Halver', on Fri, 18 Mar 2005:

Who the hell counts in base 4 these days? (;-)

--- If you can do it digitally, the easiest way would be to use a comparator to square up the incoming signal, then use the comparator's output signal as the clock of a "D" type flip-flop wired as a divide-by-two, and then filter the output square wave either passively or actively. If you use a dual op-amp you can use one section for the comparator and the other for the filter.

-- John Fields

There is an analogue sum available Bill, that may (or may not) be persuaded to cooperate.

A sub-harmonic oscillator works in a similar manner. Use a tetrode, ground the plate, use about 30V for the screen, input signal goes to the plate. The control grid/cathode is set up as a Hartlet oscillator; it cannot oscillate at such a low screen voltage - which is set for it to be a little less that what is requred for oscillation. The signal on the plate adds energy, and the oscillator section will "slowly" be excited into oscillation; power is extracted from the grid at some negative gain.

Incorrect; over 40 years ago we did that at Sylvania with a tetrode. See a previous response by me earlier in this thread.

A "starved" sub-harmonic oscillator is rather simple; see an earlier response by me for more details.

Tony,

Try Sin(2A)=-2*Sin(A)*Sin(A-pi/2), this is an inverting summer with phase shifter and multiplier in feedback path summed with Sin(2A). If input is Apk*Sin(2A) then output should be sqrt(Apk)*Sin(A).

You can sync a Wien Bridge oscillator - the HP204C sinewave oscillator had a sync terminal which injected into the oscillator. Some of the sync signal did appear on the output, depending on the size of the sync input. HP say it will lock on harmonics of the sync input, but no mention of subharmonics. However, a rectifier would divide by two and give you a guaranteed to lock signal.

Roger Lascelles