Full Wave Rect. For increasing Frequency?? - Page 1
Forum section: electronics / general

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 17, 2020, 12:28:26 pm
content: Electronics is fun, but I'm toying mentally with something (possibly old!) to increase Frequency.This is an exercise in theory, as much as practicality/need. For no other 'need' except I want to....Full-Wave Rectification, obviously results in the bottom half of the original sinusoidal Waveformbeing moved above. Ok, there is a waveform now, all above 0-v, that although it has a roundedpeak, and a more generally 'sharpened' bottom, it is LIKE a 'pseudo' sine-wave, all above Zero.Now lets utilize a basic cct to shift that waveform DOWN, so that it virtually becomes a sort ofsinusoidal (rounded top, pointed bottom) waveform with a 'zero' voltage centre-Line.The result?? You have effectually doubled the Frequency!! Now 'Rectify' again.. etc...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 17, 2020, 12:59:35 pm
content: Note that you need a load impedance, to keep the waveform always pushed down towards zero -- otherwise the voltage rides over the valleys without dropping, and you don't get nearly as much AC output.If we put a load resistor on there, we lose as much signal power; same as a resistor-biased class A amplifier.A resistor isn't strictly required, but this is representative of how the method yields fairly poor efficiency.If we cascade a bunch of stages, pretty quickly the signal power goes away and we wonder what we are even trying to do...If we can afford amplification, it's still quite reasonable though. And indeed, if we use nonlinear amplifying devices, we can use two in parallel, bias them somewhere into cutoff, and drive them so that they conduct on opposite phases of the input -- the two, FET gates say, are driven oppositely, but their drains are connected in parallel, giving an active full wave rectification, as it were.What about waveform quality? It works nicely for one stage, but you can easily see that the second stage will be "flipping" the spiky valley into another peak, alongside a smooth sinus peak. It's not at all repetitive. And the 3rd stage will be even weirder, etc. This method doesn't work very well in general, but if we can afford to filter out the higher harmonics, we can smooth that negative valley into another sinus hump, and when paired with the positive hump, it looks alright again.How much filtering can we afford? If the input signal has a frequency range of less than half an octave, then the doubled output will have a range of less than one octave, and we can cleanly select the doubled fundamental while rejecting all higher harmonics.We can generalize this further to any harmonic distortion network and filter; say we have an amplifier with a very narrow conduction angle, i.e. it makes a low duty cycle, spiky waveform from a sine wave input -- if we drive a 3rd or even 5th or higher harmonic bandpass filter with this clipped signal, we can make the filter resonate. The filtering won't be perfect, so there will be some amplitude modulation -- the signal rings down between hits, it's like striking a bell at a subharmonic rate. We can make the filter arbitrarily tight to fix this, or we can pair it with an amplitude limiter.These frequency multiplier circuits were in common use back in the days of vacuum tubes -- when a radio's crystal might be only a few MHz yet a stable VHF or higher carrier is needed, or to convert signals between bands for various purposes, or to widen a signal's bandwidth (FM modulation is difficult; a phase modulator is simple but gives very little deviation; by multiplying it up, then shifting it back down with hetrodyning, wideband (broadcast) FM can be made).Tim

author: Circlotron
author_rank: Super Contributor
timestamp: June 17, 2020, 01:26:43 pm
content: I think you can put a sine wave into both inputs of an analog multiplier and get a double frequency sine wave output with DC offset. Capacatively couple the output into another multiplier and the sky is the limit. Forget the exact details. Someone will know.

author: TimFox
author_rank: Super Contributor
timestamp: June 17, 2020, 02:09:50 pm
content: Closer to your full-wave rectifier, MCL packages diode bridges that are specified for use as frequency doublers.Seehttps://www.minicircuits.com/WebStore/Multipliers.htmlWith proper diode drive and matching, unwanted harmonics can be reduced.A true multiplier, as illustrated above, produces a good sine wave but most balanced modulators are switching, rather than true multipliers.

author: fourfathom
author_rank: Super Contributor
timestamp: June 17, 2020, 03:29:16 pm
content: Yes, the Mini-Circuits parts are one implementation. In case you can't find the internal schematic it probably looks something like this:This is a two-diode full-wave rectifier driven by a trifilar-wound center-tapped transformer. There will obviously be loss and the output frequency will contain both odd and even harmonics of the doubled frequency. If the diodes are well-matched there will be good suppression of the input signal. A doubler like this is usually followed by a filter and amplifier.

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 19, 2020, 01:23:12 pm
content: Quote from: Circlotron on June 17, 2020, 01:26:43 pmThank you, and 'T3sl4co1l' and theothersfor your responses!Yes, since posting my 'thoughts' I realized that I was trying to "re-invent the wheel" hahaha...I will 'play' with all that on the bench, just for fun, but now my old & failing 'brain' can't remembersomething else? (I should know!! grrr..)....AC impedance (like in a Transformer) is based on (yes the frequency) but basically a VARYING fluxdensity, (variable/moving lines of magnetic force), to produce 'Back-EMF' opposing the original forcewhich produced it, but also to induce such variations into the Secondary. That's a Given!!...But does the 'variation' 'have' to cross Zero, as in a typical sine Wave? If, in the example of a fullwave rectifier output, continuously varying the voltage from the 'peaks' to the sharp transitions at'zero' volts, (no negative on the 'wave'), and THIS was output into another transformer primary,would not the 'physics' of 'AC' on a winding still be in effect?? As in... a continuously varying fluxresulting in a primary 'impedance', and generation of a voltage on the secondary??I 'should' know this!!! but my body/brain is failing!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2020, 02:39:19 pm
content: An ideal transformer will transmit any amount of flux (integral volts*time), but a real one is limited on this property. Consequently, you can't pass DC through a transformer (at least, not easily; using switches to synchronous AM de/modulate a signal, you can create a "DC transformer"), where "DC" is some lower cutoff frequency determined by the transformer's parameters.In the above waveform for example, you'd have to use some sort of coupling network to separate the DC bias from the red trace, before coupling it to a transformer. A typical case would be a load resistor across the bridge output, then a series coupling capacitor, then the transformer winding.The blue trace is balanced (it spends as much volt*time above zero as below) so can be transformer coupled easily enough.Tim