Varactor phase shift and SCR phase degree control

[markphaser]

Is there a way i can change the capacitors in my phase shift network with Varactors? so i can Adjust the Varactors with a Voltage supply changing the Capacitance?

Can i use a "SCR" component to have waveshape?

Can i use a "SCR" to give different phase degrees?

Can i use a "SCR" to control the phase degrees?

[R.G.]

Is there a way i can change the capacitors in my phase shift network with Varactors? so i can Adjust the Varactors with a Voltage supply changing the Capacitance?

Not to any significant degree. Look up the datasheets of some varactor diodes. The capacitances are in the picofarads, too small to use in conventional phasers.

Can i use a "SCR" component to have waveshape?

No. SCRs are silicon semiconductors. They do not have waveshape any more than apples have orange juice.

Can i use a "SCR" to give different phase degrees?

No.

Can i use a "SCR" to control the phase degrees?

Not to any practical degree.

You found the word "SCR" along with "phase control" right?

[markphaser]

Thanks R.G

You found the word "SCR" along with "phase control" right?  Yea i did

Use can Trigger the SCR with a pulse or square wave and it gives different degrees or starts "conducting" at a degree

[markphaser]

varactor diodes the capacitances are in the picofarads, too small to use in conventional phasers.

Can i use Varactors in "series" to increase the capacitances?

Or i would need a "Custom" Varactor diode to have the capacitance in the microfarads?

What would the Varactors doping process do different and how? the PN junction needs to be Bigger?

[markphaser]

So i have to get this Custom made to have a large depletion layer region (large area)?

[R.G.]

Can i use Varactors in "series" to increase the capacitances?

No. (a) capacitors in series do not add; capacitors in parallel add; (b) no, you cannot practically get enough varactors in parallel to make a phaser with them

Or i would need a "Custom" Varactor diode to have the capacitance in the microfarads?

You would need an impractically large varactor - so large that it's probably impossible to build one free enough of defects.

What would the Varactors doping process do different and how? the PN junction needs to be Bigger?
So i have to get this Custom made to have a large depletion layer region (large area)?

(a) Come on now, be realistic - you would not understand differences in doping processes. And yes, in all matters to deal with capacitance, it's the capacitor area that matters if you can't change the plate spacing. Yes, you would have to get a custom varactor; as I said, no, it' not practical to make even one big enough for a phaser.

[markphaser]

Thats R.G for correcting me

If i make the resistor large enough can't i use the (pf) Varactor? to adjust the phase shift?

Thats really to bad they don't make (uf) Varactors this would be cool to put in phase shifters to adjust the capacitance

[markphaser]

Here is the phase control:

http://scitec.uwichill.edu.bb/cmp/online/el10c/Lecture%20G%20Other%20Semiconductor%20Devices/Lecture1/Otherzc.GIF

[stm]

I might be opening a can of worms here but...

When I worked for the AM/MW transmitter industry we used MVAM108 varactor diodes to tune the frequency synthesizer PLL.  These varactors are quite unique in the sense that they have very high capacitance (for a varactor) with a tuning range from 20pF to 450pF.  They have one of the largest capacitance variation ratios (nearly 20:1) I've seen in varactors, in comparison to the more typical 5:1 or so.  I had a hard time trying to find a suitable replacement after they were discontinued--couldn't find another device with such special characteristics.  In the end we purchased a large lot from a surplus broker.

Some info here:  http://www.angelfire.com/electronic2/index1/mvam108.html

We even used them to implement a voltage controlled third order passive filter suitable for RF (shunt C - series L - shunt C), so the bottom line is it might be *possible* to implement a phaser with said varactors, however not *practical* since they are discontinued and their current high cost as NOS.

[markphaser]

Thanks

I was wondering if i could use a  Varactors phase control on a LFO circuit ? thats uses a Lamp?

[Ge_Whiz]

Come on, R.G. - a couple of thousand varactors in parallel ought to do it. Oh, for each stage I mean...

Mr Markphaser - you have, I'm afraid, completely misunderstood the use of the 'phase' in relation to SCRs.

[markphaser]

What does it do then?

[Ge_Whiz]

In simple terms, a SCR can be triggered to switch at, or interrupt, an AC waveform (usually at high voltage) at the same point of each half-cycle in order to provide effective power control, e.g. for mains light dimmers. It does not influence the phase of the waveform at all.

[markphaser]

Its seems like it changes the phase. It my book it shows graphs of phase control phase shifts from triggering and dimming the light

It dims the light at certain phases of the waveform so if the LFO is hooked up to a Lamp then it could dim the lamp at certain phases by a SCR phase control circuit?

[Rob Strand]

>   These varactors are quite unique in the sense that they have very high capacitance (for a varactor) with a tuning range from 20pF to 450pF.

Those wide capacitance varactors tended to be for AM band apps.   Yes they are hard to get and relatively expensive.  You can of course use these in a voltage controlled oscilllator.  The *possible* frequencies will be in the 300kHz to 30MHz you would have to divide down the output  to get to LFO rates.  There's several issues which make this a poor solution:
- if you divide the frequency down it is most efficiently done using digital dividers, then you would end-up with a square wave which is not easy to shape over a wide range of frequerncies.
- instead of a divider you could use an divider feeding an EPROM lookup table for the waveshape which then feeds some sort of D to A.
- The frequency range variation is limited.  You would need an LC oscillator which will only give you about a 4:1 frequency range.  Varactors don't work well in RC oscillators because the capacitance is non-linear hence the voltage swing is limited.
- The varactors around these days have very low capacitances and very small capacitance ranges.
- Using a varactor means  your control is a voltage to the varactor.  For an LFO this has absolutely no advantage over using a straight-out variable resistance pot for most application.  Why go through the interim step of voltage control!

All in all the method has very little to offer.

[Transmogrifox]

Its seems like it changes the phase. It my book it shows graphs of phase control phase shifts from triggering and dimming the light

It dims the light at certain phases of the waveform so if the LFO is hooked up to a Lamp then it could dim the lamp at certain phases by a SCR phase control circuit?

Markphaser, you still are greatly misunderstanding phase shift, and I sense that you don't fully understand the nature of the AC waveform, whether power, signal, or digital switching.  If you understood it, you would not ask the questions that you ask. 

For guitar signals:  You cannot hear phase shift.  If you use an all-pass filter to mess up the phase relationship of different frequencies in you guitar signal, you won't hear the difference.  If you modulate this with an LFO you wouldn't hear it.  You don't hear any sign of phase shifting until you mix the input signal with the output and the out of phase components cancel, thus creating a notch filter.  Then when you modulate that, you hear a notch filter moving around.  You still don't hear the "phase shift".

For SCR's and phase:  An AC voltage ranges from -170Volts to +170 Volts  (this is 120Volts RMS= 170/sqrt(2)) ).

Suppose it starts at 0V.  It climbs as time passes up past 10, then 20....50, 70...165, 170...then down again 165, 70 40 25 14 10...0, -5, -16, -65, -140, .....-170...then back to 0 in the same manner.  It does that 60 times a second.  This is one cycle.  Phase refers to what point in the cycle is what voltage, or what part of the cycle, rather, "phase" of the cycle it is at.  If you "phase shift" it forward, it's like letting it start a little later.  If you phase shift it backward, it's like it started the cycle a little earlier in time.

If you switch it on and off real quick when it's near 0 volts, of course the light doesn't turn on!  If you switch it on when it's at 170 volts....then "let there be light".  If you switch it on at 50 volts for a brief pulse, then "let there be dim light".  You just repeat these "turning it on" pulses at the same point in each cycle so you equivalently have a pulse string at 25 volts, or a pulse string of 50 volt pulses, or 80 volt pulses depending on what "phase" of the 60 Hz cycle at which you sample the 60 Hz waveform.  I glanced over some code for one of Microchip's uC's for a light dimming circuit.  They used the 60 Hz power through a resistor directly onto the uC comparator input as a 0 crossing detector to sync the "sampling" times to the power input.  For a light dimmer, you can actually sample twice:  Once when the waveform reaches its 170Volt peak, and once on the -170 Volt peak (for max bright).   This corresponds to sampling at 90 degrees, and 270 degrees.

As you hopefully see, this is a nonlinear process, and would make your guitar sound more like a "noise swash", or an "Ugly Face" or 'PWM' type fuzzy noise making effect--not a phaser.  Modulated, it would be like and ugliness volume swell (now hopefully some of you noise fans are getting ideas).

Have you ever watched the windshield wipers on a school bus?

[polaris26]

Not at baseband - but have you thought about modulating a carrier,  phase shifting a sideband, and then demodulating??  At RF the capacitances wouldn't need to be so large... but still not a practical way to get the job done...

Dave

Is there a way i can change the capacitors in my phase shift network with Varactors? so i can Adjust the Varactors with a Voltage supply changing the Capacitance?

[R.G.]

Not at baseband - but have you thought about modulating a carrier,  phase shifting a sideband, and then demodulating??  At RF the capacitances wouldn't need to be so large... but still not a practical way to get the job done...

Some things are best left buried. That thread is six years old. Let it cool off for another few thousand years first.