Phase Shift Sinewave Oscillator Construction

[Peterae]

I spent a bit more time on the osc tonight, i now have a good handle on the function and have added pots.

It is now adjustable between 17kHz to 70 kHz,so i have a Frequency adjust pot, threshold pot which sets the THD/Oscillation point and an Amplitude pot.

I am happy with the circuit and just need to wait for all my precision bits to arrive, i will solder the bits to the bottom of the 3 pots and use 18 turn pots glued together, then i will epoxy them up and seal in alu foil with a supply, gnd and output pin.

I can now move on and work on the 0-360 degree phase control circuit, i want 2 of these, i will feed the sine into 2 shifters so i can then control each Channel +/-180 degrees.

[Peterae]

I build one stage of the jfet phase shifter which should give me a 0-180 variable shift.

I could not get it operating in multisim and it does not work on breadboard either right now

The circuit uses 2n3070 but these are impossible to obtain so i ended up getting some 2n5457 jfets.

On breadboard i either get 180 phase shift or 0 phase shift but distorted waveform, it suddenly flips phase at a certain point on the pot.

I am guessing the bias point must be different using the 2n5457 devices.

Looks like i need to read up and biasing or maybe try a pot in the source and see if i can find a spot where it works.

Data sheet 2n5457
http://www.futurlec.com/Transistors/2N5457.shtml

attached data sheet for 2n3070

[muDped]

Yes, biasing is probably causing your
unusual mode of operation.  A variable
10 MegOhm resister in the upper arm
of the biasing circuit may do the trick.

Each stage of the two stage phase shifter
operates as a phase splitter.  The signals
which appear at Source and Drain should
be equal in amplitude and opposite polarity
without any distortion.

The phase shift adjust variable resistance
portion (1.0 MegOhms) of the combiner
couples more or less of the Source signal
to be combined with the Drain signal at the
Gate of the stage following.  Zero resistance
equals 0 degrees and maximum resistance
equals 180 degrees for each of the two stages.

You'll get it going with just a little tinkering!

Be sure that the input signal (for test purposes)
doesn't overdrive the JFETs into distortion.

The type of JFET used shouldn't be critical.

[Peterae]

Thanks for the explanation Dumped.
Very helpfull 

[Peterae]

OK the reason the simulators were not working on the phase shifter was the 10nF was too large for my 35kHz frequency, 2n2 seems to work well but it would appear that a 1 meg pot is not needed more likely 50K should do it on the simulation anyway.

Just been trying to work out the coupling capacitor value between my oscillator stage and the input of the shifter from what i see i use this formulae

C = 1/(2 * pi * F * R)

If i use 35kHz as my frequency and 1meg as the input impedance of my shifter i get 4.54pF seems very low to me.

[Peterae]

Got the phase shifter working well with 2n2 caps instead of 10nf, a 10K pot is good, as it turns out my bias is only slightly off, there is a slight amplitude difference between the sine on the source and drain so therefore when i vary i get a slight variation in amplitude, so easily fixed, thanks again Dumped your explanation of how the circuit works meant i had it working in a couple of minuets 

[Peterae]

I have now refined my plan and end target design.

the whole idea of this build is to generate a sine variable in frequency, split the output into 2, then feed 2 x 0-360 degree phase shifters, so i will have 2 identical phase adjustable sinewaves, i will add a switch which can switch in a schmitt trigger circuit if needed, the output of the 2 sines/pulses will then each feed a transistor amp stage which both drive LT700 transformers, these are impedance matching transformers, these have a primary impedance of 1.2KOhm and a secondary center tapped 3.2Ohm, i am not currently sure what frequency i will be able to drive these up to but cannot find anything available that will operate at higher frequency's.

The idea being that i will be able to drive 2 transformers out of phase using either sine or pulsed to see if there are any signs of interesting effects to study, this is a sort of analogue version of my digital controller.

So next step is to get an adjustable Schmitt Trigger working, i will place a pot in place of the 1K resistor this will allow me to adjust the hysteresis and therefore the pulse width, combined with the already available 100k for setting the trigger threshold voltage.

[Peterae]

The Schmitt seems to simulate pretty well.

Not yet sure about the final amp stage, i have found a diagram as follows, maybe over complicated for what i need, ultimately i would like to be able to drive bifilar coils instead of a speaker, i would think that as long as my bifilar impedance at the driven frequency is about 3 Ohms then i should be OK.

I have seen single transistor LT700 driver stages, are the benefits of using the below diagram going to justify the complexity of the output stage.?
Does anyone have any idea if this will work at 35kHz? I will probably need to build it and see i guess.
http://www.hoofbags.me.uk/geekiestuff.html

[Peterae]

I built the amp in the last post seems to work, i used a 3ohm resistor as a load but have a problem i get some sort of oscillation on the output stage.

The scope shot green is across the primary of the first transformer or LT44
the yellow trace is across the secondary of the final stage transformer with a 3.4 ohm load across.

So does any one have any idea what may be causing this oscillation.

[Peterae]

The other boards are coming on as well, still waiting for my caps from hong kong for the sine board.

I have built and tested both phase delay boards now they work pretty well, i still have a slight amplitude variation between 0 and 360 degree shift and did not get to the bottom of this, but i think this should not be much of a problem.

So the boards picture we have sine osc left, 2 phase boards in middle and amp on right.

A vero layout of the phase delay board
A vero layout of sine osc although i have not tested this yet

[poynt99]

So does any one have any idea what may be causing this oscillation.

That's the busker amp?

You're most likely getting a 180º phase shift at the oscillation frequency. The common bandaid for that is a series RC "snubber" network on the output to ground, usually a 1 Ohm and 0.1u or 10 Ohm and 0.1u or so. Do some research and experimentation with the values.

You can check this page for more details:
http://sound.westhost.com/amp_design.htm#op-stability

[Peterae]

That's the busker amp?

That's right

Cheers Darren i will have a read in the morning.
Drat i didn't want to add components on the low impedance coil because i want to drive bifilar coils without added components, although maybe they wont affect it.

[muDped]

...

So does any one have any idea what may be causing this oscillation.

Your scope shots may provide the clues:

The input signal to the Push-Pull Output Stage
seems to have some sort of parasitic oscillation
riding on it.

What's showing up in the output may simply
be this parasitic amplified.

Something to look at is the input power from
your supply - does this "oscillation" also appear
on your DC input as "ripple?"  If so, you may be
experiencing the old "motorboating" effect.

Ripple on the DC supply can feed back into the
prior stages to produce spurious oscillations.

Try loading the circuit lightly to see if that makes
any difference.

[Peterae]

Thanks Dumped
I will look at that also, it maybe this scope, the Hantek DSO has always suffered a noisy trace on mv settings even when shorted.
It does seem strange the circuit does not use a zobel network, and must assume it works without one, although i will also look at this also.

[poynt99]

Many amps have the network, and this one especially should also. It's a cobbled together design, not of the utmost fidelity, and stability is probably hit and miss.

[Peterae]

Do you know of any instances of stabilizing it on the primary side of the output transformer, surely if i filter on the primary it will have the same effect, i am really trying to avoid extra components on the secondary that could interfer with the coils i am connecting to it.

[poynt99]

Do you know of any instances of stabilizing it on the primary side of the output transformer, surely if i filter on the primary it will have the same effect, i am really trying to avoid extra components on the secondary that could interfer with the coils i am connecting to it.

Sorry Peter, I thought that was implied. Yes, the Zobel network would go on the output of T3/T4, in parallel with the 2.2-4.7 Ohm resistor. Changing the value of that resistor may have an effect on the oscillation as well, but you're safer if you include the Zobel network.

[Peterae]

Oh that's great news, sorry my confusion all i kept seeing was resistor capacitor from the speaker  
That's cheered me up a bit.

This amp is rough, i understand that but i wanted something cheap to build for a trial, i will need to switch to something more robust at some point.

[Peterae]

I lost Patience today waiting for my caps Grrr and used caps i have in stock tonight to get my sine oscillator working, it works well, I ended up placing a 680Ohm resistor in series with the 1k threshold pot, this gives me an effective frequency range of 25kHz to 65kHz, i may extend this to just over 70kHz but am not too worried right now and added 4n7 caps for coupling each phase delay to the sine source.

I decided to leave the amp stage for a few days and get the osc system up and running, this way i can use my final circuit for testing the amp.

I am tempted to build an agc control to keep the amplitude of the various waveforms the same, otherwise i will may end up with problems matching the 2 separate waveforms.
I will be able to switch between sine or square as well and each circuit mismatches the amplitude, i also have the problem that the phase shift circuits slightly alter the sine amplitude as the phase shifts, if i do add a transistor agc circuit i can do away with the amp volume pot as well.
I will need to do tests to see how well this will work with sine and squarewave inputs and how well it controls the amplitude of each input signal.

[Peterae]

Thanks for your help chaps, i wired the whole lot up in a box and now when i drive the amp from my sine osc and phase shifter i do not get the nasty oscillations.  

May be my old signal generator picking up noise or something.

[Peterae]

Well after many hours i have failed to get the above circuits working as i want them, i've learnt a lot and have been lead in the direction that i should have followed in the first place.
I have been following Spherics approach to delaying one pulse before another pulse i realize i should be following the approach that SM took

I originally got the idea from electron circuits which use vacuum rectifiers like the 5U4 GB or 5AR4 etc.
If the plate voltage is not rectified then it is AC with a potential 60 Hz frequency.
That combines with the 5 volt 60 Hz in the coil of the htr transformer and generally amounts to nothing. In fact the power of the 5 volt transformer amounts to nothing.
It is an insignificant power supply except when the two transformers get slightly out of phase with each other, or when they are connected in reverse of one another.
Then you can measure all kinds of things going on.
You can generate all kinds of hash and multiple frequencies, and I do mean all kinds.

The next build will be to drive 2 transformers out of phase using the phase shifter, but using AC mains as the sine source, i will drive a 450-0-450 transformer from the AC mains supply and drive a 5V 3Amp transformer using the mains phase shifted frequency.

So my first and only problem with this build is to develop a Class A power amp that will drive a winding to give me 5V AC @ 3Amps.

Anyone know what the chances of success would be using a fet as a class A to drive a transformer winding, as opposed to using a transistor class A?
Are there any benefits for using a Fet instead of a Transistor.

I have chosen to drive the filament transformer using a class A because i do not want any chances of switching spikes in the waveform.

I am also hoping to upconvert the 50Hz mains to the 3rd harmonic and be able to switch this in to drive the filament phase controlled winding.

[Peterae]

After a bit of thought about the Class A amp i wonder if anyone has ever used a variable voltage regulator, LM138 is a 5Amp device and varies between 1.2V and 32V if i drive the adjust pin using my sine wave i maybe able to drive a transformer winding.

This thread is now continued here for further testing of SM's heater valve phase tests.
http://www.overunityresearch.com/index.php?topic=1498.msg22804#msg22804