Help me understand the Micro Vibe

[antonis]

Yes, the former..
No, the later..
(guess why.. no hint)

[jlo]

Yes, the former..
No, the later..
(guess why.. no hint)

Does it have to do with the diodes?
I don't understand why its a dual pot in the first place...

[antonis]

For exactly the reason you can't consider both gangs in parallel..

We deal with an LOW frequency oscillator.. This means Caps DO exhibit resistance (or else, RC time constants should be zero and also Diode pair should be shorted..)

Diodes, as they are, don't allow C20 cap plates voltage to be raised over 600mV, either on charging or discharging mode..
So, LFO can't swing Q12 Emitter higher or lower than this..

[jlo]

For exactly the reason you can't consider both gangs in parallel..

We deal with an LOW frequency oscillator.. This means Caps DO exhibit resistance (or else, RC time constants should be zero and also Diode pair should be shorted..)

Diodes, as they are, don't allow C20 cap plates voltage to be raised over 600mV, either on charging or discharging mode..
So, LFO can't swing Q12 Emitter higher or lower than this..

I see.  So the diodes are oriented opposite each other to affect charging AND discharging

[jlo]

Revisting R.G.'s tech article. 
"The two diodes across the center capacitor limit the size of the LFO output waveform. An oddity of this particular way of building a Univibe is that the LFO amplitude goes up with increasing speed, even with the diodes there."

So is it a dual pot because each pot controls each side of the waveform?  Each with its own upper and lower time constraints?

[antonis]

I've noticed when there isn't an answer on time (clearly specified by yourself) you retrace well established texts..
(making me to set a delay of 48 hours, at least, for my answers..) 

[jlo]

I've noticed when there isn't an answer on time (clearly specified by yourself) you retrace well established texts..
(making me to set a delay of 48 hours, at least, for my answers..) 

Apologies.  I never expect an answer and I do truly appreciate anytime someone takes the time out of their busy life to help.   Its probably my learning style that doesn't translate well in the format of posts.   A lot of times I'm thinking "out loud".   

[PRR]

I have never seen quite that oscillator anywhere else. (Maybe one of the gurus here has.)

An oscillator is generally an amplifier and a time-network. A Sine oscillator is a little tougher.

This one is notable that the amplifier is unity voltage gain. (High current gain though.) Compare with a Wien which needs voltage gain of 3, or a phase-shift which generally needs voltage gain of 27(?).

A sine oscillator generally wants at least two reactances. This can be an L and a C. Coils are brutal in audio so we try with 2 or 3 Cs and some Rs. Wien and phase-shift are popular. The string of 2 or 3 identical caps and two ganged variable resistors is obviously setting the frequency here, but it is an odd duck. There are much odder ducks. Life is too short.

These oscillators normally start on random noise. (Performance LFOs may add a kick-start.) If the amplifier gain exceeds the reactance-network loss the output WILL build up "to infinity". Before infinity the amplifier clips, which makes the *average* gain somewhat less, and limits the final amplitude. Either by clipping or by de-biasing the amplifier to a lower-gain condition.

Rather than clip to the supply rails we can clip with diodes, which may be less traumatic to the amplifier.

And there is the Becham/Hewlett lamp (a slow limiter) and much fancier limiters. These can give point-oh THD but LFOs usually do not have to be so sweet.

[jlo]

Ok I think Im understanding the LFO a bit better.  So does the Darlington create a 180 phase shift and then each RC creates 90 + 90 so that the output is in phase with the input with a full 360 shift.  When its fed back it creates/maintains the oscillation.  If we want to change the frequency we use the dual pot so that the 90 degree shifts are equal?

[antonis]

Emitter followers (Darlington or not) are not famous enough for phase shifting..
(maybe that's the reason for also called "Voltage followers"..)

[jlo]

Emitter followers (Darlington or not) are not famous enough for phase shifting..
(maybe that's the reason for also called "Voltage followers"..)

I see.  That 180 shift is with a common emitter .   So then the output of the Darlington is in phase with the input but gets shifted 180+180? 

[antonis]

So then the output of the Darlington is in phase with the input but gets shifted 180+180?

Philosophical brainstorm: Can you point any reason for shifting 360^o the output..??

[jlo]

So then the output of the Darlington is in phase with the input but gets shifted 180+180?

Philosophical brainstorm: Can you point any reason for shifting 360^o the output..??

I was just inferring based on RC phase shift oscillator.  But that uses a common emitter transistor...

[PRR]

We do not need phase shift. A "follower" with gain of +1.1, connect out to in, it will scream.

The oddity is that this amplifier has gain like 0.98. They found voltage gain in a passive R-C network. I've modeled it and yes, there's a sliver of voltage gain (and larger impedance dip) at one frequency.

[jlo]

We do not need phase shift. A "follower" with gain of +1.1, connect out to in, it will scream.

The oddity is that this amplifier has gain like 0.98. They found voltage gain in a passive R-C network. I've modeled it and yes, there's a sliver of voltage gain (and larger impedance dip) at one frequency.

Can you please explain the RC network and the dual pot?   And how do you get the additional gain?

[jatalahd]

Can you please explain the RC network and the dual pot?   And how do you get the additional gain?

Yes.

As a pre-requisite you need to understand the RC phase-shift oscillator in its more common form (feedback from collector to RC stage). This infomation is available in the internet (just search by "RC phase-shift oscillator"). The following information presented here is top secret and will be buried in this thread and never found again

There are four different feedback topologies in amplifier design: series-shunt, shunt-series, shunt-shunt, series-series (use internet search if you want to know more). The "normal" RC phase-shift oscillator uses the shunt-shunt feedback model. The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:



In this image, we have the signal source added and the feedback voltage Vf is summed as "inverted" to the input. Please ignore the RC/RL, since this model can be applied to the common-emitter amp as well but not now. Hopefully this is clear so far. In the oscillator, the signal source can be removed, since the input signal is taken 100% as a feedback signal from the output.

Next we draw the Uni-Vibe RC network on top of the same feedback diagram (NOTE: redrawing circuits in standard form is a huge help to understand them)



Now it is starting to look as the "normal" RC phase-shift oscillator. So we have 180 degree shift already at the input side of RE and another 180 shift from the RC network. This sums up either as 180 - 180 = 0 OR 180 + 180 = 360 = 0 at the frequency of oscillation. When there is positive feedback (0 degree shift) the thing will oscillate. But wait...

There is also a theory called the "Barkhausen stability criterion". It says that for oscillation to happen, the LOOP GAIN of the circuit must be equal or larger than -1. There is so much false information floating around saying that the gain of the circuit must be so-and-so much for oscillations to happen (to make up the loss of the RC feedback network). This is not so. The basic emitter follower circuit can easily have a LOOP GAIN of over 100 (although the voltage gain is ALWAYS less than 1), and there is no magic related to this. The loop gain is calculated differently than "normal" gain. So in this case the Barkhousen stability criterion is fullfilled and also overly ensured by the darlington's high current gain (these days a single high-Beta tranny like BC549C would be enough here).

By adjusting the resistances, you adjust the cut-off frequencies of the RC-pairs, thereby affecting the oscillation frequency. Not any different than in the normal RC phase-shift oscillator.

And if your next question is how to calculate the loop gain for this specific circuit, then I am sorry to say but you are not ready for it. I did it using numerical math tools and utilizing matrix algebra. There is no simple equation to give.

[antonis]

There is no simple equation to give.

Additionally to the above, there isn't a compact formula for BOTH necessary & sufficient oscillation criterion..
(Barkhausen's criterion is necessary  but not sufficient condition where Nyquist one is the opposite - in the mean of instability..)
Logical enough, taking into account that both mathematical conditions developed aiming for ensuring stability criterions..

To make long story short, you can never be sure for a working oscillator without breadboarding it..

@jatalahd: I presume hybrid-π model analysis of CC amp confuses rather than enlighten OP..
(with no intention for knowledge underrating, jlo..)

[jlo]

There is no simple equation to give.

Additionally to the above, there isn't a compact formula for BOTH necessary & sufficient oscillation criterion..
(Barkhausen's criterion is necessary  but not sufficient condition where Nyquist one is the opposite - in the mean of instability..)
Logical enough, taking into account that both mathematical conditions developed aiming for ensuring stability criterions..

To make long story short, you can never be sure for a working oscillator without breadboarding it..

@jatalahd: I presume hybrid-π model analysis of CC amp confuses rather than enlighten OP..
(with no intention for knowledge underrating, jlo..)

Im confused most of the time!

[jlo]

Emitter followers (Darlington or not) are not famous enough for phase shifting..
(maybe that's the reason for also called "Voltage followers"..)

But the feedback is 180 shifted?

[antonis]

Emitter followers (Darlington or not) are not famous enough for phase shifting..
(maybe that's the reason for also called "Voltage followers"..)

But the feedback is 180 shifted?

Should we remane topic "Endlessness is not enough"..??

How can come 180^o phase shift between 2 IN PHASE signals without anything to lead/lag them..??