Wien Oscillator - Calc of RC values - UK Vintage Radio Repair and Restoration Discussion Forum

Bazz4CQJ · 5th Apr 2020, 10:47 pm

I wonder if someone could advise me about the RC components in a Wien oscillator?

I know that F=1/(2pi RC), but for any given frequency, any number of values of R & C would appear to be viable, but I have the feeling there has to be some “matching” between R and C and that only a limited range of values would give optimum results, e.g, the capacitor reactance at F needs to be comparable to the value of R? I cannot find anything on Goggle.

I have a small Wien module, currently fixed at 1kHz (10nF and 15k) and I’d like to make it variable, lets say 50Hz to 16kHz in two or three ranges.

Thanks

B

orbanp1 · 6th Apr 2020, 12:35 am

Hi Bazz,

It mostly depends on practical considerations.
You need to pick the variable component, the cap or the resistor, and those are dual controls.
You usually get a 10:1 spread in one range.
For variable caps you do not go beyond 1nF for mechanical reasons, like the quad variable caps of the old HP-200 series generators.
For home built ones it is more like a 2 x 500pF variable cap.
Accurate dual variable pots are expensive. The ones I have seen used in lab instruments are something like 10k wire-wound pots, 8" in dia or so, custom made.
Also, the input impedance of the amplifier part of the Wien-bridge oscillator will have an effect on the accuracy of the Wien components.
The stray capacitances, input capacitance, and input resistance of the amp should have minimal influence on the Wien components themselves. This will give constraints on the Wien component value selection.
It is also important that the amp would not have any appreciable phase shift at the operating frequencies of the generator.
From this comes the calculation of the resonant frequency: at "resonance" the phase shift of the frequency dependent part of the Wien bridge is zero.
That is when R1 * R2 = 1 / (w^2 * C1 * C2), where "w^2" is omega squared. Just do the math.

On the other hand, there is no need to reinvent the wheel, there are lots of well working Wien bridge oscillator circuits. You should study those.

I am sure others will also chime in with valuable comments.

Regards, Peter

Bazz4CQJ · 6th Apr 2020, 1:03 am

I found these values for one design;

For R =2k-100k (twin gang 100k pot with 2k fixed resistor in line)

C/nF...............F
100........... 30-300Hz
10............. 0.3-3kHz
1.............. 3-30kHz

As the resistance spans the same range for all values of C, it seems as though the C-R connection does not exist.

I had imagined that each range needed a different range for R for optimum performance.

B

Diabolical Artificer · 6th Apr 2020, 6:05 am

This might help - https://sound-au.com/project22.htm however the C component is less precise than the R ( 1% or better R's are easier and cheaper to source) . For my spot frequency Wein bridge I matched two caps on a LCR bridge then selected 1% resistors but really as far as F is concerned it doesn't matter if you test at 999hz or 1001hz, for most things that is.

Not sure how much THD an unbalanced leg contributes though, not much I would have thought. If you need a suitable lamp I have several spare, drop us a PM if interested, Andy.

turretslug · 6th Apr 2020, 12:27 pm

Quote:

Originally Posted by Bazz4CQJ

I wonder if someone could advise me about the RC components in a Wien oscillator?

I know that F=1/(2pi RC), but for any given frequency, any number of values of R & C would appear to be viable, but I have the feeling.... ....that only a limited range of values would give optimum results

You can certainly run into trouble if you try to cover an overly-ambitious span of ranges based around one gang capacitor or potentiometer- with variable resistors, at the HF end of the range span capacitors could end up needing to be just tens of pF, giving a headache of strays and matching/tracking, not to mention amplifier input and output impedances. At extreme LF ranges, capacitors end up needing to be hundreds of nF or even uF (excuse symbol...), large and expensive in close tolerance, stable non-polarised types. With a variable gang capacitor, necessarily restricted to hundreds of pF for practical purposes as outlined above, LF coverage can mean resistors of tens of megohms- the tolerance and stability problems crop up again as well as amplifier impedances. Unduly low resistors for HF coverage make the oscillation amplifier work hard.

There is a degree of choosing "sweet spot" for values, but not too onerous, just don't try and make something that does 0.1Hz to 10MHz around a single ganged variable by switched ranges!

Bazz4CQJ · 6th Apr 2020, 8:10 pm

Quote:

Originally Posted by Diabolical Artificer

This might help - https://sound-au.com/project22.htm however the C component is less precise than the R ( 1% or better R's are easier and cheaper to source) . For my spot frequency Wein bridge I matched two caps on a LCR bridge then selected 1% resistors.

Not sure how much THD an unbalanced leg contributes though, not much I would have thought. If you need a suitable lamp I have several spare, drop us a PM if interested, Andy.

Thanks for that offer, the unit is built and working, but fixed at 1kHz. It was actual a kit bought online for about ~£3-50

, including all the components, lamp, PCB and P&P from China, which I though was quite good (but I don't think it is still available). I don't need a high spec unit, just something that is "close" to a sine wave, and as judged by eye, it does that.

Looking at the ranges quoted in my last post, and doing some calculations with the components used, it's clear that the ranges do not correspond to the full resistance swing; it looks as if it drops off at the extremes. I have a Wayne Kerr LCR bridge, so I can do some component matching. I've had to send off for a dual gang pot, so now waiting for that to arrive.

B

Skywave · 7th Apr 2020, 1:55 pm

A good write-up of the theory of the Wien Bridge Oscillator is here:
https://www.electronics-tutorials.ws...en_bridge.html

Many years ago I built a variable freq. A.F. osc. based on the Wien Bridge. By using a dual-gang reverse-law log. pot., the resulting freq. scale was more linear than that obtainable with a dual-gang linear-law pot.

ISTR that the two Cs and the two Rs do not necessarily need to be equal. If they're not, then fosc = 1/2*pi*√(R1*R2*C1*C2), which reduces to the usual eqn. if R1 = R2 and C1 = C2.

Al.

5th Apr 2020, 10:47 pm	#1
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Wien Oscillator - Calc of RC values I wonder if someone could advise me about the RC components in a Wien oscillator? I know that F=1/(2pi RC), but for any given frequency, any number of values of R & C would appear to be viable, but I have the feeling there has to be some “matching” between R and C and that only a limited range of values would give optimum results, e.g, the capacitor reactance at F needs to be comparable to the value of R? I cannot find anything on Goggle. I have a small Wien module, currently fixed at 1kHz (10nF and 15k) and I’d like to make it variable, lets say 50Hz to 16kHz in two or three ranges. Thanks B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

6th Apr 2020, 12:35 am	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Wien Oscillator - Calc of RC values Hi Bazz, It mostly depends on practical considerations. You need to pick the variable component, the cap or the resistor, and those are dual controls. You usually get a 10:1 spread in one range. For variable caps you do not go beyond 1nF for mechanical reasons, like the quad variable caps of the old HP-200 series generators. For home built ones it is more like a 2 x 500pF variable cap. Accurate dual variable pots are expensive. The ones I have seen used in lab instruments are something like 10k wire-wound pots, 8" in dia or so, custom made. Also, the input impedance of the amplifier part of the Wien-bridge oscillator will have an effect on the accuracy of the Wien components. The stray capacitances, input capacitance, and input resistance of the amp should have minimal influence on the Wien components themselves. This will give constraints on the Wien component value selection. It is also important that the amp would not have any appreciable phase shift at the operating frequencies of the generator. From this comes the calculation of the resonant frequency: at "resonance" the phase shift of the frequency dependent part of the Wien bridge is zero. That is when R1 * R2 = 1 / (w^2 * C1 * C2), where "w^2" is omega squared. Just do the math. On the other hand, there is no need to reinvent the wheel, there are lots of well working Wien bridge oscillator circuits. You should study those. I am sure others will also chime in with valuable comments. Regards, Peter Last edited by orbanp1; 6th Apr 2020 at 12:52 am.

6th Apr 2020, 1:03 am	#3
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Re: Wien Oscillator - Calc of RC values I found these values for one design; For R =2k-100k (twin gang 100k pot with 2k fixed resistor in line) C/nF...............F 100........... 30-300Hz 10............. 0.3-3kHz 1.............. 3-30kHz As the resistance spans the same range for all values of C, it seems as though the C-R connection does not exist. I had imagined that each range needed a different range for R for optimum performance. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

6th Apr 2020, 6:05 am	#4
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,670	Re: Wien Oscillator - Calc of RC values This might help - https://sound-au.com/project22.htm however the C component is less precise than the R ( 1% or better R's are easier and cheaper to source) . For my spot frequency Wein bridge I matched two caps on a LCR bridge then selected 1% resistors but really as far as F is concerned it doesn't matter if you test at 999hz or 1001hz, for most things that is. Not sure how much THD an unbalanced leg contributes though, not much I would have thought. If you need a suitable lamp I have several spare, drop us a PM if interested, Andy. __________________ Curiosity hasn't killed this cat...so far.

7th Apr 2020, 1:55 pm	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Wien Oscillator - Calc of RC values A good write-up of the theory of the Wien Bridge Oscillator is here: https://www.electronics-tutorials.ws...en_bridge.html Many years ago I built a variable freq. A.F. osc. based on the Wien Bridge. By using a dual-gang reverse-law log. pot., the resulting freq. scale was more linear than that obtainable with a dual-gang linear-law pot. ISTR that the two Cs and the two Rs do not necessarily need to be equal. If they're not, then fosc = 1/2pi√(R1R2C1*C2), which reduces to the usual eqn. if R1 = R2 and C1 = C2. Al.