Author Topic: Help With Sine Wave Wien Bridge RC Oscillator with output Frequency 1N~2N  (Read 2696 times)

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Offline muhamadamru

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Hello :
I have a Subject in the faculty as project we need to do an Sine Wien Bridge RC  Oscillator PCB Boards :
1)Variable Oscillation frequency Between 1N Khz ~ 2N Khz (where N is my number ).
2)Output Impedance 1+N Kohm.
3) The Amplitude of the Oscillation will be automatically controlled with J-FET
4) The output amplification will must be (0~((N+10)/4)) [V].

We are limited by a small 45mmX45 mm PCB. 
the initial alpha schematic is in the attachment:

I just started and i decided to take them Step by Step  :

i decided to Choose LM741 Op-amp. Now to get the oscillation frequency in a range between N~2N i must vary
the R1, R2 resistors simultaneously or to vary the C1, C2 capacitor simultaneously but with the exact amout for both of the to maintain the oscillation condtion F(w) * Av = 1 .

So is there any method to achive this Goal ? 
 

Offline LvW

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So is there any method to achive this Goal ?

Which goal? Just tuning the oscillation frequency?
There are double-pot`s and double variable caps.
 

Offline TimFox

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Another method is decade switches.  Typically, 1% (or better) resistors on two or three frequency switches, with the decades wired in parallel since frequency is proportional to 1/R, with 1% (or better) capacitors (factor of 10 apart) on the range switch.
 

Offline Kleinstein

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Digital pots are also an option, if frequencies are not too high.
 

Offline muhamadamru

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Digital pots are also an option, if frequencies are not too high.
The size of the board is very small and i don't know how to use them , i think Digital means they need to be programmed which i have no experince
 

Offline muhamadamru

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Another method is decade switches.  Typically, 1% (or better) resistors on two or three frequency switches, with the decades wired in parallel since frequency is proportional to 1/R, with 1% (or better) capacitors (factor of 10 apart) on the range switch.
Is there a Double POts SMD ?
 

Offline muhamadamru

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Another method is decade switches.  Typically, 1% (or better) resistors on two or three frequency switches, with the decades wired in parallel since frequency is proportional to 1/R, with 1% (or better) capacitors (factor of 10 apart) on the range switch.
I Don't know that those Decade Switches are ?!!?
What i know is :
The frequncy is 1/2pi R C
R : is formed from 2 resistor , if i changed them simultaneously my transfer function will remain 1/3 but the freq will be changed
So what i need is to change both of the resistors at the same time and maybe between a given range which is a new thing for me
like between 3Khz and 6Khz which i don't know yet how to do it 
 

Offline Zero999

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Why choose the LM741? Virtually no one, except students use it these days. There are much better and cheaper alternatives.

The schematic you've shown, has no amplitude stabilisation.

The frequency could be easily adjusted using a dual ganged potentiometer.
« Last Edit: October 11, 2015, 07:56:59 pm by Hero999 »
 

Offline muhamadamru

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Why choose the LM741? Virtually no one, except students use it these days. There are much better and cheaper alternatives.

The schematic you've shown, has not amplitude stabilisation.

The frequency could be easily adjusted using a dual ganged potentiometer.
I am student and also noob if there is a better alternatives please tell me
how to make with a stable amplitude ?
 

Offline Zero999

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Please read:
http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf
https://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdf

Also a good read:
http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

There are many replacements for the LM741 which are often cheaper. The TL072 is a good general purpose replacement and the NE5532 is good for audio.
 

Offline LvW

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Re: Help With Sine Wave Wien Bridge RC Oscillator with output Frequency 1N~2N
« Reply #10 on: October 12, 2015, 06:32:53 am »
I am student and also noob if there is a better alternatives please tell me
how to make with a stable amplitude ?
Are you required to use the WIEN type oscillator?
There are other types of oscillators offering single element tuning.
 

Offline TimFox

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Re: Help With Sine Wave Wien Bridge RC Oscillator with output Frequency 1N~2N
« Reply #11 on: October 12, 2015, 03:51:58 pm »
Another method is decade switches.  Typically, 1% (or better) resistors on two or three frequency switches, with the decades wired in parallel since frequency is proportional to 1/R, with 1% (or better) capacitors (factor of 10 apart) on the range switch.
I Don't know that those Decade Switches are ?!!?
What i know is :
The frequncy is 1/2pi R C
R : is formed from 2 resistor , if i changed them simultaneously my transfer function will remain 1/3 but the freq will be changed
So what i need is to change both of the resistors at the same time and maybe between a given range which is a new thing for me
like between 3Khz and 6Khz which i don't know yet how to do it
A decade switch is a set of 10-position switches (typically rotary switches) with two isolated switching poles.  In this case, you want to switch the conductance = 1/resistance to get a proportional change in the frequency given by your equation, since the frequency is proportional to 1/R.  I have also seen 1-2-4-8 sequences using slide or toggle switches to get this switchable conductance, but the rotary switches are more convenient.  If you use more poles on the rotary switches, you can get a 1-2-4-4 sequence (e.g., 100k, 50k, 25k, 25k) to span one decade from (1/100k) to (10/100k) with two sections of four poles and popular resistor vaues.
 

Offline Kleinstein

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Re: Help With Sine Wave Wien Bridge RC Oscillator with output Frequency 1N~2N
« Reply #12 on: October 12, 2015, 06:29:44 pm »
Digital pots are usually made to be controlled from a ┬ÁC or similar. There are a few with control via a realtively simple Up/down interface. However they may need debouncing for simple contacts. Usually they require the signal to be withing the supply range of that chip, which often is rather low. So something like a 5 V supply an OP would be a good idea.

SMD dual pot are rare - I have not seen them. There are resonably small TH versions, that don't need much board area - but high.
 


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