Help With Sine Wave Wien Bridge RC Oscillator with output Frequency 1N~2N - Page 1
Forum section: electronics / beginners

author: muhamadamru
author_rank: Contributor
timestamp: October 11, 2015, 01:02:24 pm
content: 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-FET4) The output amplification will must be (0~((N+10)/4)) .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 varythe 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 ?

author: LvW
author_rank: Frequent Contributor
timestamp: October 11, 2015, 01:36:47 pm
content: Quote from: muhamadamru on October 11, 2015, 01:02:24 pmWhich goal? Just tuning the oscillation frequency?There are double-pot`s and double variable caps.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2015, 02:12:35 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 11, 2015, 04:40:06 pm
content: Digital pots are also an option, if frequencies are not too high.

author: muhamadamru
author_rank: Contributor
timestamp: October 11, 2015, 07:04:19 pm
content: Quote from: Kleinstein on October 11, 2015, 04:40:06 pmThe 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

author: muhamadamru
author_rank: Contributor
timestamp: October 11, 2015, 07:05:28 pm
content: Quote from: TimFox on October 11, 2015, 02:12:35 pmIs there a Double POts SMD ?

author: muhamadamru
author_rank: Contributor
timestamp: October 11, 2015, 07:08:50 pm
content: Quote from: TimFox on October 11, 2015, 02:12:35 pmI Don't know that those Decade Switches are ?!!?What i know is :The frequncy is 1/2pi R CR : is formed from 2 resistor , if i changed them simultaneously my transfer function will remain 1/3 but the freq will be changedSo 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 melike between 3Khz and 6Khz which i don't know yet how to do it

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2015, 07:13:52 pm
content: 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.

author: muhamadamru
author_rank: Contributor
timestamp: October 11, 2015, 07:37:31 pm
content: Quote from: Hero999 on October 11, 2015, 07:13:52 pmI am student and also noob if there is a better alternatives please tell mehow to make with a stable amplitude ?

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2015, 08:16:20 pm
content: Please read:http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdfhttps://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdfAlso a good read:http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdfThere 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.

author: LvW
author_rank: Frequent Contributor
timestamp: October 12, 2015, 06:32:53 am
content: Quote from: muhamadamru on October 11, 2015, 07:37:31 pmAre you required to use the WIEN type oscillator?There are other types of oscillators offering single element tuning.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2015, 03:51:58 pm
content: Quote from: muhamadamru on October 11, 2015, 07:08:50 pmA 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2015, 06:29:44 pm
content: Digital pots are usually made to be controlled from a µC 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.