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Old 22nd April 2008, 03:43 PM   #1
Bengali is offline Bengali  United States
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Default Signal Generator Help

Hi,

I'm building this circuit:

http://www.redcircuits.com/Page82.htm

What do you suggest to use on the output of this circuit?

I would like to be able to attenuate the level and perhaps even double the output level. How to do this without affecting the low distortion performance?

Looks like it was designed to put out a fixed 1VRMS signal, according to the notes: The 10K trimmer must be set to obtain a 1V RMS output.


Thanks.
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Old 23rd April 2008, 12:05 PM   #2
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Looks to me like the circuit should be able to put out a good deal more than 1VRMS. Suggest using a 10 turn pot for the output - that way you can get a lot of adjustability. Have NO idea what the third op amp is being used for or if it's necessary.

Charles
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Old 23rd April 2008, 12:23 PM   #3
AndrewT is offline AndrewT  Scotland
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D5 stabilises the output voltage by feeding back into R9.

If the TL084 were replaced with a higher voltage rating opamp then you could run this circuit at the voltage supply limits of the 5532/5534 of around +-21Vdc.
I suspect you could get around 2Vac from this and retain lowish distortion. You may be able to get much higher.

How much will distortion increase if the variable resistors do not track particularly well?
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Old 23rd April 2008, 12:47 PM   #4
Mooly is online now Mooly  United Kingdom
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You have an unused Op-Amp in IC3. You could buffer/amplify the output with this. Do you have instructions on how to couple the LED to the LDR, try and use similar parts to the original here, this is an old (but good) design.
Karl
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Old 23rd April 2008, 04:10 PM   #5
Bengali is offline Bengali  United States
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Thanks for your replies. After posting, I realized there is a spare op amp as you mentioned.

What do you think regarding the attached circuit?
Gain of 2 should suffice.

cap couple to be safe. if i use 220uF and the output pot is 10K,
cutoff frequency should be around 14hz?

the notes mention to seal the led and ldr in light free enclosure. I was going to face them together and use heat shrink tubing.
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Old 23rd April 2008, 04:50 PM   #6
Mooly is online now Mooly  United Kingdom
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Hi, Not exactly, but nearly what I had in mind, you dont need the pot at the output of the buffer. You can set the level with the original pot. Taking the output from the wiper gives an ill defined output impedance. If your feeding a capacitive load the output from the wiper will fall with frequency. If it were me I would add a 600ohm (330 + 270) in series with the output to give a low standardised output impedance, and this will give some protection to the Op Amp if you inadvertantly connect the output to a voltage source. Again if it were up to me I would not A.C. couple.
Regards Karl
Edit, Untill you know the output swing from the oscillator you can't really set your feedback resistor, alternatively when it's all working you could replace the feedback resistor with a switched attenuator to give known values such as -20db and -40 db, could be very useful.
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Old 23rd April 2008, 05:44 PM   #7
Bengali is offline Bengali  United States
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Hi Mooly,

Thanks. I will take out the cap and 10K pot. I didn't like the frequency falling either.

I wonder why the note says you MUST adjust the 10K pot for 1VRMS.

I'll add the 600ohms in series with the wiper of the 2.2K pot.
Looks like the 2.2K pot is to adjust the frequency.

you switched attenuator on the feedback is a great idea. Once I build up the circuit I tweak the output.
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Old 23rd April 2008, 05:49 PM   #8
Mooly is online now Mooly  United Kingdom
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R7 the PRESET pot, that will be the one to adjust for 1 vrms output, presumably measured on pin 7 of IC2B. You have got your pin numbers wrong on your buffer. 600 ohm to output of this buffer NO pot.
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Old 23rd April 2008, 06:02 PM   #9
Mooly is online now Mooly  United Kingdom
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Like this.
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Old 23rd April 2008, 06:45 PM   #10
Bengali is offline Bengali  United States
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oops!

thanks for clarifying.
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