Author Topic: Adding a DC offset to a function generator output  (Read 6345 times)

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Adding a DC offset to a function generator output
« on: August 06, 2012, 03:26:25 pm »
Hi

I have a new (to me) HP 8111A function generator which can sweep its output frequency in proportion to an externally supplied 0-10V signal. The 8111A has a floating output. I have a simple 8038-based function generator which can output a triangle wave which could provide the sweep signal. However this FG has a ground referenced output (with waveforms symmetrical about ground) and no offset capability. I have a lab power supply with floating outputs.

Can I connect the power supply in series with the ground referenced FG in a way that would a) not cause anything to burst into flames and b) allow me to hook the output to the 8111A and a scope, while fulfilling the requirements of a)?

Cheers
John
 

Offline robrenz

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Re: Adding a DC offset to a function generator output
« Reply #1 on: August 06, 2012, 04:19:02 pm »
Manual here http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CFAQFjAA&url=http%3A%2F%2Fwww.home.agilent.com%2Fagilent%2Fredirector.jspx%3Faction%3Dref%26cname%3DAGILENT_EDITORIAL%26ckey%3D847585%26lc%3Deng%26cc%3DUS%26nfr%3D-11143.0.00&ei=OOofULjsLcmQ0QHGu4GYCg&usg=AFQjCNF7LbFhMVk_bjL6xh2yjeO_rPAh2g&sig2=jZfWmq_0J5QKgiQLfZ7fzw


See section 4-14 where they show VCO hookup.  You should be able to put your triangle wave FG in series with your floating dc power supply to supply the offset to get the 0.1 to 10V sweep signal you need. This would be feed into the external input of the 8111A.  Check the manual to see if the BNC shell of the input is earth grounded. In theory even if it is the other FG is ground referenced also so it should not be an issue other than possible ground loop issues. Dont forget the 50 ohm feedthrough terminations where needed.

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Re: Adding a DC offset to a function generator output
« Reply #2 on: August 06, 2012, 04:50:08 pm »
Well confession is supposed to be good for the soul. When I had the 8111A in bits I couldn't see, on the basis of a cursory inspection, how the circuit was connected to earth. I allowed this to become a 'fact' in my mind, so when I put it back together I didn't bother checking it - until just now. Hey! Whaddya know! The output BNC is connected to earth. Well, duh.

John
 

Offline robrenz

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Re: Adding a DC offset to a function generator output
« Reply #3 on: August 06, 2012, 05:45:59 pm »
You made me feel usefull ;D

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Re: Adding a DC offset to a function generator output
« Reply #4 on: August 06, 2012, 06:56:43 pm »
And sorry, meant to say; I found that manual when I first got the 8111A. How very kind of Agilent to make it available freely. I went through the 'Performance Tests' section; every parameter that I could measure (I don't have any means of measuring sine wave distortion) was still within spec. There's a fantastic section later on that covers adjusting and calibrating, which filled a fun couple of evenings, seeing if I could get it back nearly as good as it was before I started fiddling with it.

Anyway, at the risk of belabouring the point, this it how I interpret your reply:



Is that what you had in mind?

Cheers
John
« Last Edit: August 06, 2012, 06:58:27 pm by icon »
 

Offline robrenz

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Re: Adding a DC offset to a function generator output
« Reply #5 on: August 06, 2012, 08:36:54 pm »
Yes.  I am not an expert so take my advice with a grain of salt.   
But you need to know the input impedance  of the VCO input. Whatever it is will form a voltage divider with the output impedance of your cheap and nasty + PS. that will affect the actual voltage seen as a VCO input.  If the 8111A is 50 Ohm input impedance and your combo is 1 Ohm output impedance you will only get 9.8V PP into the 8111A as an example.

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Re: Adding a DC offset to a function generator output
« Reply #6 on: August 06, 2012, 09:36:23 pm »
Yes.  I am not an expert so take my advice with a grain of salt.   

Meh. You have Hardinge collets. Good enough for me.

Quote
But you need to know the input impedance  of the VCO input.

I think it has a very high input impedance. In the performance tests, you feed it 0.1V, 1V and 10V and it actually shows those voltages on the display.

I've just looked at the specification and it says 10KOhms

Cheers
John
 

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Re: Adding a DC offset to a function generator output
« Reply #7 on: August 07, 2012, 08:55:43 pm »
Hooray! Success - sweep generator:



John
 

Offline robrenz

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Re: Adding a DC offset to a function generator output
« Reply #8 on: August 07, 2012, 11:24:12 pm »
Nice! :)

Offline T4P

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Re: Adding a DC offset to a function generator output
« Reply #9 on: August 08, 2012, 03:02:09 am »
John, that's really nice
 


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