Buffer for the AWG

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Ian P
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Buffer for the AWG

Post by Ian P » Fri Dec 13, 2019 4:12 pm

I have a 2206B and for some applications the 600R output impedance of the AWG can be a bit limiting.
I understand that you may have made the design decision in order to limit the current drawn from the host computer USB port. I would have probably done the same.

So I am making myself a buffer amplifier to drive lower impedances, powered off a second USB port on the host.

I did a frequency response plot of AWG voltage output as part of my investigations and found that the output decreases at high frequencies. It starts to reduce at about 500kHz and reaches 1.88V at 1MHz.
Sinewave output from AWG, set to 2V amplitude. Measured with a very short wire between AWG output and CH1 input.
Same reduction is present using the x10 probe supplied, so it isnt some sort of termination problem.

Is this usual? I can certainly work with this small limitation now that I know about it, but I would like confirmation that it is within specification and why it is there.

Best regards
Ian

Gerry
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Re: Buffer for the AWG

Post by Gerry » Mon Dec 16, 2019 10:49 am

Hi Ian,

Yes this is usual. It is the natural roll-off of the circuitry, and is why you see frequency specifications for analogue Channel inputs quoted as 0-xHz (-3dB) (sometimes they are quoted without the '-3dB' qualification, in which case it is implied). Most simple analogue circuitry (creating a 1st order low pass filter response) starts to roll-off gently well before the -3dB point, which is why you see, for instance, a square wave start to lose its sharp edges as the frequency is increased in the Signal Generator, and the harmonics are reduced as a result.

The function Generator roll-off is typically less than that of the input channel. The reduction of -3dB is equivalent to roughly 70.8%, so 2V to 1.88V is well within spec.

Regards,

Gerry
Gerry
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Ian P
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Re: Buffer for the AWG

Post by Ian P » Wed Dec 18, 2019 3:38 pm

Thanks Gerry.
Reassured that my 2206B is behaving normally. As the bandwidth for the measuring input is quoted as 50MHz I would have expected very little of the roll-off to be contributed by that.
I think the AWG is a great feature. Most of the time I can do all my measurements with just the Picoscope!

Best regards
Ian

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