Hi Jochen,

Thanks for the kind words :-)

The main problem is not the amount of noise, but it's effect on the ad606.
I had expected Gaussian noise on the input to produce something roughly
like DC on the output, since in the datasheet, the only real difference
that they mention in the effect that Gaussian noise has vs. the effect of a
sinusoid is that it has a different log intercept. they don't mention that
the output would be a lot rougher. Is it the case in general that the
output of the amp with Gaussian noise on the input is very noisy?

I'm thinking that the problem may be that I'm dealing with noise that is
narrowband and is not white. For example, if you have white noise with an
rms amplitude of 2mV, you would expect to see a DC level out corresponding
to about -20dBm. Because the noise has some frequency components below the
low-pass video filter cutoff (internal in the ad606), you would see some
noise on the output, but nothing like a significant fraction of the average
level. However, if you feed noise which is only 100kHz wide into the ad606,
then when it gets detected, the nonlinear operation of detection will shift
some of the noise down into the 100kHz region, where it will pass right
through the video LPF and become noise on the output. Now, this happens in
the white noise example, too, but there, the portion of noise that gets
converted into a 100kHz BW by the detection system is only a fraction of
the total input noise power, but in my case, it is most of the input noise!

This is my theory of what is going on (have not had a chance to test it
yet) and I'm wondering if anyone else has seen this before?

Sean

At 12:57 PM 6/20/2002 +0100, you wrote:
>Hi Sean,
>
>I am always amazed at what you do (and the quantity), and I am glad that
>maybe I can be of help in this case.
>
>I have used the AD606 in a spectrum analyzer, which is about the
>application you use it for, because at the output of the resolution
>filter you get a series of peaks out of the input signal.
>When I had to get more than 80 dB of range out of the 606, my biggest
>problem was noise on the input pins. The bandwidth of the AD606 is
>around 70 MHz, so you can calculate the theoretical minimum by
>estimating 4kTBR (resistor noise).
>At that bandwidth this gives a BIG figure, and then you have to add the
>semiconductor intrinsic noise.
>Conclusion: Maybe the coupling between the filter and the IC is not
>optimal, maybe it is not shielded and other signals (up to 70 MHz) are
>coupled, maybe the power supply is noisy, and a lot of other
>maybes....nut as a HAM and student you will have the knowledge to test
>for them one by one and eliminate the possible sources.
>Now for the 10 dB of noise: It is always there, either the resistor
>noise plus semiconductor, or your input signal. If you look at a
>spectrum analyzer, you see the baseline very noisy, and the noise
>quantity will go down as your signal gets stronger. So there are 2 ways
>for you to score:
>1) Use a preamplifier between bandpass and log amp
>2) Use a low pass filter on the output of the log amp, on the spectrum
>analyzer this is called video filter - but it limits your response
>speed!
>
>Good luck!
>
>Jochen Feldhaar DH6FAZ

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu