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
"Sean H. Breheny" schrieb:
>
> Hi all,
>
> I have a circuit where I need to detect the amplitude of an incoming
> 10.7MHz signal. The signal consists of 10 microsecond long pulses of a
> 10.7MHz carrier. What I tried to do was to use a ceramic filter with a BW
> of 360kHz (roughly corresponding to a rise time of 3 microseconds) to pass
> the minimum BW needed to allow the pulses to pass, and then feed the result
> into an Analog Devices AD606 Logarithmic Amplifier IC. However, I
> discovered that when I try this, the background noise causes the output of
> the log amp to be very noisy at low signal levels. I had originally thought
> (from looking at the AD606 datasheet) that Gaussian noise was detected by
> the IC much the same way as a sinusoidal signal, yielding a DC voltage out
> which is proportional to the log of the power in the noise.
>
> This output noise causes a problem because the output noise level is so
> high that it extends to about 10dB higher than the average level (in other
> words, let's say that the noise power is -70dBm and that this should
> correspond to an output DC voltage of 0.5 V. What I actually see is a 0.75
> pk-pk noise signal on top of a 0.5V offset as the output, and the slope of
> the amp, in V per dB is such that the extra 0.75/2=0.375 V corresponds to
> 10dB, so if I sample the output of the log amp, I can see results which are
> as much as 10dB above or below
> -70dBm. This effectively means that my SNR needs to be > 10dB for even
> remotely reliable detection of the pulses.).
>
> Upon thinking a little further, I think my problem may be a fundamental
> one: since my log amp can respond to fast edges (and must), it is
> responding to every edge in the noise amplitude which is slow enough for it
> to respond to, and there are plenty of these since the noise has just gone
> through a ceramic filter, which removed all the really fast edges.
>
> Now, finally, to my questions :-)
>
> #1) Can someone either confirm my diagnosis of what is going on or tell me
> what I might be doing wrong?
>
> #2) Is there a better way to measure pulse amplitudes in the presence of
> noise (without having to make a matched filter or do DSP with high sample
> rates)?
>
> Thanks,
>
> Sean
>
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