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Simple discrete components logarithmic detector for spectrum analyzer?

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neazoi

neazoi

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Hi, I am looking for a simple discrete components logarithmic detector for a homemade spectrum analyzer.
The only one I have found is at the middle of this page http://lea.hamradio.si/~s53mv/spectana/sa.html but I wonder if there is a simpler one at the compromise of the dynamic range.
 

Why don't you use an integrated one ? It's precise and very cheap..
 

There are circuits of log amplifiers in the National Semiconductor applications book.
You will still need at least one op amp, and thermal drift can be a problem.



A commercial log amp chip will be compensated for thermal drift and generally solve most of the problems for you.
 

The term logarithmic detector is a bit ambiguous. The SA diagram is showing a logarithmic IF detector. It can't be replaced by a simple detector with succeeding log amplifier, because a simple IF detector doesn't have the necessary dynamic.

The discrete multi stage IF detector in figure 9 of the said link seems quite simple, don't see what's your problem with it, except for using a certain amount of components.

logdet.jpg

You may find solutions with higher per-stage gain and fewer stages, the transfer curve will have respectively higher ripple. Or use one of the available log IF detector ICs, e.g. from Analog Devices.
 

FvM said:
You may find solutions with higher per-stage gain and fewer stages, the transfer curve will have respectively higher ripple.
Click to expand...

Yes, I have seen such schematics in old magazines. 3-4 dual gate mosfets to do the job, however the extra amount of components needed, equals the amount of the detector you attached. Also I was not aware of the disadvantages of the fewer stages, I was only suspected that the logarithmig detection curve would not be so smooth. Thanks for pointing this out.

I think I am going to stick with the proposed schematic.

But does really a log detector needed in a homebrew spectrum analyzer? Would that be a problem to use a simple envelope detector (video detector), if we sacrifice amplitude resolution of the low-level signals?

Also I wonder how can I make this schematic work at 12v instead of 7.5 and also to operate at 15MHz?
 

For what it is worth, you can experiment with simple discretes but implement with something like the AD8313 for the complete SA.
 

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