True Log Amplifier Schematic

[wolfalex]

Hello,
 
  in the EEVBLOG Video #575 a nice DIY analog spectrum analyzer was shown. Im especially interested in the Logarithmic Amplifier section (which I think this kind of circuit is also called True Log Amp). I found
  a lot of theoretical information about this circuits but not really a practical circuit diagram. Does somebody probably have a circuit diagram for this kind of log amp which he did in the past? If everything fails
  I can still try to reconstruct the circuit from the video, but I am not sure how difficult this will be.

 This document also talks about this subject for example:

     http://www.analog.com/media/en/training-seminars/tutorials/MT-078.pdf

 Thank you very much.

[LaserSteve]

Take a look here, scroll down the page for a discrete LOG amp at ~9  Mhz frequencies.

http://lea.hamradio.si/~s53mv/spectana/sa.html

Dr. Vidmar basically designs world class stuff, anyways...

Modern way: Use a 10$ high bandwidth LOG Detector IC from Linear Technology or Analog devices...

http://www.linear.com/parametric/RF_Log_Detectors

http://www.analog.com/en/products/rf-microwave/rf-power-detectors/non-rms-responding-detector/ad8307.html#product-overview

http://www.analog.com/media/en/technical-documentation/data-sheets/ADL5513.pdf

If you do some digging there are some chips out there for less then 10$ with bandwidths extending well into 8-10 Ghz.

Steve

[rfeecs]

Differential pairs have a hyperbolic tangent response for voltage-in current-out.  So these are often used with a cascade of gain stages to do the log approximation over a very wide dynamic range.  See the attached figure 11 from HP Journal article about the old 8757 scalar network analyzer.

Another approach is make a variable-in-dB gain amplifier using a resistor ladder attenuator, and putting this in a feedback loop to do a log detector.  This is described in attached patent US5077541.  It has lots of schematics for the brilliant idea, authored by Barrie Gilbert.  This is used in some of the Analog Devices log detectors.

[wolfalex]

Thank you very much for the great links. This was exactly the information I was looking for. I really want to try and build one of this discrete log amps and see how well it works (at least use one on very low frequencies). I know that the finished ones are not so expensive and that it is almost impossible to built a good working one in discrete form. But its more for learning purposes and to see how simple a SA can probably be built using only standard parts.

[dom0]

The HP 3581A (and sister models) features a (slightly-better-than) 90 dB log amp working at the 100 kHz IF. The manual has a concise but good description of how it works (paragraph 4-59 and following), but the actual implementation is "boring": they made a  purpose-built hybrid chip. That machine works essentially exactly like a single-IF spectrum analyzer; but due to the low frequencies is a nice "display model" and easily studied as well.