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From: GMoretti@massey.ac.nz (Giovanni Moretti)
Newsgroups: rec.ham-radio,sci.electronics
Subject: How does one build LOG AMPS (>60dB dynamic range)
Message-ID: <596@massey.ac.nz>
Date: 27 Feb 90 01:47:04 GMT
Organization: Massey University, Palmerston North, New Zealand
Lines: 26
X-Reader: NETNEWS/PC Version 2.1

I've build a spectrum analyser (0-100MHz) that has a 10.7MHz IF (from
HAM-RADIO years ago) and it works quite well but I'm not happy with the
log-amp/detector.  

The detector at 10.7MHz is cascade of amplifier stages each with the same
gain (10dB) and a diode halfwave detector from each stage. The outputs of
the detector are summed.  Unfortunately it needs a lot preset pots to get
it working correctly and its linearity is not as high as I'd like.

Also I'd like to try making a direct conversion spectrum analyser - it
would need to be DC coupled.  The log detector would then run at a much
lower frequency.  I've tried using S-meter output of a CA3089 and its
logarithmic but doesn't have the required dynamic range (> 60dB).

How does one make real log amps that don't need lots of preset adjustment?

Ideas ??

Giovanni

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|   GIOVANNI MORETTI, Consultant     | EMail: G.Moretti@massey.ac.nz          |
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|   Palmerston North, New Zealand    | QUITTERS NEVER WIN, WINNERS NEVER QUIT |
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