Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!sumax!ole!ray
From: ray@ole.UUCP (Ray Berry)
Newsgroups: sci.electronics
Subject: Re: Audio Spectrum Analyzer
Message-ID: <1265@ole.UUCP>
Date: 8 Sep 89 18:32:48 GMT
References: <3906@mentor.cc.purdue.edu> <53928@psuecl.bitnet>
Reply-To: ray@ole.UUCP (Ray Berry)
Organization: Seattle Silicon Corporation, Bellevue, WA.
Lines: 32

>The sweepable notch filter sounds like a good idea--the fellow at
>Motorola should be thanked for that one--but I have another.

   This is a nice idea.  Perhaps the LO could be log swept.  If you could
generate a quadrature LO, you could use a pair of mixers and a pair of 
LPF's to get your 'notch' at baseband (by computing analytic magnitude).
If the LPF's were SCF's, perhaps you could even devise a means to make the 
cutoff freq track the LO such that the LP's had constant Q and approximated 
something like 1/3 octave bandwidth regardless of the LO frequency.

>It is my understanding that a lot of the commercial spectrum analyzers
>(especially the lower cost ones) are using switched-capacitor filter
>IC's.  These are really nifty IC's that give excellent filter
>performance and are easy to use.  Examples are the MF 4 and MF 10.

   Reticon makes these in a 1/3 octave flavor, 3 to a package.  Each
such chip will cover 1 octave total in 3 bands- with a single clock input
to the chip.  Dividing the clock by two (1 octave) produces the clock for
the next chip down the chain.  Dual 1/2 octave chips are also available,
as are single 1 octave devices.

>...  The big advantage I see of this approach is:  by using a
>programmable divider IC, you could keep your CPU fairly free.
>You could load a new divisor, say, 60 times a second.  That seems

   These filters have a lot of "state" info in them.  I.E., you can't
instantly change their output by banging the clock frequency around. 
You have to wait some (non-trivial) number of clock periods for the 
output to stabilize after changing the clock freq.
-- 
Ray Berry  kb7ht  uucp: ...ole!ray CIS: 73407,3152 /* "inquire within" */
Seattle Silicon Corp. 3075 112th Ave NE. Bellevue WA 98004 (206) 828-4422