> Hi, yes I've noticed this too, things like this are almost quaint and
> nearly useless except as little "thought design" problems.  My guess is
> that (a) they publish what they have sitting around from whenever, they
> (b) publish what people happen send in, and (c) a design that is a
> micro, decoupling caps, with just a few bits specific and some code
> doesn't grab any attention.  So even though (c) with some discussion as
> to why this presented solution is the best one for working design
> engineers, its just not going to happen anytime soon.
>
> Alas.

One of the magazines recently had an engineer write in saying the design
ideas were kinda dumb. They then asked him to review the new designs and
comment on them. I think his comments were pretty good. I don't remember
which magazine it was in, though.

Earlier this year I was redesigning an IR transmitter for use in movie
theaters. The existing transmitter transmitted two FM signals (2.3MHz and
2.8MHz). It used a somewhat complex varactor tuned LC oscillator for each
frequency. Distortion was pretty high. The oscillator output went through
an LC low pass filter (removing harmonics), then through a Schmitt trigger
(putting the harmonics back). I replaced the LC oscillator and surrounding
circuitry with an LTC6907. Much simpler, much lower distortion, FM noise
about 58dB below 75kHz deviation.

The circuit has two audio compressors. Pretty standard stuff. The RMS
detector of each compressor drove a pair of comparators. One to drive a
peak flash LED to set audio levels. The other comparator drove, of course,
a 555 timer that would hold the carriers up for 30 minutes after audio
disappeared. Timing 30 minutes with a 555 is a good trick!

So, this all used a bunch of parts. I put in a PIC that reads the
compressor RMS detector voltages, drives the peak flash LEDs, does the
timing for carrier on and off, etc. It also controls a third carrier
(1.8MHz) that I added to run closed captions. All that in one part.

So, now the circuit is much simpler. Of course, after I finished it, I've
thought of a few more things to simplify. Maybe next time. The PIC with
the internal RC oscillator and PLL is really nice. No external components
required. It just works.

Harold
"The ideal design has zero parts."




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