To: WSFAlist at KeithLynch.net
Date: Mon, 25 Dec 2006 02:17:08 -0500
Subject: [WSFA] resistors, capacitors, and modems
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at KeithLynch.net>

On Sat, 23 Dec 2006 11:25:03 -0500 "Mike B." <omni at omniphile.com> writes:

...> Typical resistors are only accurate to their labels +/- about
> 10-20%,
> and I believe the same is true of typical capacitors...

That was true of resistors about 50 years ago, but not now, though it
remains largely true of capacitors still.

Advances in resistor manufacturing technology, including robotics, and
the adoption of 'metal film' construction, have made 1% tolerance
resistors so cheap that there is no sizeable market anymore for 5%, 10%,
or 20% tolerance low power 'jellybean' carbon composition resistors.  A
quick check at www.digkey.com confirms this.  The electronic parts
distributor Digi-Key has 24 million 10K ohm 1/8 watt SMD 1% chip
resistors in stock, priced at about one half cent each in 5000 piece
lots, or one quarter cent each in 500K piece lots.  At those prices, the
resistors themselves almost might as well be free, as it costs more than
that, each, to solder them onto circuit boards, using robotic assemblers.

DigiKey does still sell old fashioned axial carbon composition resistors,
the technology of 50 years ago.  They have 1246 pieces of 10K ohm 1/4
watt carbon composition resistors in stock, and they are priced at 42
cents each singly, or 25 cents each in 5000 piece lots.  Presumably those
resistors would be bought by people repairing old equipment, who feel
they must get the _exact_ replacement.  More modern 5% axial carbon film
10K ohm resistors, which should work as well, are priced at 8/10 cent
each in 5000 piece lots.  Metal film resistors are more modern than
carbon film, perform better, and are lower priced, thanks to the enormous
unit volume of production and sales of the metal film resistors.

High power resistors (25 watts and up), on the other hand, are still made
the same way they have been made for most of the past 100 years: wire
wound on a ceramic tube, covered with vitreous glaze, and baked in a kiln
like a piece of pottery.  Those resistors are priced from about 50 cents
each up to several dollars each, depending on size, and the resistance
tolerance is typically 5% or 10%.  50 years ago, 1% tolerance low power
resistors were made in a similar way, as carbon composition resistors
could not be economically manufactured with tolerance tighter than 5%.
Carbon film resitors with 2% tolerance were popular in the 1980s as a
lower priced alternative to 1% metal film resitors, but now metal film is
generally both the cheapest and the best.

It is somewhat ironic that in probably a majority of low power resistor
applications, 5% or even 10% tolerance would be adequate, as was the case
50 years ago, but 1% tolerance resistors are used, because they are now
so cheap.

> >The analog filters in 300 bps full-duplex modems were actually
> quite
> >elaborate, especially the receive filters.  In the early to
> mid-1970s,
> >the filters used op-amps, and resistors and capacitors of 1%
> tolerance,
> >and they were epoxy-potted to prevent subsequent drift of
> component
> >values due to humidity.
>
> Was this the commercial quality ones, or did this apply to cheaper
> consumer models like my Cat, which was about $160 if I remember
> right
> (circa 1981)?
>

I described what I observed in the mid-1970s, when I worked for a year in
a production testing lab at a company making high quality commercial
grade full duplex 300 bps and half duplex 1200 bps modems.  Those modems
sold for maybe two or three hundred dollars apiece at that time, and they
were sold in competition to Bell modems.  Sorry I can't remember exact
prices for particular models.

If you paid $160 for a consumer modem in 1981, you probably got a modem
which performed as well as one costing two or three times as much in
1975.  The history of modems has been one of falling prices in tandem
with improving performance.  I could not speculate whether the 'Cat' had
potted filters - I've never seen one of those modems.  Eventually, analog
filters disappeared when digital filters (DSP) became cheaper.  In 1981,
I was part of an engineering team designing a high speed DSP modem which
was expected to sell for something like $1K or $2K, so it's a safe bet
that your $160 1981 modem had analog filters, not digital filters.

Ron Kean

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