VCF with inductors

[Eric Barbour]

At 12:07 PM 10/2/97 +1000, you wrote:

>...why yes, this is a saturable core reactance, as used in power control
>systems a million years ago.....as the DC saturates the magnetic core the 
>inductance falls.....but you do not get the sort of ranges you want for
>most Emusic applications......

It's true, although the "million years ago" really isn't.
To this day, many industrial electrical systems around
the world use saturable inductors for various jobs.
In spite of MOSFET PWM controllers, there are some areas
where a saturable choke or tranny just makes more sense.

To wit:

A) Most of the companies still producing vacuum-tube AM and FM
transmitters, such as Energy-Onix, Continental and CCA, still
use "Swinging" chokes in their main plate supplies. This is
a scheme often seen in large RF power amplifiers that operate
on high voltage, as it presents more of an even load to the
power supply. Draw more current (during AM modulation peaks),
and the choke saturates, dropping the plate current and
creating a compression effect and preventing over-modulation.
A simple and easy thing to do when you are operating a tube
RF final at 4000 volts or more. Think of it as "passive
signal limiting". Even in FM, where the RF signal is supposed
to be continuous, a swinging choke can help prevent "splatter"
and adjacent-band interference when the load is being
changed, such as for night antenna-pattern changes mandated
by the FCC in America.

BTW, tube transmitters are doing great business, sales up 5%
every year for the last 5 years (though mostly FM and TV--
more than 80% of AM transmitters sold today are solid-state).

B) Some modern RF designs use saturable ferrite cores to
control signal level. I recall reading a recent editorial in
one of those free engineering magazines, where a snide remark
was made about a recent graduate EE who thought he discovered
core saturation and said, "what a great idea! You can control
RF signal level without active electronics! I wonder if I can
patent this??".....a senior engineer in this guy's company had
to point out the use of IF-transformer core saturation for AGC
in a World-War-II-era textbook to this young fella.

Poor guy. 

Makes me wish that EE programs at universities would teach
ANALOG design, not semiconductor physics and computer
worship. Maybe do it the hard way--day one, start with a lab--
give each student a 6AU6 and tell him to make a simple RF
oscillator. At least it will weed out the less capable! (They
DO need to be weeded out! You should see the average Silicon
Valley EE.....he knows how to toe the digital party line, but
not how to design serious analog. A good analog engineer can
make $150,000 per year here, because he is an endangered species.
The need for analog circuits has NOT decreased, in fact it's
on the rise. All that PCS/satellite/cellular/modem stuff still
needs RF, supervisory and power-supply circuits.)

C) Apparently, some companies (even in DIGITAL America) still
make and sell saturable-reactor devices for industry. Examples:
Sola and Topaz still make those voltage-regulating power
transformers for large computer power supplies. Even though
the switching supply is SUPPOSED to smooth out brownouts,
surges and spikes, we all know that this is not entirely
possible. The regulating transformers (which use core saturation)
are said to be selling better than ever, and are often used in 
uninterruptable power systems and surge protectors.
And more conservative manufacturers sometimes use saturable-reactor
speed controls for AC motors. (less chance of inducing garbage
back into the AC mains than with electronic controllers, supposedly.)

As for a VCF using a saturated inductor.....ummm...it MIGHT work.....
As you say, the range probably won't be very wide. The right
technique for the job, eh? Even if it means vacuum tubes.....
Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA