[time-nuts] AM vs PM noise of signal sources

Bill Hawkins bill.iaxs at pobox.com
Tue Jan 9 01:11:58 EST 2018

Thank you, Charles.

What a clever way to minimize the power dissipation in Q4 with the
components of the day.

A switching regulator without the steep (and noisy) transients of
today's switchers.

Bill Hawkins 

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Charles
Sent: Monday, January 08, 2018 10:21 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] AM vs PM noise of signal sources

Bill wrote:

> What does the unijunction 2N2646 do in the oven controller?

For the following discussion, you need to refer to the *corrected*
schematic I mentioned in my last post.  If you are looking at the HP
schematic, you will wonder how the hell it works (and it wouldn't, if HP
actually built them as they drew it).

The 10544 oven control circuit uses pulse-width modulation to control
the heater in a "bang-bang" manner rather than a smooth proportional
manner.  UJT A3Q3 forms a relaxation oscillator (along with A3C1 and
A3R10), with a period of ~250uS (frequency ~4kHz) and a voltage span
from ~0v to ~8v.

This positive-going ramp is applied to the base of Darlington A3Q2
(MPSA12), which is 1/2 of a differential pair current switch along with
A3Q1 (2N3904).

The thermistor and associated op-amp circuitry set a threshold voltage
between ground and about 7v at the base of Q1.  After the relaxation
oscillator resets to ~0v, current flows through A3Q1 and A3R8, pulling
the base of Darlington A3Q4 negative and turning it on to saturation. 
The collector of A3Q4 therefore applies essentially the full oven heater
supply voltage from Pin 14 (nominally 24v) to the high side of the

The oscillator voltage ramps positive toward its ~8v maximum (the
trigger point of UJT A3Q3).  When the emitter of A3Q2, which is two
diode drops below the ramp voltage, exceeds the voltage at the emitter
of Q1 (as set by the thermistor and A3U1), Q2 steals the current that
has been flowing in A3Q1 and turns Darlington switch A3Q4 off, which
interrupts the current flowing through the heater.  Some time later
(about 250uS after the previous reset), the oscillator voltage reaches
the trigger point of the UJT and it resets the voltage on A3C1 to ~0v
and the cycle begins again.

Thus, every ~250uS the heater is on for a time (set by the thermistor
circuitry) and off for the remainder of the ~250uS.  This switching
action can be seen at the "Oven Monitor", Pin 11 (but note that the
instrument may have a capacitor to ground on the mother card side of the
oven monitor, to integrate the switching waveform for use by the
instrument's health monitor).

Best regards,


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