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

Charles Steinmetz csteinmetz at yandex.com
Mon Jan 8 23:20:39 EST 2018

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 heater.

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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