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Analyzing the MMR40 PTO – Spice vs Real World


Analyzing the MMR40 PTO – Spice vs Real World

Some waveforms at startup from the spice model (Component designations from original manual, not spice model):
J310 Gate (Q14):

Q14 Oscillator Gate Startup ~ 5.0 vpp Scale: -5.0v to 0.5v

Q14 Oscillator Gate ~ 5.0 vpp Scale: -5.0v to 1.2v

And here is the measured waveform at Q14 Gate (Ac Coupled):

Q14 Gate ~ 4.5 vpp (AC Coupled)

And DC Coupled:

Q14 Gate DC Coupled ~ 4.5vpp

J310 Source (Q14):

Q14 Oscillator Source Startup ~ 1.1vpp Scale: -0.1v to 1.2v

J310 Source (Zoomed In):

Q14 Oscillator Source ~ 0.5vpp Scale: 0.32v to 0.76v

And the Measured Waveform:

Q14 Source (AC Coupled) ~ 0.5vpp

And DC Coupled:

Q14 Source (DC Coupled) ~ 0.5 vpp

Input to R3 (which represents U6 Oscillator Input):

U6 Pin 6 Oscillator Input from PTO ~ 0.9vpp Scale: -0.45v to 0.45v

Input to U6 (Zoomed in):

U6 Pin 6 Oscillator Input ~ 0.9vpp Scale: -0.45v to 0.45v

Measured Waveform (AC Coupled):
This shows a little lower level of signal, about 0.6vpp, due to the capacitive loading of the scope probe at this point. The probe is about 10pf shunted across the 22pf capacitor on U6 pin 6. This gives us a total of about 32pf. When I changed the 22pf capacitor in spice with a 32pf capacitor, it showed 0.6vpp, just like we are measuring.

U6 Pin 6 Input ~ 0.6vpp

And DC Coupled, It has a bit of a DC bias on it, probably from U6, which I didn’t model in spice for these tests:

U6 Pin 6 Input (DC Coupled) ~ 0.6vpp

R1/C9 point on cold end of tickler coil:

Junction of R1 and C9 ~ 1.2vpp Scale: -0.1v to 1.2v

R1/C9 (Zoomed In):

Junction R1 and C9 ~0.04vpp Scale:0.512v to 0.552v

And the measured waveform (AC Coupled) a little more ripple than predicted, but still very small:

R1/C9 Junction (AC Coupled) ~ 0.2vpp

And DC Coupled:

R1/C9 Junction (DC Coupled) ~ 0.2vpp

I really like using ltSpice to simulate real world circuits. It allows me to change various values and see what effect it has on the circuit.
As you can see, the Spice results come out very close to the real world results, so using spice as an experimentation platform can save a lot of time.


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