SPICE circuit with noise on output

Thread Starter

FuseFuse

Joined Feb 8, 2017
27
Hello,

I'm trying to reproduce a circuit, whether an inverter or a power supply circuit, where a switching module generates noise on the output voltage. In a second step I want to suppress this noise with a common mode filter. Does anybody have such a circuit (.asc file) for step nr. 1? Frequencies of interest are between 10 kHz and 10 MHz.

Thank you for your replies
 

Alec_t

Joined Sep 17, 2013
12,890
You could add a behavioural voltage source in series with the output to superimpose a fixed or random frequency. I expect someone here has a random noise source model.
 

ag-123

Joined Apr 28, 2017
233
I think SMPS typically have high frequency sawtooth modulation riding on DC.
at least from a modelled run here:
https://forum.allaboutcircuits.com/...on-differential-equations.187476/post-1740574
there are some circuit examples here.
http://www.ecircuitcenter.com/Circuits.htm
However, i doubt they are 'common mode' signals. But maybe it is true for an OpAmp, I've tried playing with some stuff (LM(V)358), if after all the + and - inputs are fed with 2 sets off 'resistor dividers' where one set of them is a sensor. An imbalance in resistances, I think can still feed the difference of that 'noise' through the op amp and gets amplified to the output. The CMRR is supposed to have taken care of that, up to some voltages.
In an OpAmp they should be very small, right? + input is supposed to be equal to - input due to the high amplification.
 
Last edited:

Thread Starter

FuseFuse

Joined Feb 8, 2017
27
Hello,
maybe my request wasn't sufficiently clear before. My bad.
What I'm looking for is an existing switching circuit where common mode noise is reduced with an implemented common mode choke. As simple as that. Finding for such a circuit (please note, not a netlist) is somehow tricky on the Internet.

Thank you for your replies.
 

crutschow

Joined Mar 14, 2008
30,100
Here's a way to use the arbitrary behavior voltage generator with the random number function in LT spice to generate random noise with time:

1655732702052.png
 

ag-123

Joined Apr 28, 2017
233
well, i'm not sure about switching circuits where common mode noise is reduced.
But in terms of common mode signals as it relates to *OpAmps*, CMRR is a apparently feature of the differential amps / OpAmps.
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/op-amp-practical-considerations/
https://www.allaboutcircuits.com/te...-a-key-feature-of-instrumentation-amplifiers/
If this is the application context, then a good OpAmp probably helps, but that CMRR has limits and is normally documented in the specs. using chokes to reduce noise probably works for "high frequency" signals. In some cases, I placed a large electrolyic capacitor between the +V rail and GND. This apparently helps in some cases where possibly SMPS are the main supplies.
some of those experiments, which I once did involving a temperature sensor is this:
https://www.stm32duino.com/viewtopic.php?p=8455#p8455
In the end, the conclusion is that if you have long exposed wires, those wires can become an antenna and in this world with very high RF noise, all the different broadcast channels, add smps, induction cookers, etc in the neighborhood, those radio waves can be picked up even by the ADC of an ordinary microcontroller. Well, it is an (accidental) SDR.


In 3d printers (e.g. those derived from Marlin etc), they placed huge 10 uF capacitors at the thermistors as I'd guess these signals will be there.
That suppresses the magnitude of these signals, but that it also introduces significant delay in the time varying thermistor voltages.
This cause PID controls to oscillate somewhat I think. I'd think in this case an RF choke may help, but that there'd be resonant behaviour.,
The thermal sensor voltage in the above graph is probably around 1.77v, just look at that extreme voltage swings and it is 10 mV per deg C, so it is off by many degrees C if all that spikes are followed. RF signals are probably more than simply 'common mode' signals.
 
Last edited:
Top