# HOW TO COMPARE TWO VOLTAGES AND FREQUENCIES/PHASES

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#### pablome

##### Newbie level 6
Im getting two Inputs. One Reference and one Input to compare. Vref is around 120 AC 60 HZ Single Phase from 3 phase source bench/lab supply.
Other is from 3phase synchronous motor(only gonna use one phase from it to compare). At 1800RPM supplies 60Hz and hopefully around 120V.

I want to compare the voltages, frequency/phase with some circuit and feed it to the microcontroller(motorlla 68hc11e9)

#### pablome

##### Newbie level 6
Id be using the MCU in Expanded Chip mode.

Ok so for voltage comparing I don thave to rectify it but use A/D with offset, 5V max.
I have an offset circuit from lab exp and program that read in this value.
But my ordeal is to make a circuit that does the comparing.
At first I thought I could just get an output logic high or just low if they are same voltages or not.
They will never be exactly the same though...so Ill have to have a error window of 1% lets say; of 120V. And close to dead on frequency and phase.

For frequency...60Hz I was wondering again if there is a simple comparing circuit?
The MCU has Input Capture to read the period, or waveform. Dont know what circuit would be needed for that.

#### FvM

##### Super Moderator
Staff member
I wonder basically what you understand as comparison of AC voltage momentary values respectivly how you define the operation? If you have a meaningful definition, then you'll also find a suitable circuit to my opinion.

Normal way, as used e. g. in generator synchronisation would be to extract phase and voltage as independant parameters and have two individual controllers performing the synchronisation. Phase/frequency measurement would achieve both frequency synchronisation and phase lock. Phase/frequency information can be extracted as simple sign function with a comparator and processed by timer and/or input capture. Voltage in contrast must be acquired as an analog signal to allow effective control action.

Another option for low frequency (I would regard mains frequency as such) is to extract phase and amplitude by digital signal processing. This would need a sufficient number of samples for both input voltages, e. g. 4 per cycle, resulting in 480 SPS minimum. Of course, the difference in sampling time of both signals must be considered. I don't know, if the intended µP would be cacapable of this kind of signal processing.

#### pablome

##### Newbie level 6
what particular circuit can i find to use?

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