[SOLVED] Is it possible to use a PLL for VERY low frequency input ( < 1 Hz ) ??

[EQ6MSTR]

Hi all,

I am busy trying to design a VERY low frequency multiplier.
As input signal I have an encoder that gives me aprox. 0.3 Hz
square waves as output.
Both the outputs are 90º shifted in phase.
This frequency is not very stable, since it could drift slowly
from 0.1 to .5 Hz within something like 2 seconds.

I would like to know wether it is possible to use some sort of
PLL to achive a multiplication of these signals such that the output
would go from aprox. 100Hz. to 500 Hz in relation with the inputs.

Is this possible????
Any feedback is highly apreciated since I have very little idea
on how to tackle this problem. :-O

Best regards and thanks for your patience!

 

[D.A.(Tony)Stewart]

Re: Is it possible to use a PLL for VERY low frequency input ( &lt; 1 Hz ) ??

YOu can use a type II phase freq detector for this with a VCO. and /N counter but VCO must be variable from 100 to 500Hz with a loop filter around 1Hz.

This wont work very well as the mixer frequency is too close to the loop filter BW and so even with S&H and integrators, it will still be noisy.

But if your goal is to improve stability of tracking the phase of the encoder. a dual up/down counter approach may work better,

Integrating the error and predicting the rate of change to minimize error goes into your PID loop filter.
Any other characterisitics? Is there intertia? or other bandwidth info?

Is it bi-directional? (quadrature) or like a Doppler shift?

In reality you have very low S/N noise with such an encoder. Multiply the F also multiplies the noise so that is not what you need.

 

[EQ6MSTR]

Re: Is it possible to use a PLL for VERY low frequency input ( &lt; 1 Hz ) ??

Thank you for your input, much apreciated!
This could be bi-directional but then the signal would be ignored, so
only in 1 direction the speed changes ( which are very subtle ) have
to be multiplied to a higher frequency!
The encoder is coupled to a telescope rotating 1 time in 24 hours!
The speed changes due to mecanical errors in the drive train have to
be monitored with the encoder, but the signal has such a slow frequency
that I need some higher frequency output, following exactly the input.
When slewing, the signal is ignored, only when tracking in 1 direction.

Should I use some reference frequency at 0.3 Hz for the phase difference
detection??

 

[D.A.(Tony)Stewart]

Measuring a higher frequency on a counter after a PLL is like measuring the timer interval with a faster clock. If you need more resolution can you measure time interval to each quadrature signal edge and measure the phase digitally?

Consider VCO with control range from 100 to 500Hz out with / 1000 counter and type II mixer in PLL chip or software mixer. Then use PID filter on time interval of phase error to control VCO is the analog PLL approach.

 

[EQ6MSTR]

This is exactly what I am after, measuring with a higher frequency so that I could take a sample
of the encoder position every 0.5 or even better every 0.2 seconds and compare with the theoretical
optimal position.

Would the PLL follow the input exactly, when all setup correctly, that is, if the input
changes from let's say 0.3 to 0.4 Hz in half a second, would the output then change
from 300 to 400 Hz in the same half second?
How fast would the PLL function following the input???

Sorry for all these beginner questions, I am grateful for the patience you
have with me ( I am not an electronics guy )!