So you want to vary the VCF input on the function generator until its output is the right frequency as measured by the DAQ card?
To
get 1% tolerance, that means you have to differentiate between 1 kHz
and 1.01 kHz. This requires a frequency step size of 10 Hz. To get
this resolution, you would have to perform a FFT of 0.1 seconds of
data. To get a maximum of 40 kHz, you would have to sample at a
minimum of 80 kHz. This means performing an 8000 point FFT repeatedly.
To
get this to work you need to design an adaptive filter that takes the
FFT peak location and compares it to the desired location and uses this
as the feedback to set the voltage to the VCF of the function
generator. This would be difficult unless you have done something
similar in the past.
You might try to create a lookup
table by feeding voltages into the VCF and using the 8000 point FFT to
determine which voltage corresponds to which frequency. See what the
curve looks like and then use some interpolation to get specific values from the
table.
Message Edited by rpursley8 on 05-18-2009 12:13 PM
Randall Pursley
