Measuring frequency

Having problems ? let us know the details here
tomroth
User
Posts: 2
Joined: Wed Jul 15, 2009 2:02 am

Measuring frequency

Hi all!

I need to make a coil that has a specific inductance. I don't have that meter, but I just noticed that my 2204 can measure frequency. If I have a known Cap and connect my winding to it, I can measure the frequency and then figure out what my inductance is (in theory). I just don't know where to connect my probe and if I need to provide some sort of power to the circuit to measure it. Any ideas? Any thoughts would be greatly appreciated!

Tom

PeterF
Posts: 435
Joined: Thu Jun 07, 2007 10:53 am
Location: Cambridgeshire

Re: Measuring frequency

Hi Tom,
Your 2204 scope has a arbitrary waveform generator (AWG) which can be swept from near DC up to 100kHz. If you connect your coil in series with a known capacitor and can find the series resonant frequency you will be able to calculate the coil inductance. At series resonance, the circuit will have a low impedance. Connect the AWG output (signal & ground) across the coil and the capacitor. Also connect channel A input across the coil & capacitor just as the AWG, so that it can monitor the voltage.
Choose your capacitor such that the resonant frequency will be in the range 20 to 50 kHz with your desired coil inductance.
Set the AWG to sine wave output and sweeping from 10kHz to 100kHz with 2V amplitude. The output impedance of the AWG is 600ohms so when the circuit is resonant, the voltage measured by channel A will suddenly dip as the circuit impedance is much lower than 600 ohms.
Switch the scope channel A to "Spectrum mode" with a frequency range of 100kHz (actually 97.66 kHz is the nearest range).
As the waveform sweeps across the screen from left to right the signal will be at the top of the screen, near full scale. When the frequency hits resonance, the trace will collapse much lower and you can use a cursor to measure the exact frequency.
You may need to play around with the speed of sweep etc so that the "Dip" is easily seen, too quick a sweep will cause you to miss it. Once you have an approximate value you can turn off the sweep and manually adjust the frequency to be bang-on resonance for best accuracy.