Frequency meters

Updated June 18, 2006

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Frequency meters, also called "wavemeters", are what your grandparents used to determine the frequency of an unknown signal source. Sometimes called "gumball machines", (thanks, John!) now frequency meters just take up space in one of the lab cabinets that no one opens. Engineers will fight tooth and nail to get the $100,000 spectrum analyzer in their setup. But you can obtain similar accuracy with a frequency meter, and if you use one in your next setup, people will think you really know what you are doing. And they'll probably ask "where do you plug it in?"

Here's one we saw on Ebay recently, it probably sold for $10 or less. In this case the frequency meter is for X-band, and uses WR-90 waveguide. The scale reads out in MHz.

How does a frequency meter work? The cylindrical cavity forms a resonator that produces a suck-out in the frequency response of the unit. This you would turn the knob until a dip in the response is observed. The graduations will tell you what frequency you are at.

Waveguide frequency meters use a short circuit resonant cavity, which resonates at half-wavelength. Most wavemeters are waveguide, however, coaxial types are possible. Waveguide wavemeters can only measure frequency over their respective frequency band.

Here is a view of the above wavemeter taken apart. You can see the hole in the E-plane that couples out to the cavity. At the bottom of the cavity is the piston that changes the resonant frequency.

Wavemeters are affected by temperature changes, which slightly change the dimensions of the cavity.