Updated July 6,
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New for July 2014: Kerry explains how the HP536A Frequency meter works! Also, we now have a description of a WWII frequency meter here.
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
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.
The HP 536A Frequency Meter
By Kerry from Down Under
The tunable wavemeter with an L/C circuit was one of the earliest instruments for RF frequency measurements. As usable frequencies increased, the tuneable cavity was discovered. Many firms made microwave frequency meters with tuneable cavities; as ever, HP led the way in combining electronic and mechanical ingenuity.
It’s not difficult to see why these meters acquired the nickname of “gumball machine”. The black ring on the top is turned to rotate the dial and simultaneously move a piston up and down an internal cavity; both piston and cavity are silver-plated. The piston and its bearings and the cavity bore are so accurately machined that there is no contact between piston and cavity. The meter was inserted in the line under test and a detector was also connected; tuning to resonance produced a power “dip” of 0.6 -1 dB and the frequency was read from the scale. The 536A measures frequencies in the range of 940 MHz to 4.2 GHz. The spiral scale is, in total, about 15 feet long. A pair of red divider strips runs in a spiral groove as the dial rotates and the frequency is read between the strips on a cursor line. Here the frequency is close to 2 GHz:
Here the dial has been turned one turn; the divider strips have moved up the spiral groove and the dial now reads about 2.1525 GHz.
The 536 gave good accuracy/precision for the time; a step beyond measuring the wavelength with a slotted line! The 536A had a companion; the 537A This was about half the size of the 536 and covered the frequency range from 3.7 GHz to 12.4 GHz so that these two instruments formed a coaxial measuring system for frequencies between 960 MHz and 12.4 GHz. The 536A and the 537A cost $500 each in 1966. There was also a set of several meters of similar design but fitted with waveguide flanges; the HP 532 series. Waveguide sizes from WR187 to WR28 were fitted; a lucky owner of a complete set of eight of these meters could measure frequencies from 3.95 GHz to 40 GHz. The price of each 532 model in 1966 ranged from $200 to $400 so a complete set would have cost about $2500.