The PM4 is a waveguide dry calorimeter designed to be a primary standard for power measurements throughout the mm-submillimeter range. It is constructed with a waveguide load having a 6 second thermal time constant, and an excellent RF match. A thermal feedback circuit makes the sensor much faster (~0.1 sec TC) for most measurements. A calibration heater resistor is mounted on the load at nearly the same location that most of the input power should be dissipated. Very efficient coupling to the load may be made using standard linear tapers to any smaller waveguide band, and the response is fairly insensitive to the mode content. Input loss is minimized through the use of a very short waveguide. The PM4 is similar to its predecessor the PM3, except that all signal processing is digital, and it includes a digital interface. It is much faster in response, and more accurate than earlier models, the PM1 and PM2.

Specifications:
　• Input is WR10 waveguide (1.25 x 2.5 mm) with UG387 precision flange. Useful frequency response is 75 GHz through the submillimeter range, extending even to the visible.
　• Sensor size is 5.1 x 4.8 x 7.6 cm. 1 m cable connects to readout.
　• 1kΩ heater resistor (on the RF load) is used for DC calibration. Internal calibration check on all ranges.
　• Maximum VSWR <1.15:1 in 80-110 GHz band.
　• Input loss is <0.15 dB at 90 GHz.
　• 4 1/2 digit LED panel meter readout, with 4 power ranges. Maximum input power is 200 mW average.
　• Analog output BNC connector on back panel: 0-10V corresponds to 0-full scale meter reading.
　• RS232 data output port.
　• Calibration factor adjustment of up to ±29.9 dB using digi switches.
　• Temperature drift is compensated to <2 µW/°C.
　• The sensor has a thermal time constant (1/e) of 6 seconds. For faster response, the load is heated to a nearly constant temperature using a feedback loop. When input power is applied, the heater power is reduced, and the circuit measures the change, which is equivalent to the input power. The loop gain varies with the power to be measured, changing the response time. For highest sensitivity, no feedback is used on the lowest scale.

Typical Performance:

*Response time is given as the time from application of an input to a response at the analog output of 90% of the final reading. RMS noise is in the meter reading, with a maximum meter update rate of 3 Hz.

　• The figures above show the actual response (from analog output) to a 34 µW input on the 2 mW scale (left) and a 1 mW input on the 20 mW scale (right).
　• Changing scales causes a large thermal transient due to the change in bias heat. Switching scales to a lower power (200 mW to 200 µW) requires 15 minutes for stabilization. Settling time is 2 minutes when switching scales upward. The 5 digit display, and the very low noise and high stability, eliminate the need to change scales in most measurements.
　• Operational temperature range 10-30 °C.
　• Required power 90-240 V 50-60 Hz.

Options available at extra cost:
　• Sensor prepared for use in vacuum (operation in vacuum will alter the calibration and other specifications)
　• Transitions to other standard waveguide bands, with estimated calibration are available.

Do not over-tighten the DUT to the WR10 sensor-flange; the flange can be easily stripped which requires the entire sensor head to be replaced. VDI recommends that the Customer “permanently” attach a 1” section of WR10 waveguide to the sensor input and subsequently attach the DUT to the WR10 waveguide. The User can estimate loss as/if needed.

Virginia Diodes, Inc (VDI) acquired the Erickson power meter business in June 2009. Erickson Instruments LLC was founded in 2000 by Neal Erickson, a world leader in designs of submm frequency multipliers and other components. Its first product was a sensitive millimeter to submillimeter power meter, which has undergone many improvements over time and is now sold as the PM4.