Manual - 8500A Series Peak Power Meter - Giga-tronics

Series 8500A Peak Power Meters
The system consists of an amplitude-controllable oscillator operating at a fixed frequency of 1 GHz. A
portion of the RF output is coupled into a thermistor leveling circuit which is physically mounted inside
the metal housing of the calibrator assembly. Referring to Figure 4-9, the thermistor is maintained at a
fixed resistance by applying a dc bias current through it. This is achieved by using a self-balancing
Wheatstone bridge with the thermistor inserted in one leg of the bridge. The amount of power required to
maintain the thermistor at the fixed resistance, RT, is constant so that the RF power plus the dc power
dissipated at the thermistor equals a constant. This constant can be determined by switching the RF
power off and measuring the bridge voltage.
The oscillator output power can then be set by measuring this ambient bridge voltage and calculating the
dc voltage which would balance the bridge when the oscillator has the required power output. When this
required bridge voltage is written to the DAC on the GPIB/Cal Control PC board, the feedback causes the
oscillator to settle to the selected RF power.
The above technique allows the calibrator output power to be precisely controlled over a dynamic range
of approximately 15 dB. The dynamic range is increased by passing the oscillator power through a set of
switched attenuators. The attenuators are designed to allow attenuation values of 0, 10, 20, 30, and
40 dB. The overall gain is adjusted so that the 20 dB stage will provide a precise 0 dBm reference with a
specific power level out of the oscillator.
Integrated control of the calibrator and detector permits errors in the other attenuator stages to be
completely removed in the following manner.
With a particular attenuator switched in, the processor sets the oscillator output power to precisely 9 mW
and measures the detector voltage. The next level of attenuation is switched in, and a successive
approximation algorithm finds the oscillator power (about 90 mW) which will give precisely the same
detector voltage. By dividing the two powers, the relative attenuation of the two ranges is obtained. Since
the 20 dB range is adjusted to give a 0 dBm absolute reference, the absolute effective attenuation of each
range can be calculated.
The automatic calibration system generates a table of detector voltages that are equivalent to a known
level of input power. This is done by applying many different input powers to the detector, measuring the
amplified detector output, and storing the result. The calibration procedure is completely automatic, and
the entire process including the range switching, attenuator measurements, and detector calibration takes
less than one minute.
Since there is 15 dB of dynamic range in the thermistor leveled oscillator and 40 dB of attenuation
available for insertion, the calibrator operates over a total 55 dBm dynamic range. Below -33 dBm, the
diode detector itself is linear (square law), so the total dynamic range of the system will be 60 dB (-40 to
+20 dBm).
The frequency response of each individual RF detector is measured over its entire specified range at the
factory, and the information is stored in a ROM that is an integral part of the detector. Then, during the
measurement of an unknown signal, the frequency response information contained in the detector’s ROM
and the amplitude table generated by the calibration system are combined so that if a voltage equal to,
say, -10 dBm is generated by the calibrator, the instrument looks at that voltage, compares it to its table,
and by interpolating between data points knows that specific voltage level equals -10 dBm. In this way,
offset and gain errors in the detector amplifiers are removed and the system has the capability of
measuring power over the full dynamic range of the detector with just as much accuracy as the power
that is produced from the calibrator.
Refer now to DWG# 17097. Q1, together with the Base and Emitter Resonators, is the 1 GHz
controllable oscillator. A portion of the signal from Q1 is coupled to a thermistor leveling circuit through
the Low Pass Filter to remove harmonics, and through the quarter wave choke to allow connection to one
end of the thermistor, RT1. The other end of the thermistor is connected to RF ground through an RF
bypass line labeled FL5. RT1 samples the RF signal at the output of the low pass filter, and that sample
is sent to the control circuitry on the GPIB/Cal Control PC board. The control circuit has a feedback
system involving some amplifiers and associated circuitry as described in the GPIB/Cal Control board
4-28 Manual No. 20790, Rev C, November 1998
Superceded by Revision D, March 2009