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Microwave Measurements
by
Sk. Hedayath Basha
Assistant Professor
ECE Department
R.M.K College 0f Engineering and
Technology
1RF & Microwave Engineering - V unit

• In low frequency circuits parameters such as
voltage, current, etc can be measured and from
these impedance, power factor and phase angle
can be calculated.
• At microwave frequencies it is more convenient
to measure power instead of V and I.
• Properties of devices and circuits at microwave
frequencies i.e characterized by Sparameters,
power, frequency and VSWR and noise figure

• Direct microwave measuring devices are
vector network analyzers, spectrum analyzers
and power meters.
• Due to their complications and high cost,
microwave measurements in lab are often
carried out using 1 kHz square wave
modulating signal which modulates the
microwave test signal.

Important measurement devices
• The tunable detectors are used to
demodulate the signal and couple the
required output to high frequency scope
analyzer. The low frequency demodulated
output is detected using non reciprocal
detector diode mounted in the microwave
transmission line.

• Slotted section with line carriage is a
microwave sectioned coaxial line connecting a
coaxial E-field probe which penetrates inside a
rectangular waveguide slotted section.
• The longitudinal slot is cut along the center of
the waveguide broad walls. The probe is made
to move along the slotted wall which samples
the electric field proportional to probe
voltage.

Main purpose of slotted section with
line carriage is
1. For determination of location of voltage
standing wave maxima and minima along the
line.
2. Measure the VSWR and standing wave pattern.
3. Wavelength.
4. Impedance.
5. Reflection co-efficient.
6. Return loss measurement.

• VSWR meter is a highly sensitive, high gain,
low noise voltage amplifier tuned normally at
fixed frequency of 1KHZ of which microwave
signals modulated. This meter indicates
calibrated VSWR reading for any loads.

Power Measurement
• Power is defined as the quantity of energy
dissipated or stored per unit time.
• Microwave power is divided into three
categories –
1. low power (less than 10mW),
2. medium power (from 10mW to 10W) and
3. high power (greater than 10w)

• The general measurement technique for average
power is to attach a properly calibrated sensor to
the transmission line port at which the unknown
power is to be measured.
• There are three popular devices for sensing and
measuring average power at RF and microwave
frequencies. Each of the methods uses a different
kind of device to convert the RF power to a
measurable DC or low frequency signal.
• The devices are the diode detector, the
bolometer and the thermocouple.

Diode Detector
• The low-barrier Schottky (LBS) diode
technology which made it possible to
construct diodes with metal-semiconductor
junctions for microwave frequencies that was
very rugged and consistent from diode to
diode. These diodes, introduced as power
sensors in 1974, were able to detect and
measure power as low as −70 dBm (100 pW)
at frequencies up to 18 GHz.

Bolometer Sensor:
• Bolometers are power sensors that operate by
changing resistance due to a change in
temperature. The change in temperature
results from converting RF or microwave
energy into heat within the bolometric
element.

• There are two principle types of bolometers,
barretters and thermistors.
• A barretter is a thin wire that has a positive
temperature coefficient of resistance.
• Thermistors are semiconductors with a
negative temperature coefficient.
• Thermistor elements are mounted in either
coaxial or waveguide structures so they are
compatible with common transmission line
systems used at microwave and RF
frequencies

• Power meters are constructed from balanced
bridge circuits. The principal parts of the power
meter are two self-balancing bridges, the meter-
logic section, and the auto-zero circuit.
• The RF Bridge, which contains the detecting
thermistor, is kept in balance by automatically
varying the DC voltage Vrf, which drives that
bridge.
• The compensating bridge, which contains the
compensating thermistor, is kept in balance by
automatically varying the DC voltage Vc, which
drives that bridge.

• The power meter is initially zero-set (by
pushing the zero-set button) with no applied
RF power by making Vc equal to Vrfo (Vrfo
means Vrf with zero RF power).
• After zerosetting, if ambient temperature
variations change thermistor resistance, both
bridge circuits respond by applying the same
new voltage to maintain balance.

Thermocouple Sensors
• Thermocouple sensors have been the detection
technology of choice for sensing RF and microwave
power since their introduction in 1974.
• The two main reasons for this evolution are:
1) they exhibit higher sensitivity than previous
thermistor technology, and
2) they feature inherent square-law detection
characteristic (input RF power is proportional to DC
voltage out).
Since thermocouples are heat-based sensors, they are
true “averaging detectors.”

• Thermocouples are based on the fact that
dissimilar metals generate a voltage due to
temperature differences at a hot and a cold
junction of the two metals. The power sensor
contains two identical thermocouples on one
chip, electrically connected as in Figure

• The principal advantage, however, of the two
thermocouple scheme is that both leads to the
voltmeter are at RF ground; there is no need for
an RF choke in the upper lead. If a choke were
needed it would limit the frequency range of the
sensor.
• For a square wave modulated signal the peak
power can be calculated from the average power
measured as P Y peak av P where T is the time
period and Շ is the pulse width

Measurement of Cavity Q

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