MICROWAVE MEASUREMENTS

UNITS AND DEFINITIONS

WATT:

The International System of Units (SI) has established the watt (W) as the unit of power; one watt is one joule per second.

dB:

Relative power is the ratio of one power level, P, to some other level or reference level, Pref . The ratio is dimensionless because the units of both he numerator and denominator are watts. Relative power is usually expressed in decibels (dB). The dB is defined by

dB = 10 log10 ( P /Pref)

dBm:

dBm=10log10(P/1mW)

POWER MEASUREMENTS

The various methods used for measurement of Power of RF / Microwave signals are:

Thermistor sensors

Thermocouple sensors

Diode Sensors

THERMISTOR SENSORS

They operate by changing resistance due to a corresponding change in temperature. Widely known devices are Bolometers. Thermistors are semiconductors with a negative temperature coefficient.

For example the Aglient 478A thermistor is capable of measuring Power of signals up to 18GHz.

Agilent 478A thermistor

Thermocouple sensors

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.

Thermocouple Power meter

Power Meter -Agilent 437B

Thermocouple sensor DC output is very low-level (approximately 160 nV for 1 microwatt applied power), so it is difficult to transmit in an ordinary flexible connection cable.

One practical way to handle such tiny DC voltages is to “chop” them to form a square wave, then amplify them with an AC-coupled system. After appropriate amplification (some gain in the sensor, some in the meter), the signal is synchronously detected at the high-level AC.

This produces a high-level DC signal that is then further processed to provide the measurement result.

S Parameters

The S-parameters or Scattering Parameters describe the electrical behavior of linear electrical circuits or two port networks.

The following information must be defined when specifying a set of S-parameters:

The frequency

The characteristic impedance (often 50 Ω)

The allocation of port numbers

Conditions which may affect the network, such as temperature, control voltage, and bias current, where applicable.

S11-Input voltage reflection coefficientS12-Reverse Voltage GainS13- Forward Voltage GainS22- Output Voltage Reflection Coeffecient

Vector Network Analyzer

A network analyzer is an instrument that measures the network parameters of electrical networks. Today, network analyzers commonly measure s–parameters because reflection and transmission of electrical networks are easy to measure at high frequencies.

Parts needed in a Network Analyzer:

Signal Generator

Test Set

Receiver

Source

Signal Seperators

Processor/Display

SWR METER

PRINCIPLE OF SWR METER

The SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load (usually a radio antenna), or evaluate the effectiveness of impedance matching efforts.

A directional SWR meter measures the magnitude of the forward & reflected waves by sensing each one individually, with directional couplers.