- the basics of the way in which RF power meters operate, giving an
overview of the sensor technologies used in RF power meters
Power meters for measuring radio frequency power levels
are an essential tool for any RF design, test or repair laboratory. In fact
the use of RF power meters increases as the frequency of signals increases.
RF power measurement is a key parameter to measure to determine the
operation of a circuit at RF or microwave frequencies. This is because as
the frequency rises, detecting voltage and current is less easy to
undertake. Additionally RF power is one of the key parameters of interest as
RF circuits need to deliver power of various levels into other circuits or
loads. Accordingly these test instruments are used in conjunction with work
on radio receivers, transmitters and RF circuits of all types from wi-fi to
cellular and broadcast transmitting to domestic radio and television
receiver design.
In view of this fact, RF power is widely used in the
specification of components and interfaces. Typically input ports will have
maximum power levels specified, and outputs will be characterised in terms
of the output RF power they can deliver. Naturally other parameters
including impedance and the like are also provided, but power is one of the
most universal RF measurements.
RF power is not always easy to measure. There are several
methods of measuring RF power, each one having its own advantages and
disadvantages. Accordingly the type of power sensor used will depend upon
the type of signal to be measured. Some types of sensor technology will be
more applicable to low powers, others to modulation techniques where the
envelope varies and so forth.
Typically a power meter will comprise a unit where all
the control and processing circuitry is contained, but the power itself will
be detected in what is normally termed a sensor or "head". Thus it may be
possible for a power meter to utilise one of a number of power heads
according to the exact requirements, particularly with respect to power.
It is important to note that power meters act as a load
for the RF power which si absorbed by the head. This high power meters have
large loads that can dissipate the required level of power. Alternatively a
small portion of the power can be extracted by means of a coupler, or by
using a high power attenuator so that the power rating of the RF power meter
head is not exceeded.
RF power meter sensor technologies
The power sensors are the key element of any RF power meter, and the choice
of the type of sensor will depend of the likely applications that are
envisaged. The RF power meter technologies fall into one of two basic
categories:
- Heat based
- Diode detector based
Although both varieties of meter have been available for
many years, both technologies have been greatly refined over the years and
are able to meet very high levels of performance. In view of their different
attributes, they are also used in different types of application.
Heat based RF power meter sensors
As the name suggests, heat based sensors dissipate the power from a source
in a load and then measured the resulting temperature rise. To achieve this
these RF power meters sensors use thermistors and thermocouples.
The heat based RF power sensors have the advantage that
they are able to measure the true average power as the heat dissipated is
the integral of the power input over a period of time. As a result these RF
power sensors measure the RF power level independent of the waveform. Thus
the measurement is true regardless of whether the waveform is CW, AM, FM,
PM, pulsed, has a large crest factor, or consists of some other complex
waveform. This is a particular advantage in many instances, especially as
QAM, and other forms of phase modulation are being increasingly used and a
these do not have a constant envelope.
In view of the time constant wit these RF power sensors,
they are not suitable for measuring instantaneous values. Where these
measurements are required, other types of sensor may be more suitable.
Diode detector based RF power meter sensors
The other form of RF power sensor used in RF power meters, employs diode
rectifiers to produce an output. Again these sensors dissipate the power in
a load, but they measure the voltage, which can then be converted into a
figure for the RF power.
The RF power sensors have two major advantages. The first
is that they are able to measure signals down to very low levels of power.
Some of these diode based RF power sensors are able to measure power levels
as low as -70 dBm. This is much lower than is possible when using heat based
RF power sensors.
The other advantage of diode based RF power meter
sensors, is the fact that they are able to respond more quickly than the
heat based varieties. In some older power meters, the output from the diode
RF power sensor will be processed in a simple way, but far more
sophisticated processing of the readings can be made using digital signal
processing techniques. In this way the readings can be processed to give the
results in the required format, integrating over time is necessary, or
having faster, more instantaneous readings if needed.
Summary
Many RF power meters provide the facility for a variety of radio frequency
power sensors to be used dependent upon the exact nature measurements to be
made. While the heat based RF power sensors are more applicable to
applications where an integrated measurement is required, diode based ones
are more suitable where low level or instantaneous measurements are needed.
Accordingly it is necessary to choose the sensor dependent upon the foreseen
applications.
