Download Power Measurement Basics

RF Power Measurement Basics
Why bandwidth and speed matters
by Wolfgang Damm, WTG
Agenda
• Significance of power measurements
• RF signal theory
• Vital system characteristics
• Sampling techniques
• Product specification examples
• Questions - Answers
Understanding Important Parameters
Technologies effected by Power Measurements
Telecommunication
Mobile Devices, LNA, High Power Amplifier,
Antennas, Receiver, Filter, EMV
Life sciences
MRI, Telemetry, Skin / Deep Muscle Treatment
Aerospace / Defense
Radar, Communication, Telemetry, EMV
Automotive
EMV, Radar (Collision Avoidance Systems,
Parking Aids, Speed Control), EMV,
Communication, Telemetry
Consumer Electronics
Communication, CATV, WLAN, Microwave
Measuring Power: Essential for Circuit Design
This webinar discusses vital factors supporting
better product design and maintenance.
• Component and system output signals are critical in the design
and performance of almost all RF and microwave equipment.
• Signal measurement levels are critical at every system level –
from the single component to the overall system.
• In a system, each component must receive a proper signal level
from the previous component.
Why Measure RF Signal Levels
A component output signal
level is often the critical factor
in the design of RF and MW
equipment.
Signal content gets lost in noise, causing high BER
• Signal too low
– information gets lost in
noise
Signal information gets lost, due to clipping
• Signal too high
– Signal is clipped
– Component can be
destroyed
Too much power destroys circuitry
RF Signal Theory
Why Not Just Measure Voltage?
DC and low-frequency
measurements can be
calculated by the formula:
P=U2/R.
RL
V
I
Depending in the match
between RF or MW source
impedance and load
impedance, parts of the signal
energy is reflected.
A waveguide setup makes it
very difficult to measure
voltage.
Low frequency allow easy power measurements
Forward
Power
RI
Effective
Power
RL
~
Reflected
Power
Specifics of RF Power?
V
I
Amplitude
RF and MW systems show different
behavior: Voltage and current vary
depending on the position measured.
Power stays the same at every point.
t
In RF Domain “power” refers
usually to average power [Pavr]
Pavr
V*I
cos(φ) = phase angle between
V and I
Amplitude
PRF = Pavr = V * I * cos(φ)
t
V
Note:
Frequency of power is double the AC frequency
I
Power Units (W, dBm)
Power is energy transferred per
unit of time.
Basic power unit is Watt (W):
AC component of Power
DC Component of Power
A logarithmic (decibel) scale is
often used to compare power
levels:
Relative Power
P(dB) = 10 log (P/Pref)
Absolute Power
P(dBm) = 10 log (P/1mW)
Amplitude
1W = 1 joule / sec
t
Power Meters Measure Power Envelope
All RF power measurements relate to the power envelope.
RF Power Sensors
Types of Sensors
Thermocouple:
True RMS measurement
Relatively insensitive to temporary overpowering.
Moderate dynamic range (typically 40dB)
Slow
Diode (CW):
True RMS measurement in the square root area.
Very sensitive to temporary overpowering.
Very high dynamic range (typically 90dB)
Faster
Diode (Peak):
True RMS measurement in the square root area.
Very sensitive to temporary overpowering.
Moderate to high dynamic range (typically 40 – 80dB)
Very Fast
Sensors
Diodes are the most commonly used sensors.
The matching resistor is the termination for the RF signal. RF
voltage is turned into DC voltage at the diode. The capacitor C
smoothes the rippled output signal and serves as a low pass
filter.
Ccpl
Rsource
RL
Vsource
~
50 Ohms
C
Vout
Dual-Diode Sensors
Boonton uses dual-diode sensors. Why?
Static effects: Double the output voltage (Villard Principle)
Dynamic effects: Better suppression of harmonic content
Ccpl
Rsource
RL
Vsource
C
~
Vout
C
Fast Diode Sensors
Ccpl
Ri
Rsource
RL
Vsource
C
RC
~
Vout
C
RC
Ri
Influence of Ri, C and Rc on pulse
measurements:
traise = Ri * C
tfall = Rc * C
Vital System Characteristics
Choosing Power Meters / Sensors
• VSWR
• Maximum power level
• Dynamic power range
• Rise / fall time
• Frequency
• Bandwidth
• Sample rate / Effective sample rate
Fourier Transformation (FFT)
• Every non-sinusoid signal consists
of sinusoids of equal and higher
frequencies.
• Amplitude of the base frequency
and its multiples depends on the
form of the original signal.
• If any components of the test
setup elements have insufficient
capability to handle these
frequencies, the measurement will
be inaccurate and the signal trace
will be shown degraded.
FFT (2)
Nyquist
The Nyquist–Shannon Sampling Theorem states: the minimum
sampling frequency of a limited bandwidth, time-continuous
analog signal may be no less than twice the maximum signal
frequency in order to fully reconstruct an signal from the
acquired discrete data.
Nyquist
Nyquist’s Theorem applies also to nonbaseband signal frequencies with limited
bandwidth.
The required sample frequency depends
on the signal’s bandwidth. The sampling
rate must then be higher than twice the
occupied bandwidth.
Harry Nyquist 1889-1976
Example: a 1900 MHz signal with a bandwidth of 5 MHz would require a sampling frequency of
just above 10 MHz to provide a sufficient number of data points to fully reconstruct the signal.
Sampling Techniques
Sampling
• Sampling gathers slices of a signal envelope
at specific time intervals.
• Minimum requirements: samples fulfill
Nyquist requirements. Measurements (dots)
are mathematically and graphically
connected to rebuild the original signal.
Sample points are interpolated
with a sin(x)/x function. The
sampling rate in this example is
about 1.5 times the Nyquist
frequency.
Due to the rectangular form of the
original signal the representation
comes with a high harmonic
content.
Repetitive Random Sampling
Power meters take continuous samples independent of trigger
event. Although taken sequentially in time, they are
always completely random. Additional data points are added with every
sweep.
As a result, the waveform is completely reconstructed.
Repetitive Random Sampling
Detailed Pulses Analysis
1)
2)
3)
Product Specification Examples
Boonton Power Meters
(excerpt)
• Maximum Power Level: 47 dBm*
• Dynamic Power Range: 90dB *
• Frequency: 40 GHz*
• Bandwidth (peak): up to 50MHz
• Rise / Fall Time: down to 3ns
• Time Resolution: down to 100ps
• Effective Sample Rate: up to 10GSa/s
* Sensor dependent
For more information visit: http://boonton.com/Products/Power Meters.aspx
Boonton Systems – Sensors
(excerpt)
• Maximum Power Level: up to
47dBm
• Dynamic Power Range: up to
90dB
• Frequency: up to 40 GHz
• Bandwidth (peak): up to 65 MHz
• Rise / Fall Time: from < 7ns
For more information visit:
http://boonton.com/Products/Sensors.aspx
Conclusion
• Significance of power measurement
• RF signal theory
• Vital system characteristics
• Sampling techniques
• Product specification examples
Questions – Answers
THANK YOU !
Join us for our next Webinars:
In Building DAS Systems by Rand Skopas, WTG
Date: 10/20/2010