APPLICATION NOTE
A Measurement Technique Used to Determine the 3 dB
Modulation Bandwidth of Skyworks PS094-315 Phase
Shifter
Introduction
Modulation Bandwidth Measurement
Under the control of an external signal, a phase shifter is typically
used to shift the phase of a high frequency signal. An analog
phase shifter may also be used as a phase modulator, in which
case the external control signal is known as the modulating
baseband signal.
Modulation bandwidth is a measure of control port frequency
response. It can be measured using the test setup shown in
Figure 2.
A parameter of interest is the maximum baseband modulating
frequency, which is typically defined as the frequency at which
modulation sensitivity decreases by 3 dB. The “bandwidth” of an
electronic component is defined as ”The range of frequencies
within which performance, with respect to some characteristic,
falls within specific limits.”[1]
In this case, the modulation bandwidth is defined as the range of
frequencies in which modulation sensitivity is within 3 dB of the
peak value. In basic electric circuit theory, the bandwidth
represents the distance between the two points in the frequency
domain where the signal is 1/√2 of the maximum signal amplitude
(half power). As an example, the 3 dB bandwidth of a low pass
filter is depicted in Figure 1.
This Application Note describes a measurement technique used to
determine the 3 dB modulation bandwidth of the Skyworks
PS094-315 voltage variable phase shifter, operating in the 700 to
1220 MHz frequency band. For additional information about the
PS094-315, refer to the device Data Sheet (document # 200240).
The procedure is to phase modulate the RF signal applied to the
phase shifter with a lower frequency sine wave, then use a phase
demodulator to recover the original modulating signal. As the
modulation frequency is increased and the amplitude of the high
frequency RF signal is held constant, the 3 dB modulation
bandwidth can be determined as the modulating frequency at
which the recovered modulating baseband signal is reduced in
amplitude by 3 dB from its peak value.
In this case, the phase demodulator circuit is a double-balanced
passive mixer. It is selected so it has a wider baseband bandwidth
than that of the phase shifter.
Test Setup
A phase demodulator is constructed by using a power splitter and
mixer. The frequency of the modulating signal is varied, while
keeping its amplitude constant, starting from a frequency very
close to 0 Hz until the oscilloscope display drops by 3 dB, or to
0.707 of its low frequency value. This frequency is recorded as
the 3 dB modulation bandwidth.
Figure 1. Example of The 3 dB Bandwidth of a Low-Pass Filter
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APPLICATION NOTE • PHASE SHIFTER 3 DB MODULATION BANDWIDTH
Figure 2. Modulation Bandwidth Test Setup
Table 1. Test Equipment
Equipment
Manufacturer
RF signal generator
Rohde Schwarz
SME03
Function generator
Agilent
81150A
Power splitter
MiniCircuits
ZFSC-2-2
Line stretcher
Arra
DN2448A
Mixer
MiniCircuits
ZFM-2000
Oscilloscope
LeCroy
44XS
The RF signal generator provides a high frequency continuous
wave (CW) signal that is divided into two in-phase, equalamplitude signals by the power splitter. One of these signals is
phase modulated by the PS094-315 phase shifter. The other
signal is the high frequency reference signal for the mixer.
The amplitude of the output signal from the mixer is proportional
to the relative phase of the reference signal and the phasemodulated output signal from the phase shifter. The output signal
from the mixer is sent to an oscilloscope through a low-pass filter
to suppress higher frequency harmonics generated by the mixer.
A function generator provides the modulation signal that is applied
to the phase shifter control voltage port. The amplitude of the
output signal of the function generator was held constant by
adjusting the voltage amplitude as the frequency of its output
signal was changed. The line stretcher adjusts the phase of the
high frequency reference signal to maximize the amplitude of the
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Part/Model No.
recovered modulating signal, as measured by the oscilloscope.
The 3 dB pads are added to improve the RF match at the mixer.
Measurement Test Conditions
Measurements were performed using the test set up shown in
Figure 2 and equipment listed in Table 1. The ambient
temperature was +25 °C. The control port decoupling networks
(10 pF capacitors) were removed from the standard Skyworks
PS094-315 Evaluation Board (refer to the PS094-315 Data Sheet
for the schematic).
The modulation signal was a sine wave with an amplitude of 9 V
peak-to-peak. The carrier RF frequency was 940 MHz. The carrier
frequency output power from the signal generator was
+11.5 dBm.
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APPLICATION NOTE • PHASE SHIFTER 3 DB MODULATION BANDWIDTH
Figure 4. 100 kHz Modulation Demodulator Output Voltage Response
Figure 5. 5900 kHz Modulation Demodulator Output Voltage Response (3 dB Bandwidth)
Measurement Procedure
1. Apply the 9 V peak-to-peak sine wave with a 100 kHz
modulation signal to the phase shifter VCONTROL pin. Observe
this signal on an oscilloscope to confirm its frequency and
amplitude.
2. Set the RF signal generator to 940 MHz with output power
equal to +11.5 dBm.
3. Adjust the line stretcher to maximize the voltage at the output
of the low-pass filter. The response is shown in Figure 4.
4. Record the peak-to-peak voltage at the low-pass filter output.
It may be necessary to adjust the modulating signal amplitude
to keep its magnitude constant at 9 V peak-to-peak while
increasing its frequency.
The frequency at which the output voltage as measured at the
output of the low-pass filter is reduced to 0.707 of its peak value;
this is the upper bound of its modulation bandwidth. Since the
lower bound of the modulation bandwidth is 0 Hz, the upper
bound frequency is also the modulation bandwidth.
The response is shown in Figure 5. All of the test results are
plotted in Figure 6.
5. Incrementally increase the modulation signal frequency until
the voltage measured at the output of the low-pass filter is
0.707 of its value at 100 kHz as recorded in step 4.
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APPLICATION NOTE • PHASE SHIFTER 3 DB MODULATION BANDWIDTH
Figure 6. PS094-315 Demodulated Output Voltage vs Modulation Frequency
Conclusions
References
The 3 dB modulation bandwidth is determined by phase
modulating an input RF signal with a lower frequency sine wave,
then using a phase demodulator to recover the original
modulation.
1. The IEEE Standard Dictionary of Electrical and Electronics
Terms, current edition.
The 3 dB modulation bandwidth of the PS094-315 phase shifter
was determined to be 5900 kHz at an RF frequency of 940 MHz
using the technique described in this Application Note. This
methodology may also be used to measure the modulation
bandwidth of other analog phase shifters.
4
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APPLICATION NOTE • PHASE SHIFTER 3 DB MODULATION BANDWIDTH
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