T
oday’s wireless communications market,
from cellular phones to wireless data, is
expanding at an incredible rate. Along
with this growth comes an increasing need for
test equipment that verifies the performance
of these devices and systems. Signal generators play a multifaceted role in the development of both receivers and transmitters. They
are used for generating signals ranging from
simple sinusoidal tones for LO substitution to
fully modulated signals
for receiver testing.
This article focuses on
the importance of using a signal generator
with relatively high
spectral purity for RF
communications testing. The ideal signal
generator would provide perfect sinusoids
at carrier and sideband
frequencies, but in reality all signals have
imperfections. The
foresight to take these
flaws into account allows the engineer to select the appropriate signal generator and reduce development time.
WHAT IS SPECTRAL PURITY?
Spectral purity is the inherent frequency stability of a signal. Stability is defined over a period of time: short or long term. Long-term stability, or drift, is usually defined as frequency
changes over a period of time greater than one
second. Short-term stability is defined as fre-
quency changes over less than one second.
Current signal generator technology generally
offers good long- and short-term stability. For
wireless communications testing, short-term
stability is of greater concern. This article discusses key spectral purity components and the
importance of spectral purity in testing wireless
communications equipment. Implications of
spectral purity are briefly covered for LO substitution, phase noise measurements, receiver
performance tests and radar applications.
Phase Noise
Perhaps the most common method for
specifying the spectral purity of a signal generator is its phase noise. In the time domain,
phase noise is exhibited as a jitter in the zero
crossings of a sine wave, as shown in Figure 1.
For a high performance signal generator, the
phase noise is not usually discernible in the
time domain. In the frequency domain, the
phase noise appears as noise sidebands on the
SIGNAL GENERATOR
SPECTRAL PURITY
CONSIDERATIONS IN RF
COMMUNICATIONS TESTING
TUTORIAL
BRIAN CHENG
Hewlett-Packard Co.,
Microwave Instruments Division
Santa Rosa, CA
“...in reality all signals have
imperfections. The foresight
to take these flaws into
account allows the engineer
to select the appropriate
signal generator and reduce
development time.”
▲Fig. 1 Time domain phase noise jitter.
PHASE NOISE
V (t)
TIME (t)
carrier, as shown in Figure 2. The
US National Bureau of Standards defines single-sideband (SSB) phase
noise +(f) as the ratio of the noise
power in a 1 Hz bandwidth at a frequency f away from the carrier to the
signal power of the carrier:
+(f) is expressed as decibels relative
to the carrier per hertz (dBc/Hz). A 1
Hz bandwidth is used to allow the
phase noise in other bandwidths to be
easily calculated for comparison.
The SSB phase noise at a specified
carrier frequency is often graphically
represented on a log-log plot, as
shown in Figure 3. Phase noise can
be conveniently displayed for a wide
range of frequency offsets by using a
log scale on the frequency axis.
+(f) =
noise power in a 1 Hz
bandwidth at a frequency
f (Hz) away from the carrier
power level of the carrier
Spurious: Harmonics,
Subharmonics and Nonharmonics
Spurious signals are frequency
spikes that appear in the spectrum.
These spectral components may be
divided into three categories: harmonic, subharmonic and nonharmonic, as shown in Figure 4.
Harmonics are generated by device nonlinearities in the signal generator and are integer multiples of
the carrier frequency. For example, a
100 MHz carrier frequency will have
harmonics at 200 MHz, 300 MHz
and so on. The amplitudes of the harmonics (relative to the amplitude of
the carrier signal) are determined by
the nonlinear characteristics of the
components in the signal generator.
Subharmonics are generated when
frequency multiplying to create the
carrier frequency. The frequency being multiplied may leak through the
signal path and appear at the output.
For example, a 500 MHz signal multiplied by two to arrive at a 1 GHz
carrier frequency might appear as a
subharmonic.
Nonharmonics are frequency components that do not appear related to
the carrier frequency. Although signal
generator designers can determine
the location of these spurious signals,
they are unpredictable to the user.
Today’s signal generators are able to
suppress harmonics, subharmonics
and nonharmonics to a level acceptable for most applications.
Residual FM
Residual FM is another method
commonly used to specify the frequency stability of signal generators.
Residual FM includes the effects of
both spurious signals and phase
noise. It is the integral of the SSB
curve with limits set by the post-detection bandwidth. Common bandwidths are 300 Hz to 3 kHz and 20
Hz to 15 kHz.
SPECTRAL PURITY
CONSIDERATIONS
IN RF RECEIVER DESIGN
A spectrally pure signal generator
provides high value to those designing and verifying analog and digital
communications devices. As an example, a simple communications receiver, shown in Figure 5, is used to illustrate the effects of phase noise and
spurious signals on practical applications and measurements. Three major applications discussed here are
LO substitution, phase noise measurements and receiver performance
tests. All of these applications require
the use of a signal generator with sufficient spectral purity.
LO Substitution
In receiver development, as well
as transmitter development, a spectrally clean LO is required for upconversion and downconversion of signals. A signal generator is often used
to substitute an onboard LO for testing and system troubleshooting.
Looking at the downconversion in the
receiver, the importance of spectral
purity for LO substitution is readily
apparent. Suppose that two signals
are present at the input of the receiver, as shown in Figure 6. These signals are mixed with an LO signal
down to an intermediate frequency
(IF) where highly selective IF filters
separate one of the signals for amplification, detection and baseband processing. If the desired signal is the
larger signal, there is no difficulty in
recovering it.
On the other hand, a problem
might arise if the desired signal is the
smaller of the two because any phase
noise on the LO signal is translated
directly to the mixer products. Notice
that the translated noise in the mixer
output completely masks the smaller
signal. Even though the receiver’s IF
filtering might be sufficient to re-
TUTORIAL
▲ Fig. 2 A frequency carrier (a) without
and (b) with phase noise sidebands.
▼
Fig. 3 A typical phase noise plot.
▲
Fig. 4 Harmonic, subharmonic
and nonharmonic signals.
▼
Fig. 5 A simple communications receiver.
AMPLITUDE
f
0
(a)
(b)
FREQUENCY
AMPLITUDE
f
FREQUENCY
+(f)
FREQUENCY OFFSET FROM CARRIER (f)
SUB-
HARMONICS
AMPLITUDE
0.5 f
0
CW OUTPUT
PHASE
NOISE
f
0
FREQUENCY
HARMONIC
SPUR
NON-
HARMONIC
SPUR
2 f
0
RF
MIXERLOFILTER
0
PRESELECTOR
AMLIFIER
A/D TO DSP
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