Time-Domain Measurement Method to Guard
Against Preamplifier Saturation
Matthew J. Jackson #1
#
EMC Laboratory, DENSO International America, Inc.
24777 DENSO Drive, Southfield, MI 48033, USA
1
matthew_jackson@denso-diam.com
Abstract—This paper investigates the cause of preamplifier
saturation, discusses the 1 dB compression point and considers
the ramifications of collecting data with a saturated amplifier.
Two common saturation analysis methods, the attenuator check
and band-pass filter, are considered as ways to guard against
preamplifier saturation. After a brief discussion on the
amplifier’s input signal and its composition, a time-domain
measurement method, consisting of seven steps, is introduced as
another way to prevent preamplifier saturation.
I. INTRODUCTION
When performing radiated emission measurements, many
test standards require the measurement system noise floor to
be at least 6 dB lower than the applicable limits [1]. Such a
requirement allows for sufficient margin above the noise floor,
and below the limit, to measure RF emissions.
Problems start to arise when interacting factors such as a
stringent limit, large antenna factor/cable loss and high
measurement system noise floor make it impossible to satisfy
the 6 dB requirement. Potential solutions for complying with
the ambient requirements include using: a measuring
instrument with a better noise figure, a more efficient antenna,
a shorter cable or a low-loss cable. Unfortunately, all of these
options are either expensive or logistically not possible. Many
electromagnetic compatibility (EMC) labs do not have the
resources available to acquire new equipment; they must meet
the ambient requirements using equipment they already have.
The typical solution to such a dilemma is to install a low
noise preamplifier between the antenna and measuring
instrument (e.g. spectrum analyzer or receiver). However, this
approach is frequently cautioned against because of the
potential of overload (saturation) to either the preamplifier or
measuring instrument [2]. This concern necessitates that steps
be taken to verify the measurement system is not subject to
overload when using a preamplifier.
A discussion on preventing analyzer/receiver overload is
beyond the scope of this paper. The purpose of this paper is to
focus on the steps that can be taken to verify a preamplifier is
not subject to overload. Of specific interest is using a timedomain measurement method to guard against preamplifier
saturation.
II. AMPLIFIER SATURATION
All amplifiers have a linear dynamic range. This is the
range over which the output power varies linearly with respect
978-1-4244-6307-7/10/$26.00 ©2010 IEEE
to the input power such that:
(1)
Pout (dB ) = Pin (dB ) + Linear Gain (dB )
As the output power increases to near its maximum capability,
the amplifier will begin to saturate [3]. Saturation occurs when
there is a nonlinear relationship between the input and output
power of the amplifier.
An amplifier’s maximum output is limited by its supply
voltage. Fig. 1 shows a single transistor amplifier model to
illustrate this point.
+VCC
R1
R3
C2
C1
RL
VIN
R2
R4
C3
Fig. 1. Common-emitter amplifier
The maximum peak output voltage from the amplifier in Fig.
1 would be the supply voltage (+VCC). If the level of an input
signal is too high, the amplifier output will attempt to exceed
the supply voltage and be cut off (clipped). The result is an
output signal that is lower than it should be. Fig. 2 shows an
example of a clipped sinusoidal test signal:
Amp Input
Amp Output
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
Fig. 2. Clipped sinusoidal signal
Therefore, in order to avoid amplifier saturation, the input
signal must have a level that is low enough to keep the
amplifier in its linear dynamic range.
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A. The 1 dB Compression Point
Quantifying when an amplifier will begin to saturate is
accomplished by determining the output power at 1 dB
compression. The 1 dB output compression point of an
amplifier is defined as the output power level at which the
gain deviates from the small signal gain by 1 dB [3]. Fig. 3
shows the 1 dB compression point of a Miteq AM-1300
amplifier.
1dB Compression: 1 MHz
Amplifer Output
Theoretical
1dB Down
20
The linear gain of the preamplifier is determined by
characterizing the device over a given bandwidth, and is used
as a correction factor in (3) to calculate Ef. What, then, is the
impact on measured data when using a preamplifier that is
saturated? Consider the following example.
C. Saturation Measurement Example
Say there is an input signal, Pin, that will saturate a
preamplifier. If this input signal is attenuated by 10 dB the
preamplifier will no longer saturate. The amplifier’s output
with an attenuated input is determined by:
Pout (1) (dB ) = Pin (dB) − 10 dB + G1 (dB )
15
(4)
Pout (dBm)
10
Where G1 is the linear gain of the preamplifier determined by
characterizing the device. The analyzer or receiver will
measure Pout(1) and the following correction factors will be
applied to recover Pin:
5
1 dB Compression Point
0
-5
Pin (dB ) = Pout (1) (dB ) + 10 dB − G1 (dB)
-10
(5)
-15
Fig. 3. 1 dB compression point
Now, make the same measurement without attenuating the
amplifier’s input by 10 dB. The amplifier’s output without an
attenuated input is determined by:
Because of the nonlinear relation between the input and
output power at this point, the following relationship holds [3]:
Pout ( 2) (dB) = Pin (dB) + G2 (dB)
-40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10
Pin (dBm)
(2)
Pout 1 dB = Pin 1 dB + Linear Gain − 1 dB
The 1 dB compression point will become more important
when explaining the time-domain measurement method to
guard against preamplifier saturation.
B. Making Measurements with a Preamplifier
It would be helpful at this point to briefly review how
measurements are made using a preamplifier. Consider the
following measurement system:
Ef (dBμV/m)
AF
Chamber Wall
Because the input signal, Pin, is not attenuated in (6) the
preamplifier will saturate. So, G2 is the nonlinear gain of the
preamplifier. The analyzer or receiver will measure Pout(2) and
the following correction factors will be applied to recover Pin:
Pin (dB) = Pout ( 2) (dB) − G1 (dB )
(7)
Notice that G1 is used to recover Pin in (7). This is because the
test equipment control software will use the gain from the
amplifier’s characterization (G1) when correcting the
measured data. However, because the amplifier was saturated
when Pout(2) was measured, its gain, G2, was less than G1 such
that:
G2 < G1
C1
(6)
(8)
C3
C2
It should be apparent from (6), (7) and (8) that the reported
value of Pin in (7) would be lower than the reported value of
Pin in (5). Therefore, the impact on measured data, when using
a saturated preamplifier, is incorrectly reporting lower
emission levels.
Vf (dBμV)
G (dB)
f = 500 MHz
Fig. 4. Radiated emissions measurement system
Where Ef is the electric field signal, AF is the antenna factor,
C1-3 are the cable losses (in dB), G is the preamplifier gain (in
dB) and Vf is the voltage measured by the analyzer or receiver.
Ef is related to Vf as follows:
E f = V f + AF + C1 + C 2 + C3 − G
(3)
III. COMMON SATURATION ANALYSIS METHODS
Before measuring RF emissions using a preamplifier, it is
first necessary to evaluate whether the preamplifier will
saturate during testing. The following two methods are
commonly used to guard against preamplifier saturation.
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A. Attenuator Check
The attenuator check evaluates whether a preamplifier is
saturated by running consecutive measurement scans with
different amounts of input attenuation to the preamplifier. If
the difference in magnitude response between two scans is
equal to the amount of attenuation between the scans, then the
preamplifier is not saturated. However, if the difference is less
than the amount of attenuation between the scans, the
preamplifier is saturated.
While the attenuator check works in theory, it can be very
difficult to evaluate the results because preamplifier saturation
most often occurs when testing broadband devices that
randomly emit high level emissions. A noise source that does
not cause a preamplifier to saturate 100% of the time may
yield data that looks “good” except for a few random points.
Was the preamplifier saturated during those few points? Or
was the dwell time not long enough to record the max
emission? It is questions like these that complicate the
attenuator check method.
B. Band-pass Filters
The other commonly used method for guarding against
preamplifier saturation is filtering. Preamplifiers are specified
to amplify content in a given bandwidth (e.g. 10 kHz – 1
GHz). Spectral content that is outside the measurement
window, but within the preamplifier’s bandwidth, may
saturate a preamplifier. These high level emissions in
irrelevant frequencies can be removed by installing band-pass
filters at the preamplifier’s input.
Fig. 5 shows the spectrum of a continuous time signal. Say
the content at f3 is causing the preamplifier to saturate while
measuring f1 and f2. Installing a band-pass filter that removes
the content at f3 will prevent the preamplifier from saturating.
the content causing saturation is located? Which frequencies
need to be filtered out? Depending on the noise source this
information can be very difficult to ascertain.
3) Insertion Loss: The band-pass filter will introduce more
loss into the system. This loss may be substantial enough to
negate the gain from the preamplifier.
4) Impedance Matching: Care must be taken to ensure the
impedance is matched between the antenna, band-pass filter
and preamplifier.
IV. TIME-DOMAIN MEASUREMENT METHOD
Recall that an amplifier operates by applying a linear gain
to an input voltage (see Fig. 1). If the level of an input signal
is too high, the amplifier will saturate. Therefore, in order to
avoid amplifier saturation, an input signal must have a level
that is low enough to keep the amplifier in its linear dynamic
range. This implies that amplifier saturation can be prevented
by measuring the input signal in the time-domain, and
verifying its level is not too high.
A. Understanding the Input Signal
Before exploring the time-domain measurement method, it
is first necessary to understand the composition of the input
signal, v(t), to the preamplifier. Fig. 6 shows a noise source
that emits RF emissions (electromagnetic waves).
Fig. 6. Preamplifier input/output signal
These emissions propagate through the air and are received by
the antenna system. The signal, v(t), is the electric field
incident wave voltage generated by the antenna and is
determined by:
Band-pass
Filter
N
v(t ) = A0 +
∑A
(9)
k cos(2π f k t + φ k )
k =1
-f3
-f2 -f1
f1
f2
f3
f
Fig. 5. Spectrum of continuous time signal
As with the attenuator check method, there are also a few
drawbacks that arise when using a band-pass filter to prevent
preamplifier saturation.
Where N, A0, Ak, fk and φk are parameters that specify the
signal v(t). Taking the Fourier transform of v(t) would yield
the following (two-sided) spectrum:
1) Knowing When to Use: A band-pass filter is only
required if there is a saturation problem. Unfortunately, this
method does not check for preamplifier saturation, it only
prevents it. Therefore, another method must first be used to
evaluate the preamplifier for saturation.
2) Knowing Where to Use: The objective of using a bandpass filter is to remove high level emissions in irrelevant
frequencies. But, how does one know where, in the spectrum,
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α0
α1
α
*
α1
*
N
-fN
αN
…
-f1
f1
…
Fig. 7. Generic spectrum of v(t)
fN
f
Where the DC term (corresponding to zero frequency) is
given by:
α 0 = A0
(10)
the input power, at 1 dB compression, has to be reduced in
order to bring the preamplifier back into its linear dynamic
range. Typically Scf should be 3-5 dB.
2) STEP 2: Convert Pin to a peak voltage (Vpk):
The coefficients for the positive frequencies are given by:
V pk = ±
1
α k = Ak e jφk , k = 1, ... , N
2
(11)
and the coefficients for the negative frequencies are the
complex conjugates of (11).
From (9) it is clear that the amplitude of v(t) is directly
related to the amount of content being measured (N), the
magnitude (Ak) of each sinusoid and the phase (φk) of each
sinusoid. This implies that the amplitude of multiple sinusoids,
when nearly in phase, will sum and contribute to the overall
level of v(t). This means the potential for preamplifier
saturation is greater when there are many high level spectral
components in the frequency domain. Preamplifier saturation
is not always caused by just one dominate signal; many small
signals can combine to produce a large v(t).
Therefore, it is necessary to measure the input signal to the
preamplifier and verify its level will not cause saturation. This
is best done in the time-domain using an oscilloscope.
B. Making the Time-Domain Measurement
Fig. 8 shows the measurement setup that will be used to
evaluate v(t).
(Z )(0.001)⎛⎜⎝10 (Pin 10) ⎞⎟⎠
(13)
Where Z is the input impedance of the preamplifier (typically
50 Ω). The peak voltage calculated in (13) is the maximum
input signal level the preamplifier can have without saturating.
3) STEP 3: Correct for the insertion loss of cables, C2 and
C3. Ideally, the oscilloscope would be connected to C1 in Fig.
8. However, this is not possible because moving the
oscilloscope into the chamber will introduce additional RF
emissions. Therefore, the limit against which the time-domain
measurements will be compared is determined by:
Vlimit =
V pk
10 ((C2 +C3 ) 20 )
(14)
Where C2 and C3 are the max insertion losses (in dB) for each
cable over the frequency range of interest.
4) STEP 4: Select an oscilloscope with an input sensitivity
that can accurately measure Vlimit. An oscilloscope with an
input sensitivity of at least 1 mV/div should be sufficient since
most Vlimit values will be approximately ±20 mV. Also verify
the bandwidth of the oscilloscope is at least equal to the upper
frequency limit of the preamplifier, and the sampling rate of
the oscilloscope complies with the Nyquist-Shannon sampling
theorem (fs > 2∗fmax). Most oscilloscopes today have a
sampling rate that is at least five to ten times the bandwidth.
5) STEP 5: Adjust the oscilloscope settings: set input
impedance of oscilloscope to 50 Ω, turn ON max and min
voltage measurements, turn ON measurement statistics (if
applicable), set oscilloscope to trigger on rising and falling
edges and turn ON infinite persistence. While the antenna
should function as a suitable “band-pass filter” for the
oscilloscope, the scope may also be set to have its input
bandwidth limited (not available in all oscilloscopes).
Fig. 8. Time-domain measurement setup
Notice the preamplifier is not installed in the system and the
analyzer/receiver has been replaced with an oscilloscope. The
following steps should be completed to complete the timedomain measurement:
1) STEP 1: Calculate the max input signal level (Pin) that
will not saturate the preamplifier:
Pin = Pout 1 dB − G + 1 − S cf
6) STEP 6: Setup the DUT, antenna and oscilloscope
according to Fig. 8. For each DUT orientation, DUT mode,
antenna and antenna polarization, adjust the oscilloscope’s
horizontal and vertical scales and monitor v(t) for
approximately 5-10 minutes, or until no higher levels are
recorded. See Fig. 9 for a screenshot of a sample measurement:
(12)
Where Pout 1 dB is the rated output power at 1 dB compression,
G is the rated gain (in dB) and Scf is the saturation correction
factor. The saturation correction factor describes how much
482
V. CONCLUSION
This paper has shown how an amplifier operates by
applying a linear gain to an input voltage. If the level of an
input signal is too high, an amplifier’s output will attempt to
exceed its supply voltage and saturate. Saturation can be
prevented by first determining the maximum input signal level
an amplifier can have without saturating. Then, measure in the
time-domain the actual input signal, and verify its level never
exceeds the amplifier’s maximum permissible input level. An
input signal that does not exceed the max input limit will not
cause an amplifier to saturate.
Fig. 9. Sample v(t) measurement
At the conclusion of each measurement, compare the
measured max and min voltages against the preamplifier’s
Vlimit.
ACKNOWLEDGMENT
I would like to thank the late Bob Peters who helped me to
think outside the frequency domain by adamantly insisting the
voltage applied to an amplifier’s input is what really matters
when trying to guard against saturation.
7) STEP 7: Determine if the preamplifier will saturate. If
the measured max or min from step 6 exceeds Vlimit, the
preamplifier will saturate. In such a case, the preamplifier
should not be installed in the measurement system.
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REFERENCES
[1] Vehicles, boats and internal combustion engines – Radio disturbance
characteristics – Limits and methods of measurement for the protection
of on-board receivers, CISPR 25, 2008.
[2] Component and Subsystem Electromagnetic Compatibility: Worldwide
Requirements and Test Procedures, Ford ES-XW7T-1A278-AC, 2003.
[3] Miteq Amplifier Specification Definitions, Miteq, 2007.