application
Noise Figure Measurement Accuracy
Application Note 57-2
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Table of contents
Chapter 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Accurate noise figure measurements mean money . . . . . . . . . . . . . . . . . . .5
What will reading this note do for you? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Chapter 2
What factors affect noise figure measurement accuracy? . . . . . . . . . . . . . . . .7
Sources of error that can be eliminated . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Sources of error that cannot be eliminated . . . . . . . . . . . . . . . . . . . . . . . . .12
Chapter 3
Methods to calculate accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Cascade noise figure accuracy equation . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Measurement simulation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Chapter 4
A practical look at noise figure accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
The uncertainty versus measurement system noise figure (NF2) curve .15
Noise figure accuracy misconceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Chapter 5
Improving measurement accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
1. Eliminate all removable errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
2. “Increase” device gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
3. Reduce measurement system noise figure (NF2) . . . . . . . . . . . . . . . . . .18
4. Reduce individual uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Appendix
Amplifier and transistor curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Mixer and receiver curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
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Chapter 1
Introduction
Accurate noise figure measurements mean money
Anyone involved in the low-noise microwave business
knows that noise figure is a “money number.” When
measuring and specifying noise figure, the more
accurate a noise figure measurement is, the smaller
the uncertainty guardband needed on the noise figure
specification. The smaller the uncertainty guardband,
the lower the noise figure specification. The lower
the noise figure specification the higher the price
charged.
Low-noise device and system manufacturers want
accurate noise figure measurements for higher
production yields. If a manufacturer of 2 dB noise
figure amplifiers measures to an accuracy of .5 dB,
the amplifiers must measure at most 1.5 dB to be
sure the customer gets 2 dB. If the measurement
accuracy can be tightened to 0.2 dB, amplifiers
measuring 1.8 dB can be shipped.
There are other reasons why accurate noise figure
measurements are important. In addition to improved
yield, manufacturers also want accurate noise figure
measurements so they can price their product higher.
Users of low-noise devices and systems want accurate
measurements to verify they get the performance
they paid for.
Although accurate noise figure measurements are
very important, measurement accuracy is seldom
calculated. Since many things affect a noise figure
measurement, calculating accuracy is complicated
and time consuming.
What will reading this note do for you?
This note is designed to show you these things:
1. Many things can affect the accurate measurement
of noise figure (Chapter 2).
2. You can estimate your measurement accuracy
using statistically-generated curves (Chapter 3).
3. Although it appears complicated, noise figure
accuracy can be practically understood (Chapter 4).
4. You can improve your noise figure measurement
accuracy in a systematic way (Chapter 5).
HP 8970B Noise Figure Meter (10 to 1600 MHz).
HP 8970T Noise Figure Measurement System (10MHz to 18 MHz).
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Chapter 2
What factors affect noise figure measurement accuracy?
Because noise figure is an extremely sensitive (i.e.,
low-level) measurement, many more factors affect
its accurate measurement than other, higher power
measurements.
People measuring noise figure often make the mistake
of considering only one, two, or a few factors when
estimating measurement accuracy. Most often,
considering only a few factors does not adequately
represent true accuracy.
This chapter discusses many error sources in noise
figure measurements. These error sources can be
divided into two groups.
1. Sources of error that can be eliminated
2. Sources of error that cannot be eliminated
Sources of error that can be eliminated include:
a. Dirty or bad connectors (they cause reflections
and make the measurement susceptible to stray
signals)
b. EM susceptance (stray signals are measured as
noise power)
c. Impedance change between “on” and “off”
(especially a problem with transistor
measurements)
d. Using insertion gain instead of available gain (this
causes an error when correcting for second stage
noise contribution)
e. Noise figure and gain discontinuities in the
measurement system (this can cause an error if
the noise figure meter does not tune exactly to
the same frequency between calibration and
measurement)
f. Jitter (the random nature of noise makes one
noise reading different than the next)
g. Double-sideband measurements (a problem with
devices whose noise figure and gain response
varies abruptly with frequency)
Sources of error that cannot be eliminated are
combined and represent the overall accuracy of a
noise figure measurement. They include:
a. Device Noise Figure and Gain (the higher these
are, the more accurate the measurement)
b. Individual Uncertainties–mismatch, ENR, and
instrumentation uncertainty (the lower these are,
the more accurate the measurement)
c. Measurement System Noise Figure (the lower this
is, the more accurate the measurement)
Sources of error that can be eliminated
Before the days of automatic noise figure meters,
eliminating some sources of measurement error was
difficult. If those sources were not minimized, they
were simply ignored and the errors accepted. Today,
with automatic meters like the HP 8970A and 8970B,
many error sources are conveniently eliminated.
There are some sources of error, however, only good
measurement practice can eliminate. Here are those
error sources.
Dirty or Bad connectors: It only takes a small
amount of dirt in a connector to cause insufficient
contact and allow extraneous signals to couple into
the measurement. (And it only takes one dirty
connector to spread dirt to many.)
If there is visible dirt on your connectors, clean them.
A cotton swab and isopropyl alcohol work well.
Connectors do not last forever; they wear. Connectors
with worn plating on the inner or outer conductors
should be replaced. This eliminates the possibility of
loose, intermittent connections.
To learn more about proper connector care, ask your
HP sales representative for Service Note 346B-4
“HP 346B Noise Source RF Connector Care”
(literature Number 00346-90023).
EM Susceptance: Most measurements are made
in environments where stray signals are present.
Since few noise figure measurers have the luxury
of screen rooms, many times these stray signals get
coupled into the measurement. Signals emitted
from computers and other instruments, fluorescent
lights, local broadcast stations, etc., can get coupled
into a measurement through non-threaded connectors
or non-shielded cables.
The HP 8970B Noise Figure Meter eliminates many sources of possible
measurement error—noise contribution from the measurement system,
noise source “off temperature” different than 290K, and compensation
for losses before or after the device under test.
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Stray signals can cause several tenths of a dB
difference from one measurement sweep to the next
(thus appearing as an unstable measurement).
Stray signals can be easily traced. Shake the
connectors in the measurement signal path and see
if the noise figure changes significantly. If so, you
may have a bad or dirty connector susceptible to
stray signals. Another test is to attach an “antenna”
(wire, etc., ) to a spectrum analyzer and look for
stray signals at the measurement frequency. A third
test, when measuring with an HP 8970A or 8970B, is
to connect to 20 MHz IF output on the HP 8970 rear
panel to a spectrum analyzer. A stray signal will
show up as a spike on the 20 MHz passband.
How are stray signal problems avoided? First, use
threaded connectors in the signal path whenever
possible (non-threaded connectors, like BNC, are
very susceptible to stray signals). Second, use double
shielded cables. Third, enclose the device you are
measuring in a shielding container (this is especially
important if you are measuring noise figure on an
open PC board).
When down-converting measurements, avoid setting
your noise figure meter to intermediate frequencies
(IFs) that could be radiated in your work environment.
Avoid computer clock frequencies. Avoid paging
system frequencies. Avoid common multiple-often
IFs (10, 20, 30 MHz); instead, use IFs like 26 or 27 MHz.
Impedance change between “on” and “off” of
the noise source: When a noise source turns on, the
noise-generating avalanche diode appears as a short
circuit. When off, it appears as an open. Most noise
sources have a matching pad at their output to
minimize the on-to-off impedance difference. In
some measurements, this pad does not provide
sufficient matching and the small impedance
difference can cause errors.
One measurement where this can cause errors is
transistor characterization. In this measurement, the
device-under-test (DUT) is subjected to multiple
input impedances and the corresponding noise
figures and gains are measured. After measuring
at several input impedances, circles of constant
noise figure are plotted and the impedance giving
minimum noise figure is calculated. Since the input
impedance needs to be care-fully controlled, an
on-to-off impedance change from the noise source
is undesirable. Transistor characterization
measurements are generally made using speciallydesigned noise sources with very small on-to-off
impedance changes (like the HP 346A pictured below).
The HP 346A Noise Source exhibits a much
lower impedance change between on and
off than normal noise sources (for
transistor characterization and
other impedance-sensitive
measurements.)
Stray signals can be seen on a spectrum analyzer.
Figure 2.1 Signals can get coupled into a noise figure
measurement from many different sources (a) flourescent lights
(b) computer and other instruments (c) local tv/radio stations
(d) remote paging systems (e) land-mobile radio.
(b)
(a)
(c)
(e)
(d)
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Using insertion gain instead of available gain: A
noise figure meter measures noise figure and insertion
gain1. The equation to correct for second stage
contribution requires available gain2, not insertion
gain. An automatic noise figure meter assuming the
insertion gain IS the available gain, uses insertion
gain in the second stage correction equation. This
assumption results in an error.
This error is generally considered to be a mismatch
error and is figured into the overall noise figure
measurement uncertainty. The error can be removed
by measuring the reflection coefficients of the device
output ( o), the source ( s) and the load ( l) and
substituting it into the equation for available gain
Available gain is then substituted into the cascade
gain equation to calculate the noise figure of the
DUT (F1).
F12is the combined DUT/measurement system noise
figure, F2is the measurement system noise figure.
All terms in the above two equations are linear, not
dB, terms. (More about Eq. 2.2 in Chapter 3.)
Noise figure and gain discontinuities in the
measurement system: Occasionally, there is a
component in the measurement system (the downconverting mixer, an amplifier, etc.) that has a noise
figure or gain discontinuity (see Figure 2.2).
Although measurement system noise figure and gain
is measured during the calibration and factored-out
during the measurement, a sharp discontinuity can
cause problems. If the measurement system does not
perfectly tune to the same frequency from calibration
to measurement, it can measure the discontinuity
during calibration and not during measurement. The
noise figure meter then corrects for more second
stage noise than actually present. This results in a
measurement error.
There are a few ways to avoid this problem. First,
see that your measurement system noise figure
and gain response is flat. To do this, measure
system noise figure and gain in very small frequency
increments (1 to 5 MHz). If the response looks fairly
flat, there should be no discontinuity problems.
Second, if a discontinuity is present, use a high gain,
low-noise preamplifier at the measurement system
input. The preamplifier reduces the noise figure
contribution of the measurement system, including
discontinuities. Third, calibrate at the frequencies
you want to measure and approach the measurement
frequency in the same direction as the calibrated
frequency (i.e., from low to high frequency). This
helps avoid non-repeatable tuning due to hysteresis
of any YIG oscillator or YIG filter in the measurement
system.
Jitter: All noise measurements, because of the
random nature of noise, exhibit some degree of
instability, or jitter. The amount of jitter is a function
of the device measured (its noise figure and gain)
and the design of the measuring instrument.
Eq. 2.1
Eq. 2.2
Frequency
(a) Tuning for Calibration (b) Tuning for Measurement
Measurement
System Noise
Figure (or Gain)
Noise Figure Meter
IF Filter
Frequency
Measurement
System Noise
Figure (or Gain)
1. Insertion gain: The gain is measured by inserting the DUT between a generator and load. The numerator of
the ratio is the power delivered to the load while the DUT is inserted. The denominator, or reference power,
is the power delivered to the load while the source is directly connected.
2. Available gain: The ratio of power available from the output of the network to the power available from
the source.
Figure 2.2 If the measurement system has noise figure or gain discontinuities, non-repeatable tuning between (a) calibration and (b)
measurement can cause errors when correcting for second stage contribution.
Figure 2.3 Microwave noise figure measurement set-up
(double-sideband measurement).
Figure 2.4 Double-sideband measurements (1) can cause sideband averaging errors, (2) allow 3rd harmonics to be down-converted
and (3) allow other spurious signals to be coupled into the measurement.
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Jitter is minimized by averaging many measurements.
(With the HP 8970A or 8970B, jitter can be reduced
to less than .02 dB.) Averaging, or smoothing,
decreases jitter at the expense of decreased
measurement speed.
Since jitter can be reduced to such a small value
(.02 dB), it will not be considered when we calculate
accuracy. If you noise figure meter cannot minimize
jitter to such a small value, you will need to add an
additional jitter error term when determining
uncertainty.
Double-sideband measurements: This section discusses down-converted microwave measurements of
amplifiers and transistors (i.e., non-frequency translating devices).
When a noise bandwidth is down-converted from a
microwave frequency to the tunable range of a noise
figure meter, the desired noise band is not the only
noise down-converted. There are two main noise
bands down-converted plus possible harmonic noise
bands. If a measurement system allows both main
noise bands to be down-converted, it is a doublesideband (DSB) measurement system. If the measurement system prevents one sideband (commonly
known as the image band ) and allows the other
noise sideband (the desired band) to be downconverted, it is a single-sideband (SSB) measurement
system. Because DSB measurements allow more
than one band of noise to be down-converted, errors
may be introduced in the measurement.
One source of error in DSB measurements involves
the two main down-converted noise bands (fLO+ f
IF
and fLO- fIF). The noise figure meter, tuned to the IF,
measures the combined noise from the two downconverted frequencies. The noise figure meter does
not “know” the noise it measures is from two bands.
Because of this, the noise figure value displayed is
an average of the two down-converted bands. If the
device response between the two sidebands is not
linear, the average value can differ from the actual
value. (see (1) in Figure 2.4). (If the device measured has a flat frequency response, like a broadband
amplifier, there will probably not be much sidebandaveraging error.)
RF IF
LO
Output
Input
Noise
Source
Calibratioin
DUT
Local
Oscillator
HP-IB
Noise Source
Drive
Noise Figure Meter
> (1)
Device
Input
(3) Spurious Signal
Leaks Through Mixer
Noise Output
From Device
Downconverted Noise
(2)
Frequency
fLO – f
IF
fLO + f
IF
f
LO
3fLO – f
IF
3fLO + f
IF
3f
LO
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