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Title & Document Type: Spectrum Analysis: Noise Measurements - Application Note 150-4

Manual Part Number: 5952-1147

Revision Date: April 1974

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-1"-- .



.,.. ~ Noise Measurements

Printed April 1974


INTR 0 D UCTI ON. """""" 1
Review of Spectrum Analyzer Basics """""""""""""""""""""""" 2
CHAPTER 1. Impulse Noise Measurements """""""""""""""""""""""""""""" 3
Dynamic Range Considerations """""""""""""""""""""""""""" 5
Summary. """ "... """" """"""""""'" 6
CHAPTER 2. Random Noise Measurement """""""""""""""""""""""""""""" 7
J)etector Characteristics ... """""""""" c """ 8
Logarithmic Shaping """""""""""'" "" ""'" 8
Averaging :. """'" 8
Random Noise Measurement-Summary """"""""""""""""""""" 9
Dynamic Range Considerations 10
Narrow Video Bandwidths 11
CHAPTER 3. Carrier-to- Noise Ratio """""""""""""" ..12

CHAPTER 4. Amplifier Noise Figure Measurements 13
Measurement Procedure... """""" """" """"""""" .14
Sensitivity Calculations "... 15
Example Measurements 15
CHAPTER 5. White Noise Loading 17
Dynamic Range ... """"""""""" """ """ 17
CHAPTER 6. Oscillator Spectral Purity
Residual AM .
... ""'"
"""""""""""" """"
Sensi tivi ty ""'" ...20
Residual Phase Modulation (PM) 21
Voltage-tuned Oscillators """""""'" 21
Phase-lock Oscillators """"""""" """'" .22
Fixed Oscillators ,... ""'" 23
Microwave Fixed Oscillators ... ..23
Measurement Notes """"""""" , 24
Alternate Technique for Residual AM 24
Alternate PM Techniques 25
Examples """"""'" """"" """""""""""""" .25

APPEND IX .. ..26
" Determination of Maximum Input Noise Power 26
Response to Noise of the Spectrum Analyzer in the Log Mode 26
Typical Noise Sidebands for Model 8553B
1 kHz 110 MHz Spectrum Analyzer 28
Typical Low Frequency Sensitivity for Model 8556A
20 Hz 300 kHz Spectrum Analyzer 28
Correction Factors (Impulse Noise) 29
Correction Factors (Random Noise) 29

Rev. 7/15/80


This Application Note deals with the measurement of noise with the spectrum
analyzer. In order to organize our discussion, some working definition of the term
"noise" is required.

When we think of noise, we usually think in terms of the effects of the noise. For
example, receiver designers may think of audible noise in a received signal; computer
designers may think of spurious "bits" caused by transients in the system.

For the purpose of this note, we shall define noise as any signal which has its energy
present over a frequency band significantly wider than the spectrum analyzer's resolu-
tion bandwidth, i.e., any signal where individual spectral components are not resolved.
This includes both desired and undesired signals. For example, white noise may be used
in audio testing as a desired signal.


Figure 1. The left photo represents a response to a CW signal present at the spectrum analyzer input.
The right photo shows a display of random noise. Signals of this type will be analyzed by the methods of
this Application Note.

Since noise is present over a wide band of frequencies, the total voltage or power
measured by the spectrum analyzer will depend on the resolution bandwidth used. For
this reison, any noise measurement must include the bandwidth in which the measure-
ment was made, e.g., dBm/Hz, volts/MHz, etc.

Two basic types of noise will be discussed in this note, random noise and impulse
noise. Random noise is generated by heat in resistors and other continuous processes.
Impulse noise is generated by switching and transient phenomena and is characterized
by the launching of discrete impulses in time.


A few points about the operation of a spectrum analyzer are pertinent to the later
1". discussion. Let's look at the basic block diagram:





Figure 2. A responseappearson the CRTwheneverF. :t:Flc = FIF.
Example:For the 8553B 110 MHz
Spectrum Analyzer, FIF 200MHz; F.
= = -
0 110 MHz; and Flc = 200 310 MHz. Then, for an input signal

at 50 MHz, the local oscillator would be tuned to 250 MHz to get a 200 MHz difference frequency and a
response on the CRT.

An input signal is mixed with a swept local oscillator in the input mixer. This
mixing product passes through the IF filters and amplifiers, and the detected output is
displayed on the vertical axis of the CRT.

If a CW signal is present at the input, and the local oscillator is swept over the
range necessary to display this signal (F. = FLO :t: F1F)' then the resultant display will
be the IF bandpass filter shape of the spectrum analyzer. Therefore, the shape and band-
width of these filters determine both the resolution of the spectrum analyzer and the
measurement bandwidth for noise measurements. t
The spectrum analyzer will accurately reproduce the amplitude of signals which
are ::;-10 dBm at the input mixer. An input attenuator ahead of the mixer allows
adjusting the input level to the proper range. Broadband signals may have considerable
total energy, while the energy at any single frequency is small. This will result in a
decreased dynamic range. This effect is discussed in more detail later in the note.



. 60 dB Bandwidth
Determined by Filter
Shope Focto<

Figure 3. The IF filter shapeof the spectrumanalyzeris traced out whenevera CWsignal is displayed.
The3 dB bandwidthis determined the setting of the bandwidthcontrol; the 60 dB bandwidthis a prop-
erty of the IF filter.


As was mentioned earlier, impulse noise is phase coherent. That is, each spectral
component at any instant is coherent in phase to all other spectral components. For
this reason, as the measurement bandwidth is doubled, the measured noise voltage

An impulse generates a voltage across the spectrum analyzer IF which is dependent
upon bandwidth. The peak voltage displayed will be dependent on the bandwidth cho-
sen. Therefore an impulse noise measurement must be nonnalized to the instrument's
impulse bandwidth, which is defined as the ideal rectangular filter bandwidth with the
same voltage response as the actual instrument IF filter. (See Figure 4.)

The units of measurement, then, will be in volts/Hz or voltage per unit bandwidth.
For example, measurements of electromagnetic interference (EMI) are usually made in
decibels referred to one microvolt per megahertz (dBp.V/MHz).

To measure the spectrum analyzer impulse bandwidths, use the following pro-

1. Connect a signal generator to the spectrum analyzer input.
2. Tune to the signal on the spectrum analyzer, and display the signal generator
output in the linear display mode.
r 3. Adjust the output amplitude of the signal generator for an 8-division deflection
at the peak of the response.
4. Reduce the scan width until the display almost fills the CRT. (See Figure 5.)
5. Measure the area under the curve by counting squares or integrating from a
photo of the display. Divide the area by 8 to obtain the impulse bandwidth.
The calibration of the horizontal axis is given by the setting of the scan width

Additional methods are discussed in some detail, and a theory of measurement is
given in Application Note 142, "EMI Measurement Procedure."


Impulse Bandwidth

,.. Figure impulse bandwidthis definedby an ideal filter
4.The with identical voltage response.


Figure 5. Displayadjustedso that the filter responsealmost fills the CRT.

The detector in the spectrum analyzer is an envelope detector. For impulse meas-
urements, this is the type of detection which is needed. The detector responds to the
peaks of the transient signals, and the CRT acts as a "peak hold" to display the
resultant output.

Note: The video filter must not be used since this peak reading capability would
be destroyed.

80, in order to measure impulse noise, we need to determine
CRT, convert to units of voltage, and normalize to some impulse bandwidth.
the response on the
Although voltage can be read directly from the analyzer in the linear display mode,
the log mode is preferred to allow a wider measurement range. The calibration in dBm
can readily be converted to voltage from the following relationship:
0 dBm (50 0) = +107 dBJkV (50 0)

To normalize to a given bandwidth, we can use a correction factor in decibels to
be subtracted from any reading. This is arrived at from the expression:
S(dBJkV IBW1) = V(dBJkV) - B(dB[BW1))
8 = Broadband spectral intensity normalized to bandwidth, BW1
V = Voltage measured on the CRT in bandwidth, BWj
B = Correction factor

" When we double the bandwidth we double the impulse noise voltage, so the
difference in dB between signals observed in two bandwidths is .1.dB = 20 log BWAI
BWB. Therefore, B can be determined from the following relationship:
BWj =Spectrum Analyzer impulse bandwidth
BW 1 =Bandwidth to be normalized to
Let's normalize to a 1 MHz bandwidth with an analyzer which has a 140 kHz
impulse bandwidth. t



10 dBlDiv

0 100 MHz

Figure6. Example: mpulsenoise level at 70 MHzis -47 dBm.We add 107 dB to get +60 dBI'V. Sub-
tracting the bandwidthcorrectionfactor, we get 77.1dBp.V

140 kHz
B =20 log 1 MHz
= -17.1 dBMHz
and S =V - (-17.1 dBMHz)
Therefore, if we measure a signal at -47 dBm on the CRT in a 140 kHz impulse
bandwidth, and we desire the spectral intensity in dBp.V/MHz, we proceed as .follows:

1. -47dm/ 140 kHz +107 dBp.V/dBm = +60 dBp.V/140 kHz
r~ 2. 60 dBp.V/140 kHz - (-17.1 dBMHz) = +77.1 dBp.V/MHz


First, let's look at the means for obtaining maximum sensitivity. If we change the
bandwidth setting on the spectrum analyzer, we change the total noise voltage measured
by the analyzer. Furthermore, since making the bandwidth 10 times wider gives 10
times the noise voltage, the signal level displayed on the CRT will increase by 20 dB.

A 10 times increase in bandwidth causes the spectrum analyzer internal noise to
increase by 10 dB. (This will be discussed in the section on random noise.) Therefore,
10 dB improvement in signal-to-noise ratio can be obtained by increasing the bandwidth
by a factor of 10. Wide bandwidths should be used for impulse noise measurements.

To determine the. available dynamic range, let's take some typical numbers. For
this example, we will use the 110 MHz Spectrum Analyzer, Model 8553B.

In the 100 kHz bandwidth, the analyzer's average noise level is -100 dBm or
+7 dBp.V. The overload, or gain compression, point is -10 dBm or +97 dBp.V.

If a signal is inserted in the input of the analyzer which has a total energy of
+97 dBp.V across the frequency range from 0 to 120 MHz (the cutoff frequency of the
input filter), we can calculate the worst case dynamic range. We will use a typical
number of 140 kHz impulse bandwidth in the 100 kHz IF bandwidth position.
120 MHz
= 58.7 dB
". B = 20 log
140 kHz


t) )

+!I7dBpV 1 dB Gain Compression

MaximumInput for
+67 dBpV 70 dB Spurious.free
Maximum Acheivabla
Measuremant Range
for 300 kHz Bandwidth

+12dBpV Noi.. LeY" in 300 kHz Bandwidth

Figure 7. Maximumachievablemeasurement angewould be realizedby limiting input noise to 300 kHz
bandwidthbefore the input mixer of the spectrumanalyzer.For actual analyzerwithout accessories,nput
bandwidthequals120 MHz.
+97 dB~V/120MHz~ +47 dB~V/300kHz
Measurement Range = +47 dB~V -12 dB~V = 35 dB (worst case)

+97 dB}.' - (58.7dB [140kHz]) =
V 38.3 dB}! V /140 kHz

The signal-to-noiseratio would then be 38.3 dB}!V -7 dB}.'V equals 31.3 dB. .
If broadband noise exists over a wide enough band, it becomes impossible to de-
tect the level on the CRT, and the dynamic range becomes effectively zero. For example,
if the +97 dB}.' signal existed over a 3600 MHz band, the signal level would be less
than 7 dB}.' /140 kHz, and no signal would be detected.


Measure the signal level in dBm.

Add 107 dB to get dB}.' .

Nonnalize to the proper impulse bandwidth.






Random noise consists of frequency components which, as the name implies, are
random in amplitude and phase. Measurement of random noise, then, depends on some
statistical basis. Normally, the process consists of integration or averaging and taking
the rms value of this averaged result.
Since the spectral components are random in phase, doubling the measurement
bandwidth will not double the measured voltage, but instead doubles the measured
power. Therefore, random noise is usually 5pecmed as some noise power per unit band-
width, e.g., dBm/Hz. The normalizing bandwidth is called the random noise bandwidth
or noise power bandwidth. For HP analyzers, this is approximately 1.2 times
the 3 dB bandwidth.

The definition of the noise power bandwidth is similar to the impulse bandwidth.
It is the ideal rectangular filter bandwidth with the same power response as the actual
instrument IF filter.

The best way to measure the noise power bandwidth is by the method previously
described for the impulse bandwidth; except that all vertical coordinates should be
squared to give a power display. This would necessitate graphing the curve by hand
to get the desired results or doing a numerical integration.

A simpler method which gives adequate results is to measure the 3 dB bandwidth,
,,- , and multiply by 1.2. To measure the 3 dB bandwidth, use the following procedure:

1. Connect a signal generator to the spectrum analyzer input, and connect the
auxiliary output of the generator to a frequency counter.
2. Tune to the signal on the spectrum analyzer, and display the signal generator
output in the linear mode.


n . Noise Power Bandwidth

,'~ Figure8. Thenoisepower andwidthis definedby an ideal filter
b with identical powerresponse.




~ ",

Fipre 9. The setting of the bandwidth control is the nominal 3 dB bandwidth of the spectrum analyzer,

3. Adjust the output of the signal generator for a deflection of 7.1 divisions at
the peak of the display.
4. Center the display on the CRT, and switch to zero scan.
5. Carefully tune the signal generator until the vertical deflection is 5 divisions, and
record the frequency on the counter,

6. Carefully tune the signal generator through the peak response until the deflec-
tion is again 5 divisions. Read and record the counter frequency.
7. Subtract the frequencies in steps 5 and 6 to get the 3 dB bandwidth.

Nominal values for the 3 dB bandwidth are engraved on the bandwidth knob.
This is accurate to :t:5% for the 10 kHz bandwidth only. For this reason, the 10 kHz
bandwidth can be used without further calibration in a number of cases.


Some consideration of detector characteristics is now in order. We noted in our
previous discussion that the spectrum analyzer uses an envelope detector. When used
with random noise, this creates a reading which is lower than the true rms value of the
average noise. This difference is 12.8% or 1.05 dB. (See Appendix A.)


.! Since log shaping tends to amplify noise peaks less than the rest of the noise signal,
the detected signal is smaller than its true rms value. This correction for the log display
mode combined with the detector characteristics gives a total correction of 2.5 dB,
which should be added to any random noise measured in the log display mode.


A further consideration is the integration or averaging of the random noise. In the
spectrum analyzer. this is accomplished with the video filter. A video bandwidth much
narrower than the IF bandwidth should be used. A video filter setting about 100 times
narrower than the IF bandwidth will give effective averaging. (See Figure 10.) .




a b

c d

(- Figure 1D. Thevideo filter effectively averagesrandomnoise.All four photosare taken with the 100 kHz
IF bandwidth,andthe videofilter is progressivelyswitchedthroughits four positions:OFF,10 kHz,100 Hz,
and 10 Hz.


The measurement consists of the following steps:
Measure the signal level in dBm.
Add 2.5 dB.
Normalize to the proper noise power bandwidth.

. A signal is measured at -35 dBm in a 10 kHz bandwidth. The level in
dBm/Hz is desired.
First, We add 2.5 dB to get -32.5 dBm. If the 10 kHz bandwidth is used, the
noise power bandwidth is 12 kHz. So, to normalize to 1 Hz bandwidth, we com-
pute the correction factor from:
12 kHz
10 log
1 Hz
40.8 dB

This is similar to the correction used in normalizing for impulse measurements
except the calculations reflect the power addition of random signals. The final
answer, then, is:
;--: =
-32.5 dBm/12 kHz -40.8 dB -73.3 dBm/Hz



Filure11. With a 10 kHzbandwidthsetting, the noiseat 10 MHzis -35 dBm.Applyingthe 2.5 dB correc-
tion, we get -32.5 dBm.Then,normalizingto a 1 Hzbandwidth,we get -73.3 dBm/Hz.


If we examine what happens as the spectrum analyzer bandwidth is changed, we
will see that the sensitivity for random noise measurements is independent of bandwidth.
For example, we narrow the bandwidth by a factor of 10. The analyzer's internal noise
(which is, itself, random noise) is decreased by a factor of 10, or 10 dB. At the same time,
the random noise we are measuring also decreases by 10 dB, so the signal-to-noise
ratio remains constant.

If a white noise source is applied to the spectrum analyzer with total power of
-10 dBm over the 120 MHz input range of the 0 -110 MHz spectrum analyzer, we
can calculate the available dynamic range. We can pick any bandwidth, so let's use
the 10 kHz bandwidth for simplicity. The noise power bandwidth is 12 kHz and the
spectrum analyzer sensitivity is -110 dBm. To normalize to the 12 kHz bandwidth, we
compute the correction factor from:
120 MHz
12 kHz
40 dB=
Then, -10 dBm/120 MHz -40 dB -50= dBm/12 kHz. We can measure from -50
dBm to -110 dBm, or a 60 dB total range.

-10dBm 1 dB Gain Compression

MaximumInput for
-40 dBm 70 dB Spurious.free

.. Maximum AcheiYlble
Measurement Range
for 10 Hz Bandwid1h

-14OdBm Noise LanI in 10 Hz Bandwicl1h

Filure 12. Maximumachievablemeasurement ange would be realizedby limiting input noise to 10 Hz
bandwidthbefore input mixer of the spectrumanalyzer.For actual analyzerwithout accessories,input
. bandwidthequals 120 MHz.
-10 dBm/120MHz~ -80 dBm/lO Hz
Measurement Range = -80 dBm- (-140 dBm)= 60 dB (worst case)



The video filter in the spectrum analyzer can be modified for better averaging when
narrow IF bandwidths are used. When this is done, the "display uncal" light will not
function properly. The proper scan time can be calculated, though, from the following
Scan Width per Division
BWvldeo(BWIF) ~O.35
Scan Time per Division
Note: This is an empirical relationship which is useful for most cases, but it will
not provide an exact answer.






Measurement of carrier-to-noise ratio is quite similar to measurement of random
noise power density. The measurement basically consists of:

1. Measure the carrier or desired signal level.
2. Measure the random noise and apply corrections.
3. Normalize to the desired bandwidth.

For example, it is desired to measure the video carrier-to-noise ratio of a composite TV
signal. The effective bandwidth of the received signal, then, is 6 MHz. So we will nor-
malize to this bandwidth to get the C/N ratio which will be seen by the TV receiver.

So, if the carrier appears at -25 dBm, and the noise is measured as -95 dBm in a
10 kHz bandwidth, we can make the following calculations:

1. Add 2.5 dB to the noise level.
2. Normalizeto 6 MHz bandwidth.
6 MHz
N (6 MHz) = N (10 kHz) + 10 log
1.2 (10 kHz)
N =-92.5 dBm/10 kHz + 27 dB = -65.5 dBm/6 MHz
Then, the carrier-to-noise ratio is -25 dBm to -65.5 dBm, or 40.5 dB. This method
can be applied to any input signal if the bandwidth of the intended receiver is known. )
That is, if we want to know the signal-to-noise ratio seen by a 0 - 12.4 GHz crystal de-
tector, we must normalize to a 12.4 GHz bandwidth, etc.


10 dBID..

100 kHZ/Di. 50 kHZ/Di.

Figure13. In the left photo,we measure level of an FMbroadcaststation as receivedat the spectrum
analyzerat -38 dBm.In the right photo,we addvideofiltering to averagethe noise(the modulationlooks
like noise, so the carrier level must be measuredwith the video filter Off) at -100 dBm in a 10 kHz
bandwidth.Applyingthe correctionsand normalizingto a 200 kHz transmissionbandwidth,we get about
/' a 47 dB signal-to-noise ratio for an FMreceiver.

... -
. .

The noise figure of an ampli