fundamentals of spectrum analyzer

8/22/2019 Fundamentals of Spectrum Analyzer

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Fundamentals of Spectrum Analyzer

Mr.Wang JunfengEngineer of Equipment Testing Division

State Radio Monitoring Center

[email protected]+(86)10-68368866-1807

Radio Monitoring and Spectrum Management Training

(China,23-31,May,2005)


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Contents


Why do we use a spectrum analyzer?

What can we do with a spectrum analyzer?

How many types are there for spectrum analyzers?

The classic superheterodyne spectrum analyzer

Illustration of test cases


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Analyzing a RF signal

Time domain

Oscilloscope Waveform S(t)

Frequency domain

Spectrum analyzer Spectrum F{S(t)}=S(f)

Frequency and amplitude information

Modulation domain

Vector signal analyzer Vector informationFrequency, amplitude and phase information


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Analysis in time domain

X(t) Complicated in time domain


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Analysis in time domain


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Analysis in frequency domain

F{X(t)}=X(f) Simpler in frequency domain


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Easier to verify each frequency component of thesignal looked like complicated in time domain

Easier to check what happened in band, in out-of-

band domain and in spurious emission domainEasier to allocate and assign the frequency forregulatory agency with assistance of the statisticsfrom spectrum monitoring


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Frequency and amplitude test

Channel power and spectrum power density test

Wanted signal spectrum mask test

Adjacent channel power test

Spurious emission test

Occupied bandwidth test

Spectrum monitoringEtc.


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Types of spectrum analyzers

superheterodyne spectrum analyzer

Real-time spectrum analyzer (Fourier analyzer)


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superheterodyne spectrum analyzer

Larger analyzing frequency range

Excellent sensitivity

Larger dynamic range

Swept-tuned, not a real-time equipment, not suitable for

short term phenomena


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Real-time spectrum analyzer(Fourier analyzer)

RAM FFTA

D

A

D

RAM FFT

Real-time equipment, characterizing short term phenomena

Phase as well as amplitude can be tested

Limitation of the frequency range, sensitivity, and dynamicrange


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spectrum analyzer appearance


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Block diagram of a classic superheterodyne spectrum analyzer


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RF attenuator

Protect the following circuit

Adjust the signal entering the mixer at the optimumlevel


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RF attenuator

Input impedance 50ohmsAttenuation range 0 to 50dB or moreAttenuation accuracy less than 0.5dB

Minimum Step size 5dB, 2dB or 1dB


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RF attenuator

Attenuation should be large enough to avoid mixer overload

CLRWR

A

1 RM*

*RBW 300 Hz

VBW 3 kHz

SWT 1.2 s*Ref -10 dBm

Center 500 MHz Span 75 kHz7.5 kHz/

Att 0 dB*

OVLD

PRN

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-89.85 dBm

500.037500000 MHz

Comment A: 11

Date: 27.APR.2005 16:41:38

CLRWR

A

1 RM*

*RBW 300 Hz

VBW 3 kHz

SWT 1.2 s*Att 15 dB*Ref 10 dBm


PRN

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

1

Marker 1 [T1 ]

-86.87 dBm

500.037500000 MHz

Comment A: 11

Date: 27.APR.2005 16:42:16


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RF attenuator

Suitable attenuation can ensure either the excellentlinear performance or the perfect noise floor of thespectrum analyzer.


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Pre-selector or low pass filter

Reduce the signal energy entering the mixer avoidmixer overload

Keep unwanted signal from creating unwantedresponse


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Pre-selector or low pass filter

If there is no pre-selector or low pass filter, thelarger signal will cause mixer overload when the

smaller signal is tested.


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Mixer and tunable LO

Frequency down conversion to IF

fIF =fLO-fsig or fIF =fsig- fLOfsig = signal frequency, fLO = local oscillator frequency,

fIF = intermediate frequency (IF)

Tuning the spectrum analyzer to the desired range

Ramp sweep generator controls both LO and display


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Mixer and tunable LO

Narrow IF filter Multiple mixing steps


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Mixer

fIF =fLO-fsig or fIF =fsig- fLOFor certain fIF and fLO there are always two fsig fulfill the mixingformula

For example:fIF =3.9GHz, fIF =fLO-fsig , fLO =4.3GHz, fsig=400MHz

fIF =3.9GHz, fIF =fsig- fLO, fLO =4.3GHz, fsig= 8.2GHz

A pre-selector or a low pass filter is necessary to preventunwanted response created by image frequency from happening.


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Mixer

Ideal mixing: fIF =fLO-fsig or fIF =fsig- fLOActual mixing: fIF =(fLO-fsig)+k1(fLO-fsig )

2+k2(fLO-fsig )3 +--- or

fIF =(fsig-fLO)+k1(fsig-fLO)2+k2(fsig-fLO )

3 +---

It is very important to reduce the non-linear components.It is

why that we need set a suitable attenuation to find a optimummixer level especially for harmonic measurement.


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Mixer

How to distinguish the non-linear components?


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Mixer

LO feedthrough

fIF =fLO-fsig

fsig =0

fIF =fLO


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Mixer

Ref 10 dBm Att 10 dB*

*1 PK

CLRWR

A

Start 50 Hz Stop 9.93 kHz988 Hz/

*RBW 1 kHz

VBW 3 kHz

SWT 20 ms

PRN

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

101

Marker 1 [T1 ]

3.87 dBm

150.000000000 Hz

D1 -36 dBm

Comment A: 11

Date: 27.APR.2005 16:52:47

Why?


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Mixer

The LO signal is coupled into the first IF path due to itslimited isolation.


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tunable LO

Usually tunable LO is controlled by the periodicsawtooth signal. The scan generator runs freely.


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tunable LO

What will happen if a pulse signal entering the spectrum analyzer?

Can we get the spectrum when spectrum analyzer run freely?

Ref -20 dBm Att 10 dB*

*1 RM

CLRWR

A

RBW 1 MHz

VBW 10 MHz

TRG

Center 1 GHz 1.5 ms/

SWT 15 ms

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

1

Marker 1 [T1 ]

-84.23 dBm

2.500000 ms

TRG -36.5 dBm

Date: 13.MAY.2005 10:56:14

NO!


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tunable LO

How can get the spectrum of a pulse signal or aTDMA signal?

The spectrum analyzer must be triggered by specificsignal.


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tunable LO

Tunable LO could also be controlled by other specific signal.

The scan generator is controlled by specific condition, for instancevideo signal trigger, IF signal trigger, gating trigger, and extendsignal trigger.


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tunable LO

We can get the spectrum of the pulse signal if we set the specifictrigger condition for spectrum analyzer, for instance IF signaltrigger or extend signal trigger.


*1 RM

CLRWR

A

GAT

TRG

200 kHz/Center 1 GHz Span 2 MHz

*RBW 30 kHz

VBW 300 kHz

SWT 145 ms*

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20 1

Marker 1 [T1 ]

-26.09 dBm

999.992000000 MHz

Date: 13.MAY.2005 10:57:30


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IF processing circuit

IF amplifier variable gain amplifier, keepingconstant display

Reference level max level can be displayed

RBW filter determining the signal to be displayed


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IF amplifier

The gain of the IF amplifier can be adjusted in smallstep size, so the maximum signal level can be keptconstant in the subsequent signal processing

regardless of the attenuation setting and mixer level.IF gain offset is coupled to the attenuator, so largerattenuation would bring larger noise.


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IF amplifier

Larger attenuation import larger noise


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IF amplifier

LDAN=10log(ktBN,IF/110-3W)+NFSA-2.5dB)

LDAN : displayed average noise level

k: Boltzmanns constant, k=1.3810-23W/Hz

T: ambient temperature, in K

BN,IF: noise bandwidth of IF filterNFSA: noise figure of spectrum analyzer, in dB

-2.5dB: understanding of noise by sampling detector and averaging oflogarithmic level values

For the ambient temperature 290K(17C):

LDAN=-174dBm/Hz+(10logBN,IF/Hz)+NFSA-2.5dB

Increasing the attenuation, the noise figure of the spectrum analyzerwill get larger.


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Reference level

Reference level is the max level can be displayedReference level should be large enough to avoid IFoverload

Usually reference level is coupled to the attenuationto protect the mixer and subsequent circuit.


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Reference level

CLRWR

A

1 RM*

* RBW 300 Hz

VBW 3 kHz

SWT 1.2 s*Att 15 dB*Ref -10 dBm


IFOVL

PRN

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-99.39 dBm

500.037500000 MHz

Comment A: 11

Date: 27.APR.2005 16:42:41

IF overload Reference level:-10dBm


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Reference level

CLRWR

A

1 RM*

*RBW 300 Hz

VBW 3 kHz

SWT 1.2 s*Att 15 dB*Ref -5 dBm


PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-105.42 dBm

500.037500000 MHz

Comment A: 11

Date: 27.APR.2005 16:43:16

Reference level:-5dBm


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RBW filter

RBW filter The final IF filter

Resolution 3dB bandwidth of the IF filter

Selectivity Filter wave shape factor

Response time sweep time


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RBW filter

RBW=30kHz

A

*1 RM

CLRWR

Att 10 dB*

*RBW 300 kHz

VBW 3 MHz

SWT 245 ms*Ref -20 dBm

Center 1 GHz Span 5 MHz500 kHz/

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

1

Marker 1 [T1 ]

-29.35 dBm

1.000050000 GHz

Date: 13.MAY.2005 10:50:29

Different RBW filter has different resolving capability

A

*1 RM

CLRWR

Att 10 dB*Ref -20 dBm

Center 1 GHz Span 500 kHz50 kHz/

*RBW 30 kHz

VBW 300 kHz

SWT 245 ms*

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

1

Marker 1 [T1 ]

-31.51 dBm

1.000099000 GHz2

Marker 2 [T1 ]

-31.61 dBm

999.999000000 MHz

Date: 13.MAY.2005 10:51:17

RBW=300kHz


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RBW filter

RBW is the 3dB bandwidth of the final IF filter

*1 RM

CLRWR

A



*RBW 100 kHz

VBW 1 MHz

SWT 195 ms*

PRN

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-21.27 dBm

1.000000000 GHz2

Delta 2 [T1 ]

-3.03 dB

-50.000000000 kHz

3Delta 3 [T1 ]

-3.12 dB

50.000000000 kHz

Date: 13.MAY.2005 11:13:12


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RBW filter

Selectivity

Wave shape factor ratio of 60dB bandwidth to3dB bandwidth


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RBW filter

A low factor means a better selectivity


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RBW filter

In theory, a rectangular filter has excellent selectivity.

But such a filter has a long transient response time.

Short measurement time can be achieved through

the use of Gaussian filter optimized for transients.


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RBW filter

Response timeSweep time

How long does it take to complete a sweep? RBW filter is a band limited filter and needs some time to

charge and discharge. Narrow RBW filter has higher resolving capability but needs

longer charging time. ST=k(span)/RBW2

ST: Sweep timek: constant factor (variable for different filter types)

There are different kinds of filter allow resolution, selectivity andmeasurement speed to be adapted to specificapplication.


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RBW filter

Analog IF filter

It is used to realize very large RBW, usually from 100kHz to10MHz.

Ideal Gaussian filter can not be implemented using analog filter.

It is possible for a analog filter that the transient response isalmost identical with the ideal Gaussian filter within the 20dBbandwidth.

SF=14, four filter circuit

SF=10, five filter circuitWhereas SF=4.6, ideal Gaussian filter


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RBW filter

Digital IF filter

It is used to realize narrow RBW, usually less than 100kHz.

The ideal Gaussian filter can be implemented by digital filter.

Much better selectivity can be achieved, SF=4.6.

Digital filter allows shorter sweep time than analog filter ofthe same bandwidth.


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RBW filter

Usually, sweep time is automatically coupledto span and RBWSweep time can be changed manually. But try

to avoid the Mea Uncal errorSweep time must be longer than minimumsweep time


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RBW filter

Ref -5 dBm Att 10 dB

1 RM

CLRWR

A

*

*

Center 500 MHz Span 50 kHz5 kHz/

* RBW 100 Hz

VBW 1 kHz

SWT 3 s*

UNCAL

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-107.25 dBm

500.025000000 MHz

Comment A: 11

Date: 27.APR.2005 16:49:10

CLRWR

A

1 RM*



*RBW 100 Hz

VBW 1 kHz

*

SWT 6 s

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-103.71 dBm

500.025000000 MHz

Comment A: 11

Date: 27.APR.2005 16:44:17


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RBW filter

Different RBW imports different noise level.

LDAN=-174dBm/Hz+(10logBN,IF/Hz)+NFSA-2.5dB

LDAN : displayed average noise level, in dBm

k: Boltzmanns constant, k=1.3810-23W/Hz

T: ambient temperature, in KBN,IF: noise bandwidth of IF filter, in Hz

NFSA: noise figure of spectrum analyzer, in dB

-2.5dB: understanding of noise by sampling detector and averagingof logarithmic level values.

Different RBW result in different noise level.

The RBW setting is specified in measurement.


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RBW filter

Larger RBW imports higher noise level


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Video processing circuit

Log amplifier, envelope detector, detector, video filter,display screen


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Log amplifier

Compress the larger signal and increase the smallsignal

To improve display dynamic range of the spectrum

analyzer.


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Envelope detector

Convert the IF signal to video

The envelope of the IF signal is gotten.


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Envelope detector

The envelope of one certain frequency componentcan be obtained through 0 span at this frequency.


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Envelope detector

The envelope of single time slot GSM signal


*1 RM

CLRWR

A

RBW 1 MHz

VBW 10 MHz

TRG

Center 1 GHz 1.5 ms/

SWT 15 ms

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

1

Marker 1 [T1 ]

-84.23 dBm

2.500000 ms

TRG -36.5 dBm

Date: 13.MAY.2005 10:56:14


*1 RM

CLRWR

A

GAT

TRG

200 kHz/Center 1 GHz Span 2 MHz

*RBW 30 kHz

VBW 300 kHz

SWT 145 ms*

PRN

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20 1

Marker 1 [T1 ]

-26.09 dBm

999.992000000 MHz

Date: 13.MAY.2005 10:57:30


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Envelope detector

The envelope of a sine wave signal


CLRWR

A

RBW 100 kHz

VBW 1 MHz

SWT 2.5 ms

Center 1 GHz 250 s/

*1 RM

PRN

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-21.27 dBm

1.430000 ms

Date: 13.MAY.2005 11:16:04


1 RM

CLRWR

A

*

*


*RBW 100 kHz

VBW 1 MHz

SWT 2.5 ms

PRN

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-21.26 dBm

1.000000000 GHz

Date: 13.MAY.2005 11:29:33


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video filter


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video filter

Low pass filter, reducing the impact of noise on the displayedsignal amplitude, smoothing the display


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Detector

Digital display Analog to digital, finite displaypoint for a trace, normally 625 points or more

One display point represents a frequency range

What value should be displayed for each displaypoint among the frequency range?

Put all the data into a bucket and we need a mathformula to extract the data to be displayed

Different detector types mean different math formulae


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Sample detector

The data point at the center of the bucket is displayed


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Positive peak detector

The maximum data point of the bucket is displayed


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Negative peak detector

The minimum data point of the bucket is displayed


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Normal detector

The maximum data point of the bucket is displayed at odd displaypoint and the minimum data point of the bucket is displayed ateven display point


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RMS detector

Statistic average, RMS value of all the data points in abucket is displayed.

N

i

iRMS uN

U

1

21


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Average detector

Math average

N

iiAV

u

N

U

1

1


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Difference between various detectors

Positive peak, sample and negative peak

Att 15 dB*

CLRWR

A

*1 PK

CLRWR

*RBW 100 kHz

SWT 720 ms

CLRWR

Ref -50 dBm

Center 1.90025 GHz Span 100 MHz10 MHz/

*3 MI

*2 SA

VBW 300 kHz

*

PRN

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

1

Marker 1 [T1 ]

-77.86 dBm

1.935508423 GHz

Comment A: 11

Date: 27.APR.2005 12:37:31


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Positive peak, RMS and average

Att 15 dB*

CLRWR

A

*1 PK

CLRWR

CLRWR

*2 RM

*3 AV

Ref -50 dBm


* RBW 100 kHz

VBW 1 MHz

SWT 1.2 s*

PRN

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

1

Marker 1 [T1 ]

-77.85 dBm

1.905250000 GHz

Comment A: 11

Date: 27.APR.2005 12:38:40


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Normal

Att 15 dB*

CLRWR

A

Ref -50 dBm


* RBW 100 kHz

SWT 1.2 s

VBW 300 kHz

1 AP

*

PRN

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

1

Marker 1 [T1 ]

-77.70 dBm

1.905250000 GHz

Comment A: 11

Date: 27.APR.2005 12:39:19


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How many types are there for spectrum analyzers? The classic superheterodyne spectrum analyzer

Illustration of test cases


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How does the RBW filter work?

CLRWR

A

1 RM*



*RBW 1 kHz

VBW 10 kHz

SWT 1.2 s*

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-99.87 dBm

500.037500000 MHz

Comment A: 11

Date: 27.APR.2005 16:43:42

Spectrum for FM signal 1kHz mod 3kHz deviationRBW:1kHz


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How does the RBW filter work?

CLRWR

A

1 RM*



*RBW 100 Hz

VBW 1 kHz

*

SWT 6 s

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-103.71 dBm

500.025000000 MHz

Comment A: 11

Date: 27.APR.2005 16:44:17

Spectrum for FM signal 1kHz mod 3kHz deviationRBW:100Hz


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How to set the RBW in measurement?

Spectrum mask measurement RBW is approximately3% to 5% necessary bandwidth.

Unwanted emission measurement RBW is set

according to the test specification.


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Channel power measurement

Integral bandwidth:16kHz



RBW 100 Hz

VBW 1 kHz

SWT 6 s

*

1 RM

CLRWR

A

*

*

*

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-104.43 dBm

500.025000000 MHz

CH PWR -4.25 dBm

C0C0

Comment A: 11

Date: 27.APR.2005 16:45:37


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Adjacent channel power measurement

2 adjacent channel: 25kHz 50kHz


1 RM

CLRWR

A

*

*


* RBW 100 Hz

VBW 1 kHz

SWT 18 s

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

Tx Channel

Bandwidth 16 kHz Power -4.23 dBm

Adjacent Channel

Bandwidth 16 kHz Lower -79.32 dBSpacing 25 kHz Upper -79.68 dB

Alternate Channel

Bandwidth 16 kHz Lower -79.83 dBSpacing 50 kHz Upper -79.93 dB

1

Marker 1 [T1 ]

-101.93 dBm

500.025000000 MHz

C0

C0

cu2

cu2

cu1

cu1

cl1

cl1

cl2

cl2

Comment A: 11

Date: 27.APR.2005 16:47:51


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Occupied bandwidth

99% power bandwidth


1 RM

CLRWR

A

*

*


*RBW 100 Hz

VBW 1 kHzSWT 6 s

PRN

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

1

Marker 1 [T1 ]

-102.21 dBm500.025000000 MHz

OBW 8.092948718 kHz

T1

Temp 1 [T1 OBW]

-25.59 dBm

499.995913462 MHzT2

Temp 2 [T1 OBW]

-21.82 dBm

500.004006410 MHz

Comment A: 11

Date: 27.APR.2005 16:48:29


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The End

Thanks for your attention!!

Any questions?

fundamentals of spectrum analyzer

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