agilent spectrum analyzer basics

8/3/2019 Agilent Spectrum Analyzer Basics

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Hewlett-Packard CompanyMicrowave Instruments Division

1400 Fountaingrove Parkway

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Spectrum Analysis Basics

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* The frequency domain is related to the time domain by a body of knowledge generally known as Fourier theory (named for Jean Baptiste Joseph Fourier,

1768-1830). Discrete, or digitized signals can be transformed into the frequency domain using the discrete Fourier transform.

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( ' T r p 6 h y v 7 h v p 8 H 7 T 6 ( '

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q Display fidelity

q Frequency response

q RF Input attenuator

q Reference level

q Resolution bandwidth

q CRT scaling

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Relative

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in dB



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The specification assumes the worst case situation, where frequency response varies the full amplitude range, in this case

plus 1 dB and minus 1 dB. The uncertainty between two signals in the same band (the spectrum analyzers frequency range

is actually split into several bands) is 2 x 1 dB = 2 dB since the amplitude uncertainty at each signal's position could fall

on the + and - extremes of the specification window.


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- 1 dB

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B A N D 1

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Signals in t he Same Harmonic Band



28/75

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As we mentioned before, the four items listed above involve control changes made during the course of a measurement, and

can be eliminated if they can be left unchanged.

Because an RF input attenuator must operate over the entire frequency range of the analyzer, its step accuracy, like

frequency response, is a function of frequency. At low RF frequencies, we expect the attenuator to be quite good; at 20 GHz,

not as good.

The IF gain (or reference level control) has uncertainties as well, but should be more accurate than the input attenuator

because it operates at only one frequency.

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q Resolution bandwidth

q CRT scaling

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in dB



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Absolute amplitude measurements are actually measurements that are relative to the calibrator, which is a signal of known

amplitude. 0RVWPRGHUQVSHFWUXPDQDO\]HUVKDYHDFDOLEUDWRUEXLOWLQVLGH7KLVFDOLEUDWRUSURYLGHVDVLJQDOZLWKD

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Since our unknown signal to be measured is at a different frequency, we must change the frequency control. Since it is at a

different amplitude, we may change reference level to bring it to the reference level, for best accuracy. Hence, absolute

amplitude accuracy depends on calibrator accuracy, frequency response, and reference level uncertainty (also known as IF

gain uncertainty).

( ' T r p 6 h y v 7 h v p 8 H 7 T 6 ( '

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3 dB3 dB BW

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When measuring two signals of equal-amplitude, the value of the selected RBW tells us how close together they can be and

still be distinguishable from one another (by a 3 dB dip). For example, if two signals are 10 kHz apart, a 10 kHz RBW

easily separates the responses. However, with wider RBWs, the two signals may appear as one.

In general then, two equal-amplitude signals can be resolved if their separation is greater than or equal to the 3 dB bandwidth

of the selected resolution bandwidth filter.

N OTE: Since the tw o signals interact w hen both are present w ithin t he RBW , you should use a V ideo BW about 10 times smaller than t he

Res BW to smooth t he responses.


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Pe nalty For Sw ee ping Too Fast

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SPAN 3 kHzRES BW 100 Hz

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1RWHLQWKHILJXUHWKDWWKHGLVSOD\HGsignal level does not fall with increased attenuation. Remember from the theory of

operation section that the RF input attenuator and IF gain are tied together. Therefore, as we increase the RF input

attenuation 10 dB, the IF gain will simultaneously increase 10 dB to compensate for the loss. The result is that the on-screen

signal stays constant, but the (amplified) noise level increases 10 dB.


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10 dB

Attenuat ion = 1 0 dB Attenuat ion = 20 dB

Signal-To-Noise Ratio Decreases asRF Input Attenuat ion is Increased

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energy hits the detector and the displayed average noise level (DANL) increases (or decreases) by 10 dB.

The relationship between displayed noise level and RBW is:

noise level change (dB) = 10 log(RBWnew)/(RBWold)

Therefore, changing the RBW from 100 kHz (RBWold

) to 10 kHz (RBWnew

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noise level change = 10 log (10 kHz/100 kHz) = - 10 dB.

Spectrum analyzer noise is specified in a specific RBW. The spectrum analyzers lowest noise level (and slowest sweeptime)

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Another common measurement made on an AM signal (besides determining the modulation frequency) is modulation index,

which tells us the degree of modulation (0 to 100%).

The graph on the left is a typical swept-frequency-domain plot of the amplitude modulated signal. Remember that we need

the RBW to be fmod to include sidebands), the FFT function can be accessed.

This causes the analyzer to display the frequency-domain based on the FFT of the time-domain (not by directly measuring

it). The graph on the right is an example of what you would see in the FFT-frequency-domain. The carrier is at the left edge

because it is at 0 Hz relative to itself. The baseband modulating signal (upper sideband) is to the right of the carrier, offset

from the carrier by fmod. The frequency range (span) depends on sweeptime. Just like in the swept-frequency-domain, the

markers can be used to measure carrier amplitude, m, and fmod. As shown, the delta marker reads 1000 Hz and -26 dBc.

Hence fmod = 1 kHz and m = 10%.


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W here TO I = Mixer Leve l - dBc/2

SO I = Mixer Leve l - dBc

Opt imum Mixer Leve l = DAN L - M DR

Attenua t ion = Signal - Opt imum M ixer Level

M D R = 2 / 3 ( D A N L - T O I )3

M D R = 1 / 2 ( D A N L - S O I )2

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For example, lets say we have a spectrum analyzer with a DANL = -115 dBm (1 kHz RBW), and TOI = +5 dBm. This

slide shows how to calculate maximum third-order dynamic range (MDR3), optimum mixer level, and attenuation.

Remember that for every order of magnitude decrease in RBW, the DANL decreases by 10 dB (page 40). Therefore, DANL

= -135 for a 10 Hz RBW, and third-order dynamic range improves by 13 dB, [2/3(-140)] = 93 dBc.



CMB 12/96


W here TOI = ( -30 ) - ( -70 ) /2

= + 5 dBm

3MD R = 2 /3 [( - 1 1 5 ) - ( +5 ) ]

= - 8 0 dBc ( 1 k Hz RBW )

([DPSOH&DOFXODWLRQ

Opt imum Mixer Leve l = ( -115) - ( -80 )

= - 3 5 dBm

Attenuat ion = (0) - ( -35)

= +3 5 dBm


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