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displayed. and displayed. When the frequency

difference between the input signal and the LO

frequency is equal to the IF frequency, then

there is a response on the analyzer. The

advantages of the superheterodyne technique

are considerable. It obtains high sensitivity

through the use of IF amplifiers, and many

decades in frequency can be tuned.

Also, the resolution can be varied by changing

the bandwidth of the IF filters. However, the

superheterodyne analyzer is not real-time and

sweep rates must be consistent with the IF

filter time constant. A peak at the left edge of

the CRT is sometimes called the zero

frequency indicator or local oscillator

feedthrough . It occuts when the analyzer is

tuned to zero frequency, and the local

oscillator passes directly through IF creating a

peak on the CRT even when no input signal is

present. (For zero frequency tuning, FLO=

FIF). This effectively limits the lower tuning

limit.

Spectrum Analyzer Requirements

To accurately display the frequency and

amplitude of a signal on a spectrum analyzer,

the analyzer itself must be properly calibrated.

A spectrum analyzer properly designed for

accurate frequency and amplitude

measurements has to satisfy many

requirements:

Wide tuning range

Wide frequency display range

Stability

Resolution

Flat frequency response

High sensitivity

Low internal distortion

Frequency Measurements

The frequency scale can be scanned in three

different modes full, per division, and zero

scan The full scan mode is used to locate

signals because the widest frequency ranges

are displayed in this mode. (Not all spectrum

analyzers offer this mode). The per division

mode is used to zoom-in on a particular signal.

In per division, the center frequency of the

display is set by the Tuning control and the

scale factor is set by the Frequency Span or

Scan Width control. In the zero scan mode, the

analyzer acts as a fixed-tuned receiver with

selectable bandwidths.

Absolute frequency measurements are usually

made from the spectrum analyzer tuning dial.

Relative frequency measurements require a

linear frequency scan. By measuring the

relative separation of two signals on the

display, the display, the frequency difference

can be determined.

It is important that the spectrum analyzer be

more stable than the signals being measured.

The stability of the analyzer depends on the

frequency stability of its local oscillators.

Stability is usually characterized as either

short term or long term. Residual FM is a

measure of the short term stability which is

usually specified in Hz peak-to-peak. Short

term stability is also characterized by noise

sidebands which are a measure of the

analyzers spectral purity. Noise sidebands are

specified in terms of dB down and Hz away

from a carrier in a specific bandwidth. Long

term stability is characterized by the frequency

drift of the analyzers Los. Frequency drift is a

measure of how much the frequency changes

during a specified time (i.e., Hz/hr)

Resolution

Before the frequency of a signal can be

measured on a spectrum analyzer it must first

be re-solved. Resolving a signal means

distinguishing it from its nearest neighbors.

The resolution of a spectrum analyzer is

determined by its IF bandwidth. The IF

bandwidth is usually the 3dB bandwidth of the

IF filter. The ratio of the 60dB bandwidth (in

Hz) to the 3dB bandwidth (in Hz) is known as

the shape factor of the filter. The smaller the

shape factor, the greater is the analyzer's

capability to resolve closely spaced signals of

unequal amplitude. If the shape factor of a

filter is 15:1, then two signals whose

amplitudes differ by 60dB must differ in

frequency by 7.5 time the IF bandwidth before

they can be distinguished separately.

Otherwise, they will appear as one signal on

the spectrum analyzer display.

The ability of a spectrum analyzer to resolve

closely spaced signals of unequal amplitude is

not a function of the IF filter shape factor only.

Noise sidebands can also reduce the

resolution. They appear above the skirt of the

IF filter and reduce the offband rejection of the

filter. This limits the resolution when

measuring signals of unequal amplitude.

The resolution of the spectrum analyzer is

limited by its narrowest IF bandwidth. For

example, if the narrowest bandwidth is 10kHz

then the nearest any two signals can be and

still be resolved is 10kHz. This is because the

analyzer traces out its own IF band-pass shape

as it sweeps through a CW signal. Since the

resolution of the analyzer is limited by

bandwidth, it seems that by reducing the IF

bandwidth infinitely, infinite resolution will be

achieved. The fallacy here is that the usable IF

bandwidth is limited by the stability (residual

Fm) of the analyzer. If the internal frequency

deviation of the analyzer is 10kHz, then the

narrowest bandwidth that can be used to

distinguish a single input signal is 10kHz. Any

narrower IF-filter will result in more than one

response or an intermittent response for a

single input frequency. A practical limitation

exists on the IF bandwidth as well, since

narrow filters have ling time constants and

would require excessive scan time.

Sensitivity

Sensitivity is a measure of the analyzer's

ability to detect small signals. The maximum

sensitivity of an analyzer is limited by its

internally generated noise. The noise is

basically of two types: thermal (or Johnson)

and nonthermal noise. Thermal noise

power can be expressed as:

P =K . T . B

Where:

P = Noise power in watts

K = Boltzmanns Constant

-23

(1.38*10 Joule/K)

T = absolute temperature, K

B = bandwidth of system in Hertz

As seen from this equation, the noise level is

directly proportional to bandwidth. Therefore,

a decade decrease in bandwidth results in a

10dB decrease in noise level and consequently

10dB better sensitivity. Nonthermal noise

accounts for all noise produced within the

analyzer that is not temperature dependent.

Spurious emissions due to nonlinearities of

active elements, impedance mismatch, etc. are

sources of nonthermal noise. A figure of merit,

or noise figure, is usually assigned to this

ninthermal noise which when added to the

thermal noise gives the total noise of the

analyzer system. This system noise which is

measured on the CRT, determines the

maximum sensitivity of the spectrum analyzer.

Because noise level changes with bandwidth it

is important, when comparing the sensitivity