Atten AT6011 Посібник користувача - Сторінка 8

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the operation of the input attenuators (14) can

be tested at a level of -27dBm. The spec-tral

line visible on the screen can be reduced in 4

steps of 10dB each by activating the

attenuators incorporated in the spectrum

analyzer. Each 10dB step corresponds to one

graticule division on the screen. The tolerance

may not exceed

1dB in all attenuation

positions.

Horizontal Calibration

Prior to calibration ensure that all input

attenuator switches (14) are released. The

AT6010/A T6011 must be operated for at least

60 minutes prior to calibration. The VIDEO

FILTER push button (11) must be in OFF

position, the BANDWIDTH (10) must be set

to 400KHz, and SCANWIDTH (15) set to

100KHz/div. After the center frequency is set

to 500MHz, a generator signal must be applied

to the input. The output level should level

should be between 40 and 50 dB above the

noise.

Set generator frequency to 500MHz.

Adjust the peak of the 500MHz spectral line to

the horizontal screen center using the X-POS.

control (16).

Set the generator frequency to 100MHz. If

the 100MHz spectral line is not on the 2nd.

graticule line from left, it should be aligned

using the X-AMPL. Control (17). Then the

calibration as de-scribed under be verified and

corrected if necessary. The calibrations C and

D should be repeated until optimum

adjustment is achieved.

Introduction to

Spectrum Analysis

The analysis of electrical signals is a

fundamental problem for many engineers and

scientists. Even if the immediate problem is

not electrical, the basic parameters of interest

are often changed into electrical signals by

means of transducers. The rewards for

transforming physical parameters to electrical

signals are great, as many instruments are

available for the analysis of electrical signals

in the time and frequency domains.

The traditional way of observing electrical

signals is to view them in the time domain

using an oscilloscope. The time domain is

used to recover relative timing and phase

information which is needed to characterize

electric circuit behavior. However, not all

circuits can be uniquely characterized from

just time domain information. Circuit elements

such as amplifiers, oscillators, mixers,

modulators, detectors and filters are best

characterized by their frequency response

information. This frequency informat5ion

is best obtained by viewing electrical signals

in the frequency domain. To display the

frequency domain requires a device that can

discriminate between frequencies while

measuring the power level at each. One

instrument which displays the frequency

domain is the spectrum analyzer. It graphically

displays voltage or power as a function of

frequency only on a CRT (cathode ray tube).

In the time domain, frequency components of

a signal are seen summed together. In the

frequency domain, complex signals (i.e.

Signals composed of more than one frequency)

are separated into their frequency components,

and the power level at each frequency is

displayed. The frequency domain is a

graphical representation of signal amplitude as

a function of frequency. The frequency domain

contains information not found in the time

domain and therefore, the spectrum analyzer

has certain advantages compared with an

oscilloscope.

The analyzer is more sensitive to low level

distortion than a scope. Sine waves may look

in the time domain, but in the frequency

domain, harmonic distortion can be seen. The

sensitivity and wide dynamic range of the

spectrum analyzer is useful for measuring low-

level modulation. It can be used to measure

AM, FM and pulsed RF. The analyzer can be

used to measure carrier frequency, modulation

frequency, modulation level, and modulation

distortion. Frequency con-version devices can

be easily characterized. Such parameters as

conversion loss, isolation, and distortion are

readily determined from the display.

The spectrum analyzer can be used to measure

long and short term stability. Parameters such

as noise sidebands on an oscillator, residual

FM of a source and frequency drift during

warm-up can be measured using the spectrum

analyzer's calibrated scans. The swept

frequency responses of a filter or amplifier are

examples of swept frequency measurements

possible with a spectrum analyzer. These

measurements are simplified by using a

tracking generator.

Types of Spectrum Analyzers

There are two basic types of spectrum

analyzers, swept-tuned and real-time analyzers.

The swept-tuned analyzers are tuned by

electrically sweeping them over their

frequency range. Therefore, the frequency

components of a spectrum are sampled

sequentially in time. This enables periodic and

random signals to be displayed, but makes it

impossible to display transient responses. Real

-time analyzers, on the other hand,

simultaneously display the amplitude of all

signals in the frequency range of the analyzer.

hence the name real-time. This preserves the

time dependency between signals which

permits phase information to be displayed.

Real-time analyzers are capable of displaying

transient responses as well as periodic and

random signals.

The swept-tuned analyzers of the trf (tuned

radio frequency) or superheterodyne type. A

trf analyzer consists of a bandpass filter whose

center frequency is tunable over a desired

frequency range, a detector to produce vertical

deflection on a CRT, and a horizontal scan

generator used to synchronize the tuned

frequency to the CRT horizontal deflection. It

is a simple, inexpensive analyzer with wide

frequency coverage, but lacks resolution and

sensitivity. Because trf analyzers have a swept

filter they are limited in sweep width

depending on the frequency range (usually one

decade or less). The resolution is determined

by the filter bandwidth, and since tunable

filters don't usually have constant bandwith, is

dependent on frequency.

The most common type of spectrum analyzer

differs from the trf spectrum analyzers in that

the spectrum is swept through a fixed

bandpass filter instead of sweeping the filter

through the spectrum. The analyzer is swept

through a narrowband receiver which is

electronically tuned in frequency by applying

a saw-tooth voltage to the frequency control

element of a voltage tuned local oscillator.

This same saw-tooth voltage is simultaneously

applied to the horizontal deflection plates of

the CRT. The output from the receiver is

synchronously applied to the vertical

deflection plates of the CRT and a plot of

amplitude versus frequency is displayed.

The analyzer is tuned through its frequency

range by varying the voltage on the LO (local

oscillator). The LO frequency is mixed with

the input signal to produce an IF (intermediate

frequency) which can be detected and