Atten AT5010 Instrukcja obsługi - Strona 16

Przeglądaj online lub pobierz pdf Instrukcja obsługi dla Przyrządy pomiarowe Atten AT5010. Atten AT5010 20 stron. Spectrum analyzers

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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:, 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

-23

K = Boltzmanns Constant (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 of two analyzers, to compare sensitivity

specifications for equal bandwidths. A spectrum

analyzer sweeps over a wide frequency range, but is

really a narrow band instrument. All of the signals

that appear in the frequency range of the analyzer

are converted to a single IF frequency which must

pass through an IF filter; the detector sees only this

noise at any time. Therefore, the noise displayed on

the analyzer is only that which is contained in the IF

passband. When measuring discrete signals,

maximum sensitivity is obtained by using the

narrowest IF bandwidth.

Video Filtering

Measuring small signals can be difficult when they

are approximately the same amplitude as the

average internal noise level of the analyzer. To

facilitate the measurement, it is best to use video

filtering. A video filter is a post-detection low pass

filter which averages the internal noise of the

analyzer. When the noise is averaged, the input

signal may be seen. If the resolution bandwidth is

very narrow for the span, the span, the video filter

should no be selected, as this will not allow the

amplitude of the analyzed signals to reach full

amplitude due to its video bandwidth limiting

property.

Spectrum Analyzer Sensitivity

Specifying sensitivity on a spectrum analyzer is

somewhat arbitrary. One way of specifying

sensitivity is to define it as the signal level when

signal power = average noise power. The analyzer

always measures signal plus noise. Therefore, when

the input signal is equal to the internal noise level,

the signal will appear 3dB above the noise. When

the signal power is added to the average noise

power, the power level on the CRT is doubled

(increased by 3dB) because the signal

power=average noise power.

The maximum input level to the spectrum analyzer

is the damage level or burn-out level of the input

circuit. This is (for the AT5010/11) +10dB for the

input mixer and +20dB for the input attenuator.

Before reaching the damage level of the analyzer,

the analyzer will begin to gain compress the input

signal. This gain compression is not considered

serious until it reaches 1dB. The maximum input

signal level which will always result in less than

1dB gain compression is called the linear input

level. Above 1dB gain compression the analyzer is

considered to be operating nonlinearly because the

signal amplitude displayed in the CRT is not an

accurate measure of the input signal level.

Whenever a signal is applied to the input of the

analyzer, distortions are produced within the

analyzer itselt. Most of these are caused by the non-

linear behavior of the input mixer. For the

AT5010/11 these distortions are typically 70dB

below the input signal level for signal levels not

exceeding -27dBm at the input of the first mixer. To

accommodate larger input signal levels, an

attenuator is placed in the input circuit before the

first mixer. The largest input signal that can be

applied, at each setting of the input attenuator, while

maintaining the internally generated distortions

below a certain level, is called the optimum input

level of the analyzer. The signal is attenuated before

the first mixer because the input to the mixer must

not exceed -27dB, or the analyzer distortion

products may exceed the specified 70dB range. This

70dB distortion-free range is called the spurious-

free dynamic range of the analyzer. The display

dynamic range is defined as the ratio of the largest

signal to the smallest signal that can be displayed

simultaneously with no analyzer distortions present.

Dynamic range requires several things then. The

display range must be adequate, no spurious or

unidentified response can occur, and the sensitivity

must be sufficient to eliminate noise from the

displayed amplitude range.

The maximum dynamic range for a spectrum

analyzer can be easily determined from its

specifications. First check the distortion spec. For

example, this might be "all spurious products 70dB

down for -27dBm at the input mixer". Then,

determine that adequate sensitivity exists. For

example, 70dB down from -27dBm is -97dB. This

is the level we must be able to detect, and the

bandwidth required for this sensitivity must not be

too narrow or it will be useless. Last, the display

range must be adequate.

Notice that the spurious-free measurement range

can be extended by reducing the level at the input

mixer. The only limitation, then, is sensitivity. To

ensure a maximum dynamic range on the CRT

display, check to see that the following requirements

are satisfied.

The largest input signal does not exceed the

optimum input level of the analyzer (typically-

27dBm with 0dB input attenuation).

The peak of the largest input signal rests at the

top of the top of the CRT display (reference level).