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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 AT6010/

AT6011) +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

theanalyzer itselt. Most of these are caused by

the non-linear behavior of the input mixer. For

the AT6010/AT6011 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).

Frequency Response

The frequency response of an analyzer is the

amplitude linearity of the analyzer over its

frequency range. If a spectrum analyzer is to

display equal amplitudes for input signals of

equal amplitude, independent of frequency,

then the conversion (power) loss of the input

mixer must not depend on frequency. If the

voltage from the LO is too large compared to

the input signal voltage then the conversion

loss of the input mixer is frequency dependent

and the frequency response of the system is

nonlinear. For accurate amplitude

measurements, a spectrum analyzer should be

as flat as possible over its frequency range.

Flatness is usually the limiting factor in

amplitude accuracy since its extremely

difficult to calibrate out. And, since the

primary function of the spectrum analyzer is to

compare signal levels at different frequencies,

a lack of flatness can seriously limit its

usefulness.

Tracking Generators

The tracking generator (AT6010 only) is a

special signal source whose RF output

frequency tracks (follows) some other signal

beyond the tracking generator itself. In

conjunction with the spectrum analyzer, the

tracking generator produces a signal whose

frequency precisely tracks the spectrum

analyzers tuning. The tracking generator

frequency precisely tracks the spectrum

analyzer tuning since both are effectively

tuned by the same VTO. This precision

tracking exists in all analyzer scan modes.

Thus, in full scan, the tracking generator

output is a start-stop sweep, in zero scan the

output is simply a CW signal.

The tracking generator signal is generated by

synthesizing and mixing two oscillators. One

oscillator is part of the tracing generator itself,

the other oscillator is the spectrum analyzer's

1st LO. the spectrum analyzer/tracking

generator system is used in two

configurations: open-loop and closed-loop. In

the open-loop configuration, unknown

external signals are connected to the spectrum

analyzer input and the tracking generator

output is connected to a counter. This