Product specifications

8
Narrowband detection
Narrowband detection is used when most of the pulsed-RF spectrum is outside
the bandwidth of the receiver. In other words, the pulse width is narrower than
the minimum data acquisition period with the widest available receiver band-
width. With this technique, the entire pulse spectrum is removed by filtering
except the central frequency component, which represents the frequency of the
RF carrier. After filtering, the pulsed-RF signal appears as a sinusoid or CW sig-
nal. With narrowband detection, the analyzer samples are not synchronized with
the incoming pulses (therefore no pulse trigger is required), so the technique is
also called asynchronous acquisition mode. Usually, the PRF is high compared
to the IF bandwidth of the receiver, so the technique is also sometimes called
the “high PRF” mode.
Agilent has developed a novel way of achieving narrowband detection using
wider IF bandwidths than normal, by using a unique “spectral nulling” and
“software gating” techniques that will be explained later. These techniques let
the user trade dynamic range for speed, with the result almost always yielding
faster measurements than those obtained by conventional filtering.
The advantage to narrowband detection is that there is no lower pulse-width
limit, regardless of how broad the pulse spectrum is, most of it is filtered away,
leaving only the central spectral component. The disadvantage to narrowband
detection is that measurement dynamic range is a function of duty cycle. As
the duty cycle of the pulses becomes smaller (longer time between pulses),
the power of the central spectral component becomes smaller, resulting in less
signal-to-noise ratio as shown in Figure 5. Using this method, measurement
dynamic range decreases as duty cycle decreases. This phenomenon is often
called “pulse desensitization” and can be expressed as 20*log (duty cycle) in
dB. The PNA-X employs a number of unique features to minimize this effect,
resulting in considerably less degradation in dynamic range. More details about
these features will be discussed later.
Figure 5. Duty cycle (time domain) versus signal-to-noise ratio of center spectrum (frequency
domain)