USER’S MANUAL__________________________________________________________________
158 _________________________________________________________________ M211322EN-D
5.4.4 Computation of Filter Loss
The Ps printout displays the power loss (calibration error) that results when
the given filter is applied to the given transmit burst waveform. This allows
you to correct for the difference between what a broad-band power meter
measures as the overall transmit power, and what the RVP900
narrow-band receiver will detect within its passband. The filter loss is a
subtle quantity that depends on the combined characteristics of both the
transmit waveform and the receiver matched filter.
The filter loss is zero if the burst waveform consists of a pure sinusoid at
the designated intermediate frequency. It is also very near zero as long as
most of the burst energy is confined within the passband of the RVP900
filter. The filter loss will increase as the bandwidth of the burst waveform
increases and begins to spill out of that passband. Typical losses for a
well-matched filter are in the 0.5 dB to 1.8 dB range, depending on the FIR
length and other design criteria.
As an example, consider how the RVP900 filters would respond to a
simple rectangular pulse of energy lasting T
o
seconds. For this discussion
we can ignore the sinusoidal IF carrier that must also be present within the
pulse, and just focus on the rectangular envelope. This is valid because the
signal bandwidth, and hence the filter loss, is determined entirely by the
shape of the modulation envelope. For a pulse of length T
o
to have
unit-energy it must have an amplitude of . By centering this pulse
at time zero the power spectrum is easily computed using a real-valued
integral:
Whenever any of the following states appears, it implies
that AFC is enabled and that MFC is disabled:
(NoBurst) The energy in the burst is below the minimum
energy threshold for a valid pulse. The AFC
loop remains idle.
(Wait) The burst pulse has become valid just recently,
but the AFC loop is idle until the transmitter
stabilizes.
(Track) The burst pulse is valid, and the AFC loop is
tracking in order to bring the burst frequency
within the inner hysteresis limits.
(Locked) The burst pulse is valid and the AFC loop is
locked. The burst frequency is now within the
outer hysteresis limits and has previously been
within the inner limits while tracking. This is
the stable operational mode in which data
acquisition should take place.
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