Vaisala RVP900

Chapter 4 _____________________________________________________ TTY Nonvolatile Setups
VAISALA______________________________________________________________________ 125
in Section 4.2.5 Mt<n>— Triggers for Pulsewidth #n on page 117. But
within this same system, PW–2 and PW–3 could be further configured as,
perhaps, 20 µsec and 40 µsec compressed non–linear FM waveforms. This
makes it very easy for application software such as ascope to transparently
switch between radically different Tx waveforms simply by requesting a
different pulse width for each one.
The following questions appears in the Mt<n> menu (immediately after
the Rx Intermediate Frequency question) when digital Tx waveforms are
being synthesized.
Tx Waveform – 0:CWPulse, 1:LinFM, 2:NLFM : 2
The RVP900 supports three standard Tx waveforms: a conventional fixed–
frequency CW pulse, a linear FM chirp, and non–linear FM. The CWPulse
can be used as a pulsed Doppler waveform in all the same ways that a
Klystron or Magnetron system having a traditional pulse forming network
would be used. The linear and non–linear FM waveforms, however, are
compressed pulses that are intended to be transmitted by a wide–
bandwidth Klystron/TWT/SolidState amplifier.
Bandwidth of transmit pulse: 3.25 MHz
Pulselength of transmit pulse: 15.00 usec
These questions select the bandwidth and pulse length of the Tx waveform.
The bandwidth value represents the true spectrum width of the complete
waveform, that is, including all the effects of whatever frequency
modulation and amplitude modulation the waveform happens to use. Thus,
a spectrum analyzer (or the RVP900 Ps plot) would show an overall
spectrum width equal to this desired value.
Similarly, the pulse length value represents the entire time duration of the
waveform, including whatever amplitude modulations may be included at
the tails.
Waveforms are synthesized by the RVP900 using a 16-bit TxDAC
followed by an analog bandpass filter centered at the midpoint of the IF
interval. The analog filter is necessary to remove out-of-band components
from the TxDAC, but has a side effect of introducing a bandwidth
limitation within the IF passband. The result is that the shape of very short
pulses (on the order of 100 nanoseconds) will be dominated by the impulse
response of the analog filter rather than by the exact digital synthesis. In
practice the Tx pulse width should be longer than 300 nanoseconds order
NOTE
The RVP900 internal APIs permit code developers to create arbitrary
waveforms for transmission. The three types mentioned above are the
out–of–the–box selections that are standard on all RVP900 processors.