Vaisala RVP900

Chapter 2 _______________________________________________ Introduction and Specifications
VAISALA_______________________________________________________________________ 33
to 95 MHz. In addition, there is a third 12-bit A/D video channel for output
for an auxiliary signal or clock up to 1.2 MHz.
The RVP900 digital transmitter finds a place within the overall radar
system that exactly complements the digital receiver. The receiver samples
an IF waveform that has been down-converted from RF. The transmitter
synthesizes an IF waveform for up-conversion to RF. The beauty of this
approach is that the RVP900 has complete control over both halves of the
radar, making possible a whole new realm of matched Tx/Rx processing
algorithms. For example:
- Phase Modulation—Some radar processing algorithms rely on
modulating the phase of the transmitter from pulse-to-pulse. This is
traditionally done using an external IF phase modulator that is
operated by digital control lines. While this usually works well, it
requires additional hardware and cabling within the radar cabinet, and
the phase/amplitude characteristics may not be precise or repeatable.
In contrast, the RVP900 can perform precise phase modulation to any
desired angle, without requiring the use of external phase shifting
hardware.
- Pulse Compression—There is increasing demand for siting radars in
urban areas, which have strict regulations on transmit emissions.
Often the peak transmit power is limited in these areas, so the job for
the weather radar is to illuminate its targets using longer pulses at
lower power. The problem is that a simple long pulse lacks the ability
(bandwidth) to discern targets in range. The remedy is to increase the
Tx bandwidth by modulating the overall pulse envelope, so that a
reasonable range resolution is restored. The exceptional fidelity of the
RVP900 waveform can accomplish this without introducing any of
the spurious modulation components that often occur when external
phase modulation hardware is used.
- Frequency Agility—This has been well studied within the research
community, but has remained out of the reach of practical weather
radars. The RVP900 changes all of this, because frequency agility is
as simple as changing the center frequency of the synthesized IF
waveform. Many new Range/Doppler unfolding algorithms become
possible when multiple transmit frequencies can coexist. Frequency
agility can also be combined with pulse compression to remedy the
blind spot, at close ranges, while the long pulse is being transmitted.
- COHO synthesis—The RVP900 output waveform can be
programmed to be a simple CW sine wave. It can be synthesized at
any desired frequency and amplitude, and its phase is locked to the
other system clocks. If you need a dedicated oscillator at some random
frequency in the IF band, this is a simple way to get it.