Chapter 2 _______________________________________________ Introduction and Specifications
VAISALA_______________________________________________________________________ 35
The digital approach replaces virtually all of the traditional IF receiver
components with flexible software-controlled modules. They can be easily
adapted to function for a wide variety of radars and operational
requirements.
The digital receiver approach made a very rapid entry into the weather
radar market. Up until about 1997, weather radars were not supplied with
digital receivers. Today, nearly all new weather radars and weather radar
upgrades use the digital receiver approach. Much of this rapid change is
attributed to the previous generation RVP7 and RVP8, which are the most
widely sold weather radar signal processor of all time.
The number one advantage of a digital receiver is that it achieves a wide
linear dynamic range (for example, >95 dB depending on pulse width),
without having to use AGC circuits, which are complex to build, calibrate,
and maintain. Other advantages include:
- Lower initial cost by eliminating virtually all IF receiver components
- Lower life-cycle cost due to reduced maintenance
- Selectable IF frequency
- Software controlled AFC with automatic alignment
- Programmable band pass filter
- Dual or multiple IF multiplexing
- Improved remote monitoring down to the IF level
The following sections compare the digital receiver approach to the analog
receiver approach. This illustrates the advantages of the digital approach
and what functions are performed by a digital receiver.
2.7.2 Magnetron Receiver Example
For a typical analog receiver of a magnetron system (see Figure 10 on page
35 (top portion)), the received RF signal from the LNA is first mixed with
the STALO (RF-IF). The resulting IF signal is applied to one of several
band pass filters that match the width of the transmitted pulse. The filter
selection is usually done with relays. The narrow band waveform is then
split. Half is applied to a logarithmic amplifier (LOG), having a dynamic
range of 80 dB to 100 dB, from which a calibrated measurement of signal
power can be obtained. The log amplifier is required, because it is almost
impossible to build a linear amplifier with the required dynamic range.
However, phase distortion within the log amplifier renders it unsuitable for
making Doppler measurements; therefore, a separate linear channel is still
required.
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