SIGMET RVP8 USERS GUIDE 141
1. Introduction and Specifications The RVP8 Lineage SIGMET Inc. has a 20-year history of supplying innovative, high-quality signal processing products to the weather radar community. The history of SIGMET products reads like a history of weather radar signal processing: Units Year Model 1981 Sold Major Technical Milestones FFT 10 First commercial FFT-based Doppler signal processor for weather radar applications. Featured Simultaneous Doppler and intensity processing.
Open Hardware and Software Design Compared to previous processors that were built around proprietary DSP chips, perhaps the most innovative aspect of the RVP8 is that it is implemented on standard PC hardware and software that can be purchased from a wide variety of sources. The Intel Pentium/PCI approach promises continued improvement in processor speed, bus bandwidth and the availability of low–cost compatible hardware and peripherals. The performance of an entry level RVP8 (currently dual 2.
Standard LAN Interconnection for Data Transfer or Parallel Processing For communication with the outside world, the RVP8 supports as standard a 10/100/1000 Base T Ethernet. For most applications, the 100 BaseT Ethernet is used to transfer moment results (Z, T, V, W) to the applications host computer (e.g., a product generator). However, the gigabit Ethernet is sufficiently fast to allow UDP broadcast of the I and Q values for the purpose of archiving and/or parallel processing.
1.1 System Configuration Concepts The hardware building blocks of an RVP8 system are actually quite few in number: ᪽ RVP8/IFD᪽ IF Digitizer Unit-This is a separate sealed unit usually mounted in the receiver cabinet. The primary input to the IFD is the received IF signal. In addition, the IFD has channels to sample the transmit pulse and to take in an external clock to phase lock the A/D conversion with the transmit pulse (not used for magnetron systems). ᪽ RVP8/Rx᪽ Card-A PCI card mounted in the chassis.
IFD- IF Digitizer installed in the radar receiver cabinet. This can be located up to 100 meters from the RVP8 main chassis (fiber optic connection). The DAFC (Digital AFC) is an option to interface to a digitally controlled STALO. Like the RVP7, the RVP8 provides full AFC with burst pulse auto-tracking. RVP8/Rx- The digital receiver collects digitized samples from the IFD and does the processing to obtain I/Q. It also provides two trigger connections configurable for input or output.
RVP8/Tx- The digital transmitter card provides the digital Tx waveform. A second output can be used to provide a COHO in the event that the RVP8 is used to provide the system master clock. In any case, the IF transit waveform and the A/D sampling are phase locked. SIGMET I/O-62 card for additional triggers, parallel, synchro or encoder AZ and EL angle inputs, pulse width control, spot blanking control output, etc. These signals are brought in via the connector panel.
Example 3: Dual Polarization Magnetron System In this system 2 IFD’s and two RVP8/Rx cards are used for the horizontal and vertical channels of a dual-channel receiver. The legacy RVP7 technique of using a single IFD and two IF frequencies for the horizontal and vertical channels (e.g., 24 and 30 MHz) is also supported by the RVP8. In the case of either dual or single IFD’s, there is a synch clock provided by either the STALO reference frequency (e.g., 10 MHz) or by the RVP8 itself.
COTS Accessories Aside from the basic PCI cards required for the radar application, there are additional cards that can be installed to meet different customer requirements, e.g., 10/100–BaseT Ethernet card for additional network I/O (e.g., a backup network). RS-232/RS-422 serial cards for serial angles, remote TTY control, etc. Sound card to synthesize audio waveforms for wind profiler applications. GPS card for time synch. IEEE 488 GPIB card for control of test equipment.
1.1.1 IFD IF Digitizer The IFD 14-bit IF digitizer is a totally sealed unit for optimum low-noise performance. The use of digital components within the IFD is minimized and the unit is carefully grounded and shielded to make the cleanest possible digital capture of the input IF signal. Because of this, the IFD achieves the theoretical minimum noise level for the A/D convertors. There are 4 inputs to the IFD: IF video signal.
1.1.2 Digital Receiver PCI Card (RVP8/Rx) The RVP8/Rx card receives the digitized IF samples from the IFD via the fiber optic link. The advantage of this design is that the receiver electronics (LNA, RF mixer, IF preamp, and IFD) can be located as far as 100–meters away from the RVP8 main chassis. This makes it possible to choose optimum locations for both the IFD and the RVP8, e.g., the IFD could be mounted on the antenna itself, and the processor box in a nearby equipment room.
Calibration Plot for RVP8/IFD The figure above shows a calibration plot for a 14-bit IFD with the digital filter matched to a 2 microsecond pulse. The performance in this case is >100 dB dynamic range. The RVP8 performs several real time signal corrections to the I/Q samples from the Rx, including: Amplitude Correction- A running average of the transmit pulse power in the magnetron burst channel is computed in real-time by the RVP8/Rx.
᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽ ᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽᪽ The built–in filter design tool makes it easy for anyone to design the optimal IF filter to match each pulse width and application. Simply specify the impulse response and pass band and the filter appears. The user interface makes it easy to widen/narrow the filter with simple keyboard commands. There is even a command to automatically search for an optimal filter.
1.1.3 Mother Board or Single-Board Computer (SBC) The dual-CPU Pentium mother board or single-board computer (SBC) acts as the host to the Linux operating system and provides all of the compute resources for processing the I/Q values that are generated by the RVP8/Rx card. Standard keyboard, mouse and monitor connections are on the Rx backpanel, along with a 10/100/1000 BaseT Ethernet port.
be precise or repeatable. In contrast, the RVP8/Tx 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 that also happen to 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 somehow illuminate its targets using longer pulses at lower power.
1.1.5 I/O-62 PCI Card and I/O Panel The SIGMET I/O-62 is a short format PCI card that provides extensive I/O capabilities for the RVP8. A typical installation would have one I/O-62 and an RVP8 Connector Panel shown above. The Softplane᪽ is used to interconnect the I/O 62 with other SIGMET PCI cards. Note that the identical card is used in the SIGMET RCP8 radar/antenna control processor which in general does not use the Softplane᪽ connection. The I/O-62 has a single 62-position, high-density “D” connector.
Run Time FPGA Configuration The SIGMET I/O-62 card is built around a 100K–Gate FPGA which, in addition to driving the I/O signals on the 62-position connector, also coordinates the PCI and Softplane᪽ traffic. These chips are SRAM–based, meaning that they are configured at run time. This allows the FPGA code to be automatically upgraded during each RVP8 code release without needing to physically reprogram any parts. The board’s basic I/O services use up only 40% of the complete FPGA.
1.2 Comparison of Analog vs Digital Radar Receivers 1.2.1 What is a Digital IF Receiver? A digital IF receiver accepts the analog IF signal (typically 30 MHz), processes it and outputs a stream of wide dynamic range digital “I” and “Q” values. These quantities are then processed to obtain the moment data (e.g., Z, V, W or polarization variables). Additionally, the digital receiver can accept the transmit pulse “burst sample” for the purpose of measuring the frequency, phase and power of the transmit pulse.
1.2.2 Magnetron Receiver Example A typical analog receiver for a magnetron system is shown in the top portion of Figure 1–1. The received RF signal from the LNA is first mixed with the STALO (RF–IF) and the resulting IF signal is applied to one of several bandpass filters that match the width of the transmitted pulse. The filter selection is usually done with relays. The narrow band waveform is then split.
RVP8 User’s Manual October 2006 Introduction and Specifications Figure 1–1: Analog vs Digital Receiver for Magnetron Systems 160
1.2.3 Klystron or TWT Receiver and Transmit RF Example A typical analog receiver for a klystron system is shown in the top portion of Figure 1–2. The arrangement of components is similar to the magnetron case, except that the COHO operates at a fixed phase and frequency, a phase shifter is included for 2nd trip echo filtering and there is no AFC feedback required.
1.3 RVP8 IF Signal Processing 1.3.1 IFD Data Capture and Timing The RVP8 design concept is to perform very little signal processing within the IFD digitizer module itself. This is to minimize the presence of digital components that might interfere with the clean capture of the IF signals. The digitized IF and burst pulse samples are multiplexed onto the fiber channel link which provides the digital data to the RVP8/Main board at approximately 540-MBits/sec.
1.3.2 Burst Pulse Analysis for Amplitude/Frequency/Phase The burst pulse analysis provides the amplitude, frequency and phase of the transmitted pulse. The phase measurement is analogous to the COHO locking that is performed by a traditional magnetron radar. The difference is that the phase is known in the digital technique, so that range dealiasing using the phase modulation techniques is possible.
1.3.3 Rx Board and CPU IF to I/Q Processing Figure 1–3: IF to I/Q Processing Steps The RVP8/Rx board performs the initial processing of the IF digital data stream and outputs “I” and “Q” data values to the host computer via the PCI bus. In addition, the frequency, phase and amplitude of the burst pulse are measured. The functions performed by the processor are: Reception of the digital serial fiber optic data stream.
Computation of “I” and “Q” quadrature values (also performed during the band pass filtering step). Transmit burst sample frequency, phase and amplitude calculation I and Q phase and amplitude correction based on transmit burst sample. Interference rejection algorithm. AFC frequency error calculation with output to IFD for digital or analog control of STALO (for magnetron systems). The advantage of the digital approach is that the software algorithms for these functions can be easily changed.
1.4 RVP8 Weather Signal Processing The processing of weather signals by the RVP8 is based on the algorithms used in the previous generation RVP7 and RVP6. However, the performance of the RVP8 allows a different approach to some of the processing algorithms, especially the frequency domain spectrum processing. All of the algorithms start with the wide dynamic range I and Q samples that are obtained from the Rx card over the PCI bus.
1.4.1 General Processing features Figure 1–4 shows a block diagram of the processing steps. These are discussed below. Autocorrelations The autocorrelations R0, R1 and R2 are produced by all three processing modes. However, the way that they are produced is different for the three modes, particularly with regard to the filtering that is performed. Pulse Pair Mode — Filtering for clutter is performed in the time domain. Autocorrelations are computed in the time domain.
Time (azimuth) Averaging The autocorrelations are based on input “I” and “Q” values over a selectable number of pulses between 8, 9, 10, ...,256. Any integer number of pulses in this interval may be used including DFT/FFT and random phase modes. Selectable angle synchronization using the input AZ and EL tag lines assures that all possible pulses are used during averaging for each, say, 1 degree interval. This minimizes the number of “wasted” pulses for maximum sensitivity.
Thresholding The RVP8 calculates several parameters that are used to threshold (discard) bins with weak or corrupted signals. The thresholding parameters are: Signal quality index (SQI=|R1|/R0) LOG (or incoherent) signal-to-noise ratio (LOG) SIG (coherent) signal-to-noise ratio CCOR clutter correction These parameters are computed for each range bin and can be applied in AND/OR logical expressions independently for dBZ, V and W.
Unambiguous PRF1 PRF2 Range (km) 3 cm 5 cm 10 cm 500 1000 2000 375 750 1500 300 150 75 11.25 22.50 45.00 18.75 37.50 75.00 37.50 75.00 150.00 500 1000 2000 400 800 1600 300 150 75 15.00 30.00 60.00 25.00 15.00 100.00 50.00 100.00 200.00 Three Unfolding Times Uf ldi Four Times Ti 1.4.2 RVP8 Pulse Pair Time Domain Processing Pulse pair processing is done by direct calculation of the autocorrelation.
1.4.4 Random Phase Processing for 2nd Trip Echo Second trip echoes can be a serious problem for applications that require operation at a high PRF. Second trip echoes can appear separately or can be overlaid on first trip echoes (second trip obscuration). The random phase technique separates the first and second trip echoes so that: In nearly all cases, the 2nd trip echo can be removed from the first trip even in the case of overlapped 1st and 2nd trip echoes. The benefit is a clean first trip display.
1.5 RVP8 Control and Maintenance Features 1.5.1 Radar Control Functions The RVP8 also performs several important radar control functions: Trigger generation- up to 6 programmable triggers. Pulsewidth control (four states controlled by four bits). Angle/data synchronization- to collect data at precise azimuth intervals (e.g., every 0.5, 1, 1.5 degrees) based on the AZ/EL angle inputs. Phase shifter- to control the phase on legacy Klystron systems.
1.5.2 Power-Up Setup Configuration The RVP8 stores on disk an extensive set of configuration information. The purpose of these data is to define the exact configuration of the RVP8 upon startup. The setup information can be accessed and modified using either a local keyboard and monitor, or over the network. For multiple radar networks, the configuration management can be centrally administered by copying tested “master” configuration files to the various network radars.
1.6 Support Utilities and Available Application Software The RVP8 system includes a complete set of tools for the calibration, alignment and configuration of the RVP8. These includes the following utilities: ascope-a comprehensive utility for manual signal processor control and data display of moments, times series and Doppler spectra. ascope includes a realistic signal simulator capable of producing both first and second trip targets. Recording/playback of time series and moments is included as well.
1.7 System Network Architecture The RVP8 provides considerable flexibility for network operation. This allows remote control and monitoring of the system from virtually anywhere on the network, subject to the user’s particular security restrictions. Unlike the previous generation RVP7, which used a SCSI interface, the RVP8 uses a network interface exclusively. The “dsp lib” runs locally on the RVP8 and a utility, called DspExport, exports the library over the network using a TCP/IP socket.
1.8 Open Architecture and Published API SIGMET recognizes that certain users may require the ability to write their own signal processing algorithms which will run on the RVP8. To accommodate this, the RVP8 software is organized to allow separately compiled plug-in modules to be statically linked into the running code.
1.9 RVP8 Technical Specifications 1.9.1 IFD Digitizer Module, Rev E or later Input Signals ᪽ IF Received Signal: 50᪽, + 6.5 dBm full–scale, +20dBm absolute max ᪽ IF Burst or COHO: 50᪽, +6.5 dBm full–scale, +20dBm absolute max ᪽ Optional Reference Clock: 2–60 MHz –10 to 0 dBm IF Ranges: ᪽ 12—34 MHz, 38—70 MHz Linear Dynamic Range ᪽ 85 to >100dB depending on pulsewidth/bandwidth filter A/D Conversion Resolution 14 bit with jitter <2.5 picosec Sampling rate 67 to 79 MHz (selectable, standard is 71.
1.9.2 RVP8/Rx PCI Card, Rev C or later Pulse Repetition Frequency ᪽ 50 Hz to 20 KHz +0.1%, continuously selectable. IF Band Pass Filter ᪽ Programmable Digital FIR with software selectable bandwidth. Built-in filter design software with graphical user interface. Impulse Response ᪽ Up to 3024 FIR filter taps, corresponding to 75 ᪽sec impulse response length for 72 MHz IF samples at 125 meter range resolution. These very long filters are intended for use with pulse compression.
1.9.3 RVP8/Tx PCI Card Analog Waveform Applications Digitally synthesized IF transmit waveform for pulse compression, frequency agility, and phase modulation applications. Master clock or COHO signal to the radar; can be phase locked or free running, arbitrary frequency. Analog Output Waveform Characteristics ᪽ Two independent, digitally synthesized, analog output waveforms (BNC).
1.9.4 SIGMET I/O-62 PCI Card Short format PCI card with 62-position “D” connector. Multiple cards may be installed. Includes D/A, A/D, discrete inputs and outputs (TTL, wide range, RS422, etc.) See summary table below. I/O pin assignment mapping by softplane.conf file. Standard or custom remote backpanels available. ᪽ ESD protection using Tranzorb᪽ silicon avalanche diode surge suppression and high-voltage tolerant components.
1.9.5 I/O-62 Standard Connector Panel Mounts on front or rear of standard 19” EIA rack Connects to I/O-62 via 1:1 62–pin 1.8–m cable (provided). Provides standard inputs and outputs required by most weather radars such as triggers, polarization control, pulse width control and antenna angles. Az and El synchro and reference inputs (nominal 100V 60 Hz) 3 internal relays and 4 12V relay control signals for switching external devices. Programmable scope test points with source waveforms selectable in software.
1.9.6 RVP8 Processing Algorithms Input from Rx Board 16-bit I/Q samples Optional dual-channel I/Q samples (e.g., for polarization systems or dual frequency systems) IQ Signal Correction Options Amplitude jitter correction based on running average of transmit power from burst pulse.
W Spectrum width, 8 or 16 bits I/Q Time series, 16 bits each per sample DFT Doppler Spectrum output option in DFT mode, 16 bits per component Optional: ZDR, PHIDP, RHOHV, LDR, RHO, 8 or 16 bits Data Quality Thresholds Signal–to–noise ratio (SNR) Used to reject bins having weak signals. Typically applied to dBZ. Signal quality index (SQI) Used to reject bins having incoherent signals. Typically applied to mean velocity and width.
1.9.7 RVP8 Input/Output Summary Ethernet Input/Output from Host Computer ᪽ Data output of calibrated dBZ, V and W during normal operation. Full I/Q timeseries recording with a separate tsarchive utility, or through a customer’s application using a public API. Signal processor configuration and verification read–back is performed via the Ethernet interface. RS-232C Serial Data I/O ᪽ For real time display/monitoring or data remoting.
1.9.8 Physical and Environmental Characteristics Packaging ᪽ Motherboard Configuration 4U rack mount with 6 PCI slots ᪽ Custom PC configurations available or packaged by customer. ᪽ Dimensions of standard 4U chassis 43.2 wide x 43.2 long x 17.8 cm high 17 wide x 17 long x 7.00 inch high ᪽ Dimensions IF Digitizer 2.5 wide x 10.9 long x 23.6 cm high 1 wide x 4.3 long x 9.3 inch high ᪽ Redundant Power Supplies. Three hot–swap modules with audio failure alarm.
