RVP8 User’s Manual October 2004 Plot–Assisted Setups 4. Plot-Assisted Setups The IFD receiver module replaces virtually all of the IF components in a traditional analog receiver. The alignment procedures for those analog components are usually very tedious, and require continued maintenance even after they are first performed. Subtle drifts in component specifications often go unnoticed until they become so severe that the radar’s data are compromised.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.1 P+ — Plot Test Pattern The RVP8 can produce a simple test pattern to verify that the display software is working properly. From the TTY monitor enter the “P+” command. This will print the message “Plotting Test Pattern...” on the TTY and then produce the plot shown in Figure 4–1.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.2 General Conventions Within the Plot Commands The “Pb”, “Ps”, and “Pr” commands all have a similar structure to their TTY user interface. Each command begins by printing a list of subcommands that are valid in that context. These subcommands are single keystrokes that are executed immediately by the RVP8 as they are typed. The “ENTER” key is not required.
RVP8 User’s Manual October 2004 Plot–Assisted Setups The “Pb”, “Ps”, and “Pr” commands are intended to be used together for the combined purpose of configuring the RVP8’s digital front end. You may, of course, run any of the commands at any time; but the following procedure may be used as a guideline for first time setups. The full procedure must be repeated for each individual pulsewidth that the radar supports. 1. Use Mb to set the system’s intermediate frequency (See Section 3.2.6). 2.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.3 Pb — Plot Burst Pulse Timing For magnetron radars the RVP8 relies on samples of the transmit pulse to lock the phase of its synthesized “I” and “Q” data, and to run the AFC feedback loop. The “Pb” command is used to adjust the trigger timing and A/D sampling window so that the burst pulse is correctly measured. 4.3.
RVP8 User’s Manual October 2004 Plot–Assisted Setups It is possible to independently choose a subinterval of burst pulse samples that are used by the AFC frequency estimator. Thus, the AFC feedback loop is not constrained to use the same set of samples that are chosen for the FIR filter window. The FIR window typically is longer than the actual transmitted pulse, and thus, the samples contributing to the frequency estimate will include the leading and trailing edges of the pulse.
RVP8 User’s Manual October 2004 L/l & R/r Plot–Assisted Setups These two commands shift the entire group of six RVP8 triggers left or right (earlier or later in time). The lowercase commands shift in 0.025 msec steps, and the uppercase commands shift in 1.000 msec steps (approximately). The reason for shifting all six triggers at once is that the relative timing among the triggers remains preserved.
RVP8 User’s Manual October 2004 Freq Pwr DC Trig#1 BPT Plot–Assisted Setups Indicates the mean frequency of the burst, derived from a 4th order correlation model. The control parameters for this model are set using the “Mb” command (Section 3.2.6). Indicates the mean power within the full window of burst samples. DC offsets in the A/D converter do not affect the computation of the power, i.e., the value shown truly represents the waveform’s (Signal+Noise) energy.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Check that the burst pulse signal strength is reasonably matched to the input span of the RVP8/IFD’s A/D converter. The maximum analog signal level is +4dBm. Exceeding this level produces distorted samples that would seriously degrade the algorithms for phase locking and AFC. However, if the signal is too weak, then the upper bits of the A/D converter are wasted and noise is unnecessarily introduced.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.4 Ps — Plot Burst Spectra and AFC Once the transmit burst pulse has been captured the next step is to analyze its frequency content and to design a bandpass filter that is matched to the pulse. In a traditional analog receiver the matched filters use discrete components that can not be adjusted, and the transmit spectrum can not be viewed unless a spectrum analyzer is on hand.
RVP8 User’s Manual October 2004 Plot–Assisted Setups represents an integer multiple of 36MHz, either the left side or the right side will always be a multiple of 36MHz. This is important to remember when designing the matched filter, since fixed DC offsets in the A/D converters will appear aliased at these 72MHz multiples. The vertical axis of the spectrum plot is logarithmic and is marked with faint horizontal lines in 10-dB increments. An overall dynamic range of 70 dB can be viewed at once.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.4.2 Available Subcommands Within “Ps” The list of subcommands is printed on the TTY: Frequency span of the plot is 36.0 MHz to 72.0 MHz.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 2 4 – O 2 3 – N 2 2 – M 2 1 – L 2 0 – K 1 9 – J 1 8 – I 1 7 – H 1 6 – G 1 5 – F 1 4 – E 1 3 – D 1 2 – C 1 1 – B 1 0 9 8 7 6 5 4 3 2 1 0 – – – – – – – – – – – A 9 8 7 6 5 4 3 2 1 0 The Ps command continues to run normally during the AFC test mode. The customary AFC information will be replaced with a hexadecimal readout of the present 25-bit value. Your live display may look something like: Navg:3, FIR:1.33 usec (48 Taps), BW:1.
RVP8 User’s Manual October 2004 V/v Z/z % Plot–Assisted Setups “W/wN/n” subcommands to manually move to that starting point. Typing “$” would then print a dialog line in which the search span length and width are chosen. You may keep the indicated values or type in new ones, just as for all RVP8 setup questions. The search begins when the spans are accepted. The search procedure may require a few seconds to a few minutes, depending on the length and width spans that are being scanned.
RVP8 User’s Manual October 2004 FIR BW DC-Gain Freq Pwr Loss AFC Plot–Assisted Setups Indicates the length of the impulse response of the matched FIR filter. See description on Page 4–7. Indicates the actual 3dB bandwidth of the matched filter. This is the complete width of the passband from the lower frequency edge to upper frequency edge. Note that the filter’s center frequency is fixed at the radar’s intermediate frequency, as chosen in the “Mb” setup command.
RVP8 User’s Manual October 2004 Plot–Assisted Setups (NoBurst) (Wait) (Track) (Locked) The energy in the burst is below the minimum energy threshold for a valid pulse (See Page 3–25). The AFC loop remains idle. The burst pulse has become valid just recently, but the AFC loop is idle until the transmitter stabilizes (See Page 3–26) The burst pulse is valid, and the AFC loop is tracking in order to bring the burst frequency within the inner hysteresis limits.
RVP8 User’s Manual October 2004 Plot–Assisted Setups We can now examine what the filter loss (dB loss ) would be if this pulse were applied to a bandpass filter. The filter loss is simply the ratio of the power that is passed by the filter, divided by the total input power (1.0 in this case).
RVP8 User’s Manual October 2004 Plot–Assisted Setups ^ samples, but of a pure sine wave at the radar’s IF. The RVP8 determines B(f) according to its ^ sampled measurement of the transmitted waveform; however it can calculate C(f) internally based on an idealized sinusoid.
RVP8 User’s Manual October 2004 Plot–Assisted Setups other receiver and interpreting it as out-of-band input power. A consequence, however, is that the real out-of-band power is underestimated, i.e., the filter loss itself is underestimated. We recommend temporarily switching dual-receiver systems back to single-receiver mode when the filter loss is being measured. This is easily done by changing the Mc setup question back to “single”, and disconnecting the secondary burst input to the RVP8/IFD. 4.4.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Begin with the FIR length that was chosen previously in the Pb command, and use the “N” and “W” keys to set an initial bandwidth equal to the reciprocal of the pulsewidth. The main lobes of the two plots should more-or-less overlap. Experiment with changing the FIR length and bandwidth to achieve a filter with the following properties. S The filter width should be no greater than the burst spectral width.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Figure 4–5 shows a 60MHz filter with particularly poor (–42dB) DC rejection. The frequency range of the plot is 36–72MHz; hence, DC appears aliased at the right edge and we can see a peak in the filter’s stopband at DC. Contrast this with the filter shown in Figure 4–3 that has a true zero at DC.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.5 Pr — Plot Receiver Waveforms The “Pb” and “Ps” commands described in the previous sections have been used to analyze the signal that is applied to the “Burst-In” connector of the RVP8/IFD receiver module. The task that remains is to checkout the actual received signal that is connected to “IF-In”. The goal is to verify that the received signal is clean and appropriately scaled, and that nearby targets can be seen clearly.
RVP8 User’s Manual October 2004 Plot–Assisted Setups the two equal-power targets just to the left of center are approximately 18dB down from the top. The amplitude of the samples is thus 10 (*18ń20) + 0.13 , i.e., 13% of full scale. This correspondence between the LOG scale and the amplitude scale applies regardless of the plot’s zoom level. As the IF samples are zoomed up and down by factors of two, the LOG plot will shift up and down in 6dB steps.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Figure 4–7: Example of a Noisy High Resolution “Pr” Spectrum The Pr spectrum plot will properly show a 0-Hz peak from any DC offset in the A/D converter, and is thus consistent with how the DC offset is presented in the Pr sample plot. Both of these plots preserve the DC component of the IF samples so that it can be monitored as part of the routine maintenance of the receiver system.
RVP8 User’s Manual October 2004 T/t V/v Z/z % Plot–Assisted Setups starting point is displayed both in microseconds and kilometers on the TTY, and is not allowed to be set earlier than range zero. The “T” command increments or decrements the time duration of the window of IF samples. The window is not allowed to become shorter than the impulse response length of the FIR filter, since that would preclude calculating even a single LOG power point.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Total Indicates the total RMS power that is being detected by the IF-Input A/D converters. This total is computed using all of the raw IF samples in the chosen interval, and is the sum of power at all frequencies other than 0 Hz (and its aliases). Filtered Indicates the RMS power that falls only within the passband of the FIR filter for the current pulsewidth. This is computed using all of the raw IF samples in the chosen interval.
RVP8 User’s Manual October 2004 Plot–Assisted Setups 4.6 Pa — Plot Tx Waveform Ambiguity With the introduction of the RVP8/Tx Digital Transmitter PCI Card it is now possible for the RVP8 to make radar observations using compressed pulse waveforms. This opens up many new opportunities within the weather radar community for using low-power solid-state transmitters that employ very long pulse lengths (20–80μsec) .
RVP8 User’s Manual October 2004 Plot–Assisted Setups Also shown in yellow and green are the Tx/Rx responses when the overall waveform is modified by a 50KHz target Doppler shift. Real weather targets would never have such a large Doppler component, but the Pa menu allows you to study its effect anyway. An alternate form of Pa plot of the same Tx waveform is shown in Figure 4–9. The horizontal axis again represents time, but now spans the entire duration of the pulse.
RVP8 User’s Manual October 2004 Plot–Assisted Setups This waveform was designed using the “$” automatic search-and-optimize command in the Pa menu. For a given pulse length and bandwidth of the Tx waveform, this command allows you to try thousands of combinations of FM shape and amplitude shape, searching for the combination that minimizes the sum of PSL and ISL (in dB). This gives the best overall waveform for weather radar observations in which both the PSL and ISL are important. 4.6.
RVP8 User’s Manual October 2004 Plot–Assisted Setups Z/z The dynamic range of the Pa sidelobe plot is 80dB. Usually this will give plenty of room to examine the properties of the waveform. But for very wide dynamic range pulses, you can shift the plot up/down in 10dB steps using these “Zoom” keys. $ Designs an optimal compressed waveform.
RVP8 User’s Manual October 2004 Plot–Assisted Setups PSL Peak Sidelobe Level of the ambiguity diagram, expressed in deciBels relative to the main lobe level. This is the peak height of the strongest range/time sidelobe, and measures the ability of the compressed pulse to distinguish a given target from a small number of individual point targets that also lie within the pulse volume. The waveform’s ability to “see” between clutter targets is largely determined by the PSL level.
RVP8 User’s Manual October 2004 Plot–Assisted Setups internal APIs directly to design any desired transmit waveform along with the associated FIR filter to receive it. 4.6.4 Bench Testing of Compressed Waveforms Working with compressed pulse waveforms can be tricky, so it is reassuring to run some simple bench tests to verify that things are working properly. Once the Tx waveform has been designed it can be injected into the IFD for testing with the Pr command.
RVP8 User’s Manual October 2004 Plot–Assisted Setups The Ps plotting command can also be used to examine the ideal transmit spectrum and actual received spectrum of compressed pulses. An example is shown in Figure 4–11 below for a 60MHz, 40μsec linear FM pulse having a bandwidth of 2MHz. The energy in the pulse is both sharply contained within and uniformly distributed over the 2MHz frequency interval centered on the IF carrier.
