Mb — Burst pulse and AFC

Frequency and power parameters

The following parameters define the centers of the transmit and receive intermediate frequency bands. Although the Tx and Rx intermediate frequencies are usually the same, you can choose them separately so that the RF up-conversion chain for transmission can be different from the down-conversion chain for reception.

Limits: 1 ... 120 MHz

Tx Intermediate Frequency: 30.0000 MHz
Rx Intermediate Frequency: 30.0000 MHz

The intermediate frequency is derived at the receiver's front-end by a microwave mixer and sideband filter. The filter passes either the lower sideband or the upper sideband, and rejects the other.

IF increases for an approaching target: YES

Depending on the chosen sideband, an increase in microwave frequency may increase (STALO below transmitter) or decrease (STALO above transmitter) the receiver's intermediate frequency. This parameter influences the sign of the Doppler velocities computed by RVP10.

Use the same channel for Rx and Burst Pulse Sampling: YES

The question Use the same channel for Rx and Burst Pulse Sampling defines the channel used for Rx and burst pulse sampling:

• Type NO to use the default Burst Sample Input of ADC-E.

• Type YES to sample the burst pulse on the channel configured as the IF signal input. See IFDR10 inputs.

  In this configuration, the IF signal and the burst pulse signal are received on the same ADC channel.

PhaseLock to the burst pulse: YES

The PhaseLock to the burst pulse question controls whether RVP10 locks the phase of its synthesized I and Q data to the measured phase of the burst pulse.

• Type YES for an operational magnetron system, since the transmitter's random phase must be known to recover Doppler data.

• Type AUTO for phase locking only when the burst pulse power exceeds the power defined in the ‘Minimum power for valid burst pulse question.

• Type NO for non-phase modulated klystron systems in which the IFDR sampling clock is locked to the STALO.

  NO is also useful for bench testing. In these cases, the phase of I and Q is determined relative to the stable internal sampling clock in the IFDR module.

Minimum power for valid burst pulse: -15.0 

Limits: -60 ... +10 dBm

The question Minimum power for valid burst pulse is the minimum mean power required in the burst pulse for it to be considered valid and suitable for input into the algorithms for frequency estimation and AFC. For information on the reporting burst pulse power, see Burst pulse timing plot (Pb).

The mean power level of the burst is computed within the narrowed set of samples used for AFC frequency estimation. The narrow pane contains only the active portion of the burst, and thus a mean power measurement is meaningful. The full FIR pane includes the leading and trailing pulse edges and would not produce a meaningful average power. Since radar peak power tends to be independent of pulse width, this single threshold value can be applied for all pulse widths.

The value entered here should be approximately 8 dB less. This makes sure that burst pulses are properly detected even if the transmitter power fades slightly.

Design/Analysis Window– 0:Rect , 1:Hamming, 2:Blackman : 1

Choose the analysis window used in the design of the FIR matched filter and the presentation of the power spectra for the scope plots.

Settling time (to 1%) of burst frequency estimator: 5.0 sec

Limits: 0.1 ... 120 s

The burst frequency estimator uses a fourth-order correlation model to estimate the center frequency of the transmitted pulses. Each burst pulse typically occupies approximately 1 µsec. The frequency estimate feeding the AFC loop must be accurate to approximately 10 KHz.

The accuracy for the question Design/Analysis Window– 0:Rect , 1:Hamming, 2:Blackman : 1 cannot be achieved with only one pulse. However, several hundred (unbiased) individual estimates can be averaged to produce an accurate mean. This averaging is done with an exponential filter with the time constant chosen here.

AFC and MFC parameters

Enable AFC and MFC functions: YES

AFC is required in magnetron systems to maintain the fixed intermediate frequency difference between the transmitter and the STALO.

AFC is not required in coherent transmitter systems, such as Klystrom systems, because the transmitted pulse is inherently at the correct frequency.

The following AFC parameters appear only if you enable the AFC and MFC functions.

AFC/MFC control server address : 127.0.0.1

AFC/MFC control server port : 4001

These questions set the IP address and unique application port where the RVP10 server may find the STALO controller.

AFC loop

Wait time before applying AFC: 10.0 sec
AFC hysteresis –– Inner: 5.0 KHz, Outer: 15.0 KHz

Limits: 0 ... 300 s

After turning on a magnetron transmitter, it can take several seconds or minutes until the output frequency is stable. The AFC loop does not need to run during this time. Use the question Wait time before applying AFC to set a holdoff delay from the time that valid burst pulses are detected to the time that the AFC loop begins running.

In general, the AFC feedback loop is active only when RVP10 is not processing data rays. This is because the Doppler phase measurement seriously degrades when the AFC control voltage makes a change. To avoid this, the AFC loop can only run between intervals of sustained data processing. This is fine as long as the host computer allows a few seconds of idle time every few minutes. If the RVP10 were constantly busy, the AFC loop would never have a chance to run.

The loop applies active feedback when the outer frequency limit is exceeded, but holds a fixed level once the inner limit has been achieved. The hysteresis zone minimizes the amount of thrashing done by the feedback loop. The AFC control voltage remains constant most of the time; making small and brief adjustments only occasionally as the need arises.

AFC outer tolerance during data processing: 50.0 KHz

Limits: 15 ... 4000 KHz

The question AFC outer tolerance during data processing defines the frequency error tolerances for the AFC loop by placing an upper bound on the frequency error that is tolerated during sustained data processing. AFC is applied when this limit is exceeded.

AFC loop feedback computations

AFC feedback slope:    0.0100 D-Units/sec / KHz 
AFC minimum slew rate: 0.0000 D-Units/sec
AFC maximum slew rate: 0.5000 D-Units/sec
AFC span– [-100%,+100%] maps into [ -32768 , 32767 ] 

The control applied to the AFC is specified in D-Units, that is, arbitrary units ranging from –100 ... +100 corresponding to the complete span.

Since the D-Units correspond to a percentage scale, the shorter % symbol is sometimes used.

AFC feedback is applied in proportion to the frequency error that the algorithm is attempting to correct. The feedback slope determines the sensitivity and time constant of the loop by establishing the AFC rate of change in (D–Units/sec) per thousand Hertz of frequency error. For example, a slope of 0.01 and a frequency error of 30 KHz results in a control voltage slew of 0.3 D–Units per second. At that rate it would take approximately 67 seconds for the output voltage to slew one tenth of its total span (20 D–Units / (0.3 D–Units / sec) = 67 sec). AFC is intended to track slow drifts in the radar system, so response times of this magnitude are reasonable.

Note that the feedback slew is based on a frequency error that is derived from a time averaging process (see burst frequency estimator Settling Time above).

The AFC loop becomes unstable if a large feedback slope is used with a long Settling Time constant, due to the phase lag introduced by the averaging process. Keep the loop stable by choosing a small enough slope that the loop easily comes to a stop within the inner hysteresis zone.

Burst parameters

Burst frequency increases with increasing AFC voltage: YES

If the frequency of the transmit burst increases when the AFC control voltage increases, type Yes. Otherwise type No.

When this parameter is set correctly, a numerical increase in the AFC drive (D–Units) results in an increase in the estimated burst frequency. If the AFC loop is completely unstable, try reversing this parameter.

Enable Burst Pulse Tracking: YES

The parameter Enable Burst Pulse Tracking enables the burst pulse tracking algorithm (see Burst pulse tracking). The characteristic settling times for the burst are defined elsewhere in this menu, and the tracking algorithm uses dynamic thresholds to control the feedback.

Enable Time/ Freq  hunt for missing burst: Yes
 Number of frequency intervals to search: 5 
 Settling time for each frequency hop: 0.25 sec 
Automatically hunt immediately after being reset: YES
    Repeat auto-hunt every: 60.00 sec

These parameters configure the process of hunting for a missing burst pulse.

• The trigger timing interval that is checked during Hunt Mode is always the maximum ±20 μsec. No further setup parameters are needed to define the hunting process in time.

• For the hunt in frequency, the overall frequency range is always be the full -100 ...  +100% AFC span.

  The number of sub-intervals to check must be specified, along with the STALO settling time after making each AFC change.

With the default values shown, AFC levels of -66%, -33%, 0%, +33%, and +66% are tried, with a one–quarter second wait time before checking for a valid burst at each AFC setting.

Enable burst power based correction of Z0: NO

The corrected calibration reflectivity numbers to do this are stored with the time series. To apply the correction, turn on the OPTS_ZCAL flag in the SOPRM command. See XARG 6 in Setup operating parameters (SOPRM).

Time constant of mean burst pulse power estimator: 500 pulses

The question Time constant of mean burst pulse power estimator defines how many pulses are used when computing the mean burst pulse power. The per pulse power based corrections of Z₀ are the difference between the individual burst pulse power sample and this mean power.

Simulate burst pulse samples: NO

RVP10 can simulate a 1 µsec envelope of burst samples. Use the parameter Simulate burst pulse samples as a testing and teaching aid only. Never use this in an operational system.

A two–tone simulation is produced when RVP10 is in dual-receiver mode. The pulse is the sum of 2 transmit pulses at the primary and secondary intermediate frequencies. To make the simulation more realistic, the signal strengths are unequal - the primary pulse is 3 dB stronger than the secondary pulse.

The simulated burst responds to AFC like a real radar.

Frequency span of simulated burst: 27.00 MHz to 32.00 MHz

The question Frequency span of simulated burst sets the bandwidth of the simulated burst pulse.