Amplitude correction for Tx power fluctuations
RVP10 can perform pulse-to-pulse amplitude correction of the digital (I,Q) data stream based on the amplitude of the Burst/STALO input. The technique computes a (real valued) correction factor at each pulse by dividing the mean amplitude of the burst by the instantaneous amplitude of the burst. The (I,Q) data for that pulse are then multiplied by this scale factor to obtain corrected time series. The amplitude correction is applied after the Linearized Saturation Headroom correction.
Instantaneous amplitude correction is a unique feature of the RVP10 digital receiver. Bench tests with a signal generator reveal that an amplitude modulated waveform having 2.0 dB of pulse-to-pulse variation is reduced to less than 0.02 dB RMS of (I,Q) variation after applying the amplitude correction.
The mean burst amplitude is computed by an exponential average whose (1/e) time constant is selected as a number of pulses. See Mp — processing options.
A short time constant settles faster, but is not as thorough in removing amplitude variations (since the mean itself varies). Longer time constants are better in this sense, but require a few seconds before valid data is available when the transmitter is first turned on. The default value of 70 gives excellent results in almost all cases.
When RVP10 enters a new internal processing mode (time series, FFT, PPP, and so on), the burst power estimator is reinitialized from the level of the first pulse encountered, and an additional pipeline delay is introduced to allow the estimator to completely settle. Valid corrected data is produced even when RVP10 alternates rapidly between different data acquisition tasks, for example, in a multi-function Ascope display. The additional pipeline delay does not affect the high-speed performance when RVP10 runs continuously in a single mode.
For amplitude correction to be applied, the instantaneous Burst/STALO signal level must exceed the minimum valid burst power specified in the Mb setup section. If that level is not met (for example, if the transmitter is turned off), then no correction is performed. The amplitude correction feature is not in operation when receiver-only tests are performed.
The maximum applied correction is ±5 dB. If the burst power in a given pulse is more than 5 dB above the mean, or less than 5 dB below it, then the correction is clamped at those limits. The power variation of a typical transmitter is easily contained within this interval (it is typically less than 0.3 dB).