Figure 2: 3.8 GHz pulsed signals, with pulse widths ≥40 ns and PRF ≤5 MHz.
Additive Phase Noise
Phase noise in radar systems comes from various sources, not only the frequency synthesizer: most notably, the pulse modulator and power amplifiers. So, when analyzing a pulsed radar system, it is informative to assess the added phase noise from the amplifier stages. To measure additive noise, the amplifier must be operated under real conditions using a low noise, pulse-modulated signal source.
The LO output of the APPH signal source analyser can be used for this. Figure 3 shows the setup of a two-channel, cross-correlated additive phase noise measurement of an amplifier. The pulsed driving signal for the DUT is synthesized directly in the APPH and split into three paths and fed into the two REF inputs and the RF input of the signal source analyser. Besides a three-way splitter, only two mechanical phase shifters are required to tune the reference paths into phase quadrature, where the phase noise of the driving signal is cancelled and the residual noise of the DUT can be measured. The cross-spectrum measurement rejects instrument noise and substantially enhances instrument sensitivity. The APPH software guides the user through two calibration steps, making the measurement virtually as simple as an absolute phase noise measurement.