Spectral Purity (Harmonics or Spurs) Measurements
RI Measurement Approach
S Parameter Detection Hardware 100MHz to 20GHz
Relative RF Level Measurements
Single RF Tone at F1
User Specifies Harmonic Number, N or Spur Frequency F2
Measure: POUT(F1) and POUT(F1x N) or POUT (F2)
Calculate: POUT(F1) - POUT(F1x N) or POUT (F2)
Where POUT(F) = DUT Output Signal Level in dBm at Frequency F
The RI ATE system performs high performance spectral purity measurements very quickly (less than 20 msec) with the S Parameter detection hardware mentioned earlier. The user selects the RF frequency of the RF Stimulus Source (Source1) and the harmonic number or spur frequency to be measured. The harmonic number is entered as a System Scale Factor or the spur is entered as a System Frequency Offset. The system measures the signal level of both the transmitted fundamental tone and harmonic or spur signal (at the output of the DUT) by quickly tuning the system LO for the receiver to measure the fundamental signal, making an absolute power measurement, then tuning to the harmonic or spur signal and making another absolute power measurement. To improve testing times, the order in which these measurements is performed is controlled by the test plan optimizer. The difference in the signal levels is the harmonic or spur performance and the units displayed are dBc.
Test Head Configuration for Spectral Purity
The test configuration shown is a typical spectral purity measurement configuration for the RF Test Set. The test configuration is similar to making two port S Parameter measurements, but in high dynamic range instances, the extra sensitivity of the direct receive path may be needed. To measure the spectral purity of the DUT shown, the system needs to measure the levels of the DUT’s transmitted signals into port RF6 at both the fundamental frequency and the harmonic (or spur) frequency.
Both the transmitted fundamental and harmonic/spur signals are routed through port RF6 directional coupler incident arm, the electronic switch that selects the incident arm, or directly through the coupler and through the low loss mechanical port direction switch. Then both paths go through the final switch, a step attenuator, and a RF preamplifier to the single channel System Receiver (port REC) for signal processing.