Intermodulation Distortion / Third Order Intercept Measurements
RI Measurement Approach
S Parameter Detection Hardware
RF Level Measurements
Two Equal Amplitude RF Tones
Specify RF Level, Center Freq (F1) and Spacing
2nd RF Tone: F2 = F1- Spacing
3rd Order Distortion Product Term measurements
POUT = (2 × F1 - F2)
Calculate: TOI / IP3 in dBm = POUT(F1) + [POUT(F1) - POUT(2×F1 - F2)] / 2
The measurement process for performing intermodulation distortion measurements is very similar to spectral purity measurements. The RI system must apply two RF stimulus signals to the DUT and uses the S parameter measurement hardware previously discussed to make the measurements. The user specifies the RF level applied by both signals, the RF frequency (F1) of the RF Stimulus Source (Source1) and the spacing between RF tones. The RF frequency of the second RF Stimulus Source (Source 2) is defined as F2 = F1 - selected spacing. The dual tone stimulus is connected to port RF3 and, as in the spurious tests, the measurements are made by the receiver tuned to specific frequencies to generate the IM data and calculate the final product.
The ATE System will measure the third order distortion product of the DUT’s output signal at the frequency shown above. To minimize measurement time, the order in which these measurements is performed is controlled by the test plan optimizer.
Test Configuration for Intermodulation Distortion
The diagram shown is a typical intermodulation distortion measurement configuration for the RF Test Head. The system sets the Source 1/2 Combiner module in the RF Matrix to send both Source 1 and Source 2 signals to an RF power combiner after all the amplification and attenuation is set and routes the combined signal to the Test Head Source 1 input port. The Dual RF signals are routed through the Test Head to the DUT’s RF input connected to port RF3. To measure the intermodulation distortion performance of the DUT shown, the system measures the levels of two DUT transmitted signals, the signal at the fundamental frequency (F1) and the intermodulation distortion product at frequency 2×F1 - F2. These signals are routed through the incident arm of the port RF6 directional coupler, a fixed attenuator, the electronic switch which selects the incident arm, another switch, a stop attenuator, a third switch and a RF preamplifier to the single channel System Receiver (connected to port REC) for signal processing.
2 Source AMP/Attemiatpr Module Settings for Intermodulation Distortion
The diagram shows two Source TIMs in a typical intermodulation distortion measurement configuration using the Source 1/2 Combiner Module. The system will set this module in the RF Matrix to send both Source 1 and Source 2 signals through their attenuation and power amplifiers to an RF broadband power combiner and routes the combined signal to the Test Head Source 1 input port.
Measurement Configuration for Intermodulation Distortion
The intermodulation distortion measurements made by the System Receiver is similar to RF power measurements, except the system LO frequency will be tuned for one of the two stimulus signals, the signal will be downconverted to a 21.4 MHz IF. The IF signal is conditioned (attenuated, filtered or amplified) and sent to the complex/synchronous quadrature detectors. The complex detector splits the received signal into two equal amplitude and phase signals and mixes one signal with a 21.4 MHz signal in-phase with the System Receiver’s internal 21.4 MHz SOURCE to create the I (in phase) signal component and mixes the other signal with a 21.4 MHz signal which is 90° out of phase with the 21.4 MHz SOURCE to create the Q (quadrature) signal component. The resulting signals are low pass filtered and sampled by high speed sample and hold circuits. The high accuracy A to D converter digitizes the sampled I and Q signal components and sends the digitized data over the RIFL II bus to the System controller for processing. The 21.4 MHz SOURCE signals are generated from (and slaved to) the system wide 10 MHz frequency reference/time base distributed by the RIFL bus. This process will be repeated for as may of the other signals as desired then the System Software calculates the relative level of the intermodulation product with respect to the level of the fundamental signal and reports that value in dBc. If the Third Order Intercept (TOI) representation of the date is required, the TOI (a.k.a. IP3) can be calculated from the same data.