223ConceptsBaseband I/Q Inputs (Option BBA) Measurement ConceptsSelecting input probes for baseband measurementsThe selection of baseband measurement probe(s) and measurement method isprimarily dependent on the location of the measurement point in the circuit. Theprobe must sample voltages without imposing an inappropriate load on the circuit.The system supports a variety of 1 MΩ impedance input passive probes as well asthe Keysight 1153A active differential probe using the InfiniMax probe interface.The Keysight 1153A active probe can be used for both single-ended and differentialmeasurements. In either case a single connection is made for each channel (on eitherthe I or Q input). The input is automatically configured to 50 Ω single-ended typemeasurement and the probe power is supplied through the InfiniMax interface. Theprobe can be configured for a variety of input coupling and low frequency rejectionmodes. In addition, a wide range of offset voltages and probe attenuation accessoriesare supported at the probe interface. The active probe has the advantage that it doesnot significantly load the circuit under test, even with unity gain probing.With passive 1 MΩ probes, the probe will introduce a capacitive load on the circuit,unless a higher attenuation is used at the probe interface. Higher attenuation helpsisolate the probe, however, it reduces the signal level and degrades thesignal-to-noise-ratio of the measurement. Passive probes are available with a varietyof attenuation values for a moderate cost. Many Keysight passive probes can beautomatically identified by the system, setting the input impedance required as wellas the nominal attenuation. For single-ended measurements a single probe is used foreach channel. Other passive probes can be used, after manually setting theattenuation and probe impedance configurations.For full differential measurements, the system supports probes on each of the fourinputs. The attenuation for each of the probes should be the same for good commonmode rejection and channel match.Supported probesThe following table lists the probes currently supported by Option BBA:Probe Type Model Number DescriptionActive 1130A 1.5 GHz differential probe amp(No probe head)1131A a InfiniMax 3.5 GHz probe1132Aa InfiniMax 5 GHz probe1133Aa InfiniMax 7 GHz probePassive 1161A Miniature passive probe, 10:1, 10 MΩ, 1.5 m
