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To measure analog baseband I and Q accurately, keep the probes from loading the circuit, compensate passive probes before calibrating the analyzer, then calibrate the I and Q channels—including their cables and relative skew—over the modulation band. Verify the result with a known signal before trusting gain, phase, image-rejection, or EVM readings.
Why the probe can change the measurement
A probe is part of the circuit while it is connected. Its resistance, capacitance, inductance, tip leads, and cable can change a node’s amplitude, rise time, and phase, and may affect circuit operation. Tektronix guidance emphasizes this loading effect. For baseband I/Q, the risk is especially relevant when PCB source impedance differs from the analyzer’s nominal 50 Ω input.
Do not judge a probe by its DC input resistance alone. Its impedance varies with frequency, and its capacitance can load higher-frequency content even when the resistance appears high. Attenuation is another trade-off: a 10× divider can reduce loading, but it also delivers a smaller signal to the analyzer’s ADC. The probe factor must be set correctly in the instrument, and the resulting signal must remain above the analyzer’s noise floor without exceeding its input headroom.
Think of the measurement as a cascade: probe, cable, analyzer input network, ADC, and analysis software. Calibration and validation need to account for the path that is actually used, not just the analyzer input in isolation.
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Choose a probing approach for the I/Q nodes
First establish whether each signal is single-ended or differential, the node’s source impedance and common-mode voltage, and the modulation bandwidth. Then select a probe and analyzer input configuration that suit those conditions. The table compares the broad approaches; actual bandwidth, loading, voltage limits, and noise performance depend on the specific probe and instrument.
| Approach | When it may fit | What to check |
|---|---|---|
| High-impedance passive probe | When its loading, bandwidth, voltage rating, and connection geometry are suitable for the node. | Frequency-dependent input impedance and capacitance, attenuation, compensation, and whether the analyzer input and probe interface are compatible. |
| Active probe | When a lower-capacitance connection is needed and the probe’s electrical limits match the signal. | Input capacitance and impedance versus frequency, bandwidth, common-mode range, voltage limits, attenuation, and analyzer compatibility. |
| Differential probe or balanced input path | When the I or Q signal is balanced or a suitable ground-referenced connection is unavailable. | Differential and common-mode limits, input loading, bandwidth, channel configuration, and the analyzer’s differential or balanced-mode settings. |
A 10× passive oscilloscope probe is one possible choice, not a default answer. Match its bandwidth, capacitance, attenuation, interface, common-mode range, and voltage rating to the DUT and analyzer. A published 10:1 ratio by itself does not establish that a probe will load the node acceptably or preserve the signal you need to measure.
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Keep the tip leads short and preserve the intended differential geometry. Avoid adding an unplanned ground lead: it can change the return-current path and therefore the circuit being measured. Record the probe model, attenuation, cable, and analyzer input impedance so the setup can be reproduced.
Compensate passive probes before analyzer calibration
Compensation aligns the passive probe’s resistive and capacitive divider behavior around its crossover frequency. It is a prerequisite for analyzer calibration: a later correction cannot make a poorly compensated probe’s crossover response trustworthy.
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- Connect the passive probe to the analyzer or calibrator’s square-wave output, following the instrument’s probe-compensation procedure.
- Adjust the probe’s compensation control until the displayed square wave has neither overshoot nor undershoot. Undershoot indicates undercompensation; overshoot indicates overcompensation.
- Repeat for each passive probe that will be used. Do not assume that one probe’s adjustment applies to another.
Calibrate the I and Q measurement paths
Each channel needs its own complex-gain calibration: both magnitude and phase versus frequency. A calibration for I does not establish the response of Q. The Keysight procedure also calls for configuring the measurement mode and input settings, and for calibrating the I/Q cables used in the measurement.
- Configure the analyzer. Select the required differential or balanced mode, input impedance, probe attenuation, and reference impedance. Use the correct probe factor for each input.
- Calibrate the I path and Q path separately. Run the analyzer’s probe/channel calibration for each probe and channel. Use the probes and connection arrangement intended for DUT measurements.
- Calibrate the cables and relative timing. If measurement cables were not included in the probe calibration, run the instrument’s I/Q cable calibration. Measure or enter the I-to-Q skew as supported by the instrument, and select the appropriate reference impedance.
- Preserve the calibrated setup. Keep the same tips and cables, and maintain their routing. Recalibrate after changing a probe, cable, or connection arrangement that can alter the response.
The goal is matched amplitude and phase across the modulation band, with cable delay and I/Q skew accounted for. A single-frequency match is not enough if the signal occupies a wider band.
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Check whether the probes are disturbing the node
A simple loading check is to compare the trace with one probe attached against the trace after attaching a second identical probe to the same node. If the trace visibly changes, the probing arrangement is perturbing the circuit. Redesign the connection or use a lower-loading method before treating the original trace as representative. This one-versus-two-probe comparison is a sanity check, not a substitute for calibration.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Validate the calibrated setup with a known signal
Use a known modulation or loopback signal and inspect constellation shape, image rejection, EVM, and carrier or LO feedthrough. Interpret those measurements as clues about different error mechanisms:
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- Gain imbalance stretches the constellation and can create images. In an Agilent-authored 2009 example, a 1 dB I/Q gain imbalance produced EVM above 5%. The same example reports that a 0.1 dB difference can produce approximately −45 dB images and about 0.5% EVM error. These are illustrative results, not universal limits.
- Phase mismatch or skew can rotate or smear the constellation. Revisit channel phase matching and the I/Q cable-delay or skew correction across the signal band.
- DC offsets produce LO feedthrough. If feedthrough remains, investigate offsets in the signal path as well as the measurement setup.
If the known signal does not behave as expected, check the relevant calibration settings and connection path before attributing the error to the DUT. Exact error budgets depend on modulation bandwidth, source impedance, probe model, analyzer architecture, and calibration implementation; there is no single probe or residual-error threshold that fits every setup.
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