Beginning with PNA Rev.A.09.20, you can optionally measure phase with SMC. This feature is available ONLY with Opt 083 on a PNA-X or N522x model. SMC + phase is NOT offered in the N523xA.

In this topic:

See Also

SMC Phase Reference Calibration

SMC Measurements and Calibration

SMC+Phase Demo (Internet connection required)

Other FCA Topics


There are three methods used in the PNA to calibrate SMC+Phase measurements. All three methods rely on newer phase-coherent synthesizers in the PNA to produce phase capability in frequency offset measurements.

With the first two methods, during an SMC calibration, the phase delay through a Calibration Mixer is measured and compared to the known delay. The difference is used to correct subsequent SMC+Phase measurements.

A Calibration Mixer is required with the first two methods.

  1. Enter the known delay into a dialog.

  2. Uses the known delay at various frequencies from an *s2p file from a mixer characterization. You create the *S2P file from a separate Mixer Characterization. This method is NOT supported with Cal All.

  3. Uses a Phase Reference to perform a 'tier 1' calibration. A Calibration Mixer is NOT required with this process. Learn more about this process.


How to make SMC+Phase measurements

  1. On the Mixer Sweep tab, check Enable Phase, then select the Phase Reference Point.

SMC Mixer Sweep tab - Phase Settings help

Enable Phase with SMC  Check to perform phase measurements.

You can also enable phase measurements at the SMC Cal Setup dialog.

Phase Reference Point

The SMC Phase measurement technique provides for a coherent phase relationship from one frequency to the next in each sweep. However, the phase measurement of the first data point is random from sweep to sweep. This initial phase offset does not impact measurements such as group delay or deviation from linear phase. However, in order to keep a phase trace from appearing random, all phase data in the sweep is normalized against a single point. This results in a stable, normalized phase trace.

For this normalization, select the measurement point that has the best signal-to-noise ratio. The phase at the selected point will always be zero. This selection applies for both the measurement and the calibration sweeps.

The Reference Point is fixed at the middle point when segment sweep is selected.

  1. Select a Phase Format for the SMC measurement: click Response, then Format, then phase, unwrapped phase, or group delay.

  2. During SMC Cal, at the Cal Setup dialog, select the Phase Correction method as follows:

SMC Cal Setup - Phase Correction dialog help

Enable Phase Correction  Check to enable Phase measurements. This can also be done during the mixer setup.

Phase Correction

Choose one of the following methods to specify the known delay through the characterized mixer. With the first two methods, the phase delay through a Calibration Mixer is measured and compared to the known delay, either entered, or stored in an *.S2PX file.

  • Use Known Mixer Delay  Enter the fixed, known, delay through the calibration mixer.

  • Use characterized mixer  Select, then browse to the *.S2P file that characterizes the calibration mixer.  Use an *.S2PX file when making segmented SMC+Phase measurements. Learn more. Use either of the following two methods to characterize the Cal Mixer over the SMC measurement frequency range:

  1. Use the Mixer Characterization Wizard. The Cal Mixer has the same requirements as the VMC Cal Mixer. Learn more.

  2. In a calibrated VMC channel, measure the group delay of the calibration mixer, then save to an *.S2P or *.S2PX file. However, a characterized mixer is required to calibrate the VMC channel.

Measuring phase on Mixers below 55 MHz

Measuring phase on mixers below 55 MHz is challenging for the following two reasons:

  1. The PNA-X couplers have significant losses at these low frequencies.  The PNA-X automatically reduces the IFBW at low frequencies in order to compensate for the coupler roll-off. And with SMC+Phase, we recommend setting 100 averages in order to reduce the noise of the SC21 delay measurement. These two settings combine to make the measurement speed VERY slow.

  2. The PNA-X has a significant number of frequency bands below 55 MHz. SMC+Phase must have multiple measurement points within every frequency band.

Comparing SMC+Phase with VMC Phase Measurements

SMC phase measurements do NOT require a reference mixer, and are therefore easier to make than VMC phase measurements. Also the SMC calibration mixer is only required to have a known delay value, although an S2P characterization file provides more accurate results.  The Phase Reference method provides the highest accuracy phase measurements for mixer/converters.

How to improve the stability of SMC+Phase measurements

Stable phase measurements are attained by increasing Sweep Averaging, and sometimes lowering the IFBW, until you attain the desired compromise between sweep time and trace jitter (the amount of random phase change at a single data point). For SMC+Phase, the default IFBW is 10 kHz, and 1 average. During calibration, the Averaging factor is temporarily multiplied by 4 to ensure an accurate phase calibration.

The following procedure shows how to view and improve phase jitter:

  1. Create an SMC+Phase channel (Click Response, Measurement Class).

  2. Enable Phase with SMC (See above) On the phase trace (to follow) notice that the only point that has NO jitter is the data point that you selected as the Phase Reference point.

  3. Change the measurement to IPWR: (Click Response, Measure, IPWR)

  4. Change Format to Phase. (Click Response, Format, Phase)

  5. Normalize the trace. (Click Marker/Analysis, Memory, Normalize) Learn more about Normalization.

  6. Autoscale the trace. (Click Response, Scale, Autoscale).

  7. Optionally monitor the jitter with Trace Statistics (Std Dev)

  8. Increase Averaging and possibly lower IFBW to improve jitter. (Click Response, Avg, then Average and IFBW). Use a MINIMUM of 10 sweep averages (never use Point Averaging).

  9. After the adjustments are made, change the measurement back to your measurement of interest.

  10. When measuring a new DUT, restart Averaging.

How to Calibrate a 2-stage (LO) SMC+Phase Measurement

Note: The following discussion does NOT pertain when a Phase Reference cal is used to correct the SMC+Phase Measurement.

When calibrating a dual-stage SMC+Phase measurement for group delay using a characterized mixer, the channel setup requires frequency values for two LOs, but the characterized-thru mixer uses only one LO. The frequencies of LO1 and LO2 are different. There are two ways to overcome this challenge:

  1. Before the calibration, set the LO that is provided by an external source to uncontrolled. Then manually set the frequency of this external source to the LO frequency that gives the same input and output frequencies, and the same sweep direction, as the dual-stage setup. Perform the calibration under this condition. After the calibration, return the LO to controlled so that its frequency will be properly set during the measurement of the DUT.

  2. Configure a 1-stage mixer setup, with the LO set to the frequency that gives the same input and output frequencies and the same sweep direction as the dual-stage setup. Perform the calibration under this condition. Save the calibration data as a user calset. Configure the dual-stage case, and apply the 1-stage calibration.

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