SMC Phase Reference Calibration

A Phase Reference calibration is performed to simplify the SMC+Phase calibration process.

In this topic:

Other FCA Topics

Features

Requirements

Note: The U9391-60009 sine-to-square wave is recommended when using the instrument’s 10 MHz reference output as the driving source to the comb generator. Connect between the instrument’s 10 MHz output and the input to the comb generator. See the U9391 Technical Overview for more information.

Note: The 67 GHz comb generator creates a large pulse for generating useful harmonics up to 67 GHz. At low frequencies, receiver attenuation must be used to prevent overloading the PNA receiver due to the large signal. However, above 67 GHz, the extra receiver attenuation causes degradation in the signal-to-noise ratio (SNR) of the high harmonics, which would result in noisy phase calibrations. In order to prevent overloading and get good SNR, the receiver attenuation is removed and a high-pass filter is inserted between the comb generator and the PNA's test port. The calibration wizard provides extra steps to guide the user to insert the filter.

Limitations

How it Works

A Phase Reference Cal is performed, saved, and later recalled during a SMC Calibration. This is sometimes referred to as a 'tier 1" calibration. Due to stability of the PNA, the Phase Reference Cal can be performed infrequently. It is typically performed over the full frequency range of the PNA or Phase Reference so that it can be applied to all SMC calibrations that will be needed in the future.

The Phase Reference is a comb generator which produces signals at the multiple of its input frequency. By driving it with the PNA 10 MHz reference output, the Phase Reference produces signals at multiples of 10 MHz with a flat phase response.

During the tier 1 calibration, the PNA port 2 (B) receiver measures the phase at each 10 MHz comb frequency in order to generate correction values. These correction values are stored to a Phase Reference Cal Set which can be recalled and applied to correct subsequent SMC+Phase measurements.

Although only the B receiver is phase calibrated, an S-parameter calibration is performed at specified test ports and used to transfer the characterization to other PNA receivers.

A power calibration is also performed on port 1 as part of the Phase Reference cal. It does NOT have to be performed again during the SMC Cal.

Unknown Mixer Calibration (extends the phase reference calibration below 55 MHz)

By itself, the phase reference can only be used to calibrate down to 55 MHz. With one additional unknown mixer connection, the phase reference calibration can be extended to the full frequency range of the PNA-X. The unknown mixer is also used to improve the quality of the phase reference calibration below 300 MHz.

With the unknown mixer process, the phase of the unknown mixer is measured as a down converter with the output frequencies set between 10 MHz and 600 MHz. By using the data acquired using the phase reference, the system is able to make produce calibrated measurements of the mixer above 300 MHz. The phase response of the unknown mixer is assumed to be linear, so the expected phase response of the mixer is extrapolated down to 10 MHz. The variation in the measurements versus the expected phase response is attributed to, and used to correct, the phase response of the PNA receivers from 10 MHz to 300 MHz.

Unknown Mixer Properties

Hardware Setup

  1. Connect the U9391 Phase Reference to a DC power supply. See the U9391 Technical Overview for more information.

    1. The green wire is “ground”.

    2. The red wire is + 15Vdc.  The U9391C and U9391F only require +12 Vdc.

    3. The black wire is not used.

  2. Connect a cable from the PNA rear-panel 10 MHz Reference signal output to the U9391 input. The cable should be as short as possible.

Note: Note: The U9391-60009 sine-to-square wave is recommended when using the instrument’s 10 MHz reference output as the driving source to the comb generator. Connect between the instrument’s 10 MHz output and the input to the comb generator. See the U9391 Technical Overview for more information.

  1. Connect the U9391 output directly to the PNA test port 2.

  2. Insert the U9391 USB connector into any of the PNA USB ports.

  3. Because of its low-frequency roll off, the coupler on Port 2 must be reversed which bypasses the coupled arm. Flip the port 2 front loops as shown in the following image. This increases power to the receiver by approximately 16 dB at low frequencies and removes noise from the measurement.

 

Configure the Port 2 'B' receiver port front-panel loops to a vertical orientation as shown here.

Block diagram showing port 2 thru coupler main arm to B receiver.

Phase Reference Calibration Summary

The Phase Reference Cal Wizard steps will show the appropriate prompts.

 

When Unknown Mixer is enabled (Start Frequency is 10 MHz)

Phase Reference measurement and S-Parameter calibration below 50 GHz.

When mixer output is above 50 GHz, Phase Reference measurement and S-Parameter calibration.

Full band power sensor measurement.

Full band S-parameter calibration.

How to start the Phase Reference Calibration

With an Scalar Mixer / Converter + Phase measurement active...

Using front-panel
hardkey [softkey] buttons

Using Menus

  1. Press Cal

  2. then [Start Cal]

  3. then [Phase Reference Wizard]

  1. Click Response

  2. then Cal

  3. then Start Cal

  4. then Phase Reference Wizard

 

Phase Reference Cal Settings dialog box help

Use Unknown Mixer

Check to use an Unknown Mixer to calibrate the PNA receivers below 55 MHz. The Start Frequency becomes 10 MHz and can not be changed. Learn how it works.

Ports

  • Ports 1 and 2 are always selected.

  • Select port 3 and 4 to perform an S-parameter (SOLT) cal at those ports. The phase correction is transferred to all checked ports.

Omit Coupler Measurements

This choice can be selected when the stop frequency is less than or equal to 50 GHz. Enabling the checkbox simplifies the calibration procedure by reducing the number of calibration steps required, with some tradeoffs. Clearing the checkbox will always result in the most accurate phase-reference calibration.

With the checkbox selected:

  • Port 2 can NOT be used as the input to the DUT when setting up subsequent SMC+Phase measurements

  • The power-sensor calibration is performed at the end of a test cable connected to Port 1, which may degrade the accuracy of conversion loss/gain measurements due to cable movement during the calibration. This problem can be minimized by using a high-quality cable and avoiding cable movement between calibration steps.

Start / Stop Frequency

Select the start and stop frequency of the Phase Reference Cal. Subsequent SMC+Phase calibrations MUST be the same or a subset of these frequencies. Interpolation is performed when the 10 MHz 'grid' of characterized frequencies is off from measurement frequencies.

  • When Unknown Mixer is NOT checked, the lowest start frequency is 55 MHz. The lowest stop frequency is 1 GHz.

  • When Unknown Mixer IS checked, the start frequency is 10 MHz and can NOT be changed. The lowest stop frequency is 5 GHz.

Source Attenuation

Select the attenuation value for the PNA source port.

Important Source Attenuation Notes

  • This setting should match the port 1 source attenuator setting used for subsequent SMC+Phase measurements. If the settings are the same, only an S-parameter cal need be performed with the SMC+Phase calibration wizard to complete the calibration process. If the settings are different, a Cal All calibration must be performed, which requires both an S-parameter cal and another power-sensor cal. As an alternative, if you plan to make SMC+Phase measurements with different input attenuator settings, you may want to perform a couple of Phase Reference cals using these different attenuator settings.

  • The phase reference cal is not as good when using input attenuation. You will get a better calibration if you use 0 dB of source attenuation. Then use the Cal All feature to perform the 2nd tier calibration with the required input attenuation. This is because the Cal All feature includes an attenuator calibration that will properly move the phase reference cal from the 0 dB attenuator plane to the new (required) attenuator plane.

Phase Reference

When the Phase Reference is detected by the PNA, it should appear in this field. If it does NOT appear here, try a different USB port, then click Refresh.

Phase Reference Connector type and gender. When the Phase Reference does NOT mate directly with the test port, you can add a well-behaved, broadband (no filters, waveguide, etc.) adapter to the Phase Reference and ignore it. That is because the adapter will stay with the Phase Reference and the calibration plane will still be the PNA test port connectors. The adapter adds constant delay; the deviation in delay is what is being calibrated out.  Loss in the adapter is not a concern because the adapter is not used during the S-parameter and the power meter portion of the cal.

Cal Kit Connector

Select the connector type and gender of your Cal Kit.

Only ONE connector type and gender is specified. This is because the entire Phase Reference cal is performed at the PNA test ports to reduce the effects of cable flex in the characterization.

There may be times when an adapter or connector-savers are used to connect the phase reference and cal standards to the test ports. In these cases, for highest accuracy use that adapter style and gender for ALL connections to ALL test ports. The effects of the adapter will be de-embedded automatically during subsequent SMC+Phase calibrations.

Note: For highest accuracy, perform the phase reference cal at the test ports.

Cal Kit  Select the Cal Kit that will be used to perform the S-parameter Cal.

B Receiver Attenuation

The 67 GHz (U9391G) phase reference outputs too much power and overloads the PNA Port 2 (B) receiver. Use at least 16 dB and not more that 20 dB of receiver attenuation to make accurate measurements using that phase reference.

This message is shown when a 67 GHz phase reference is connected to USB.

The B receiver attenuator control is shown if your PNA has receiver attenuators. Otherwise, connect external attenuation to the U9391G output when prompted.

 

Unknown Mixer Settings (Phase Reference Cal Wizard) dialog box help

The following wizard pages appear when Unknown Mixer is checked on the previous page:

Select LO source  The LO can be an internal 2nd source or an external source.

Sources  Click to start the External Devices dialog where you can select or configure an external RF source.

LO Frequency  Select a frequency that results in the unknown mixer output of 10 MHz to 600 MHz using this formula: Input = LO + Output.

Note: For best results, use the default LO frequency which avoids the PNA-X input (source) frequency band crossings.

The Port 2 coupler must be reversed for these measurements. Learn how.

  • Connect the unknown mixer input to the between port 1 and port 2. For best results, connect the mixer output as close as possible to port 2.

  • Connect the LO of the unknown mixer.  

  • Click Measure.

    • The input and output match of the unknown mixer is measured, then, SC21 using an averaging count of 100.

    • The data point spacing of the calibration is decreased to 156 kHz. This point spacing is necessary to accurately characterize all the low frequency band breaks of the PNA-X.

 

Power Cal Settings (Phase Reference Cal Wizard) dialog box help

A power calibration is performed on port 1. This is done to simplify subsequent SMC calibrations since the power calibration will not need to be repeated.

For highest accuracy, connect the power sensor directly to the test port with no adapter.

Learn more about these settings.

 

 

Phase Reference Calibration Steps dialog box help

  1. Connect the Phase Reference directly to the port 2 test port.

    1. If an adapter is necessary, then that adapter must be left in place for the SOLT cal at port 2.

    2. When using a 67 GHz phase reference, attenuation or high-pass filter is required.

  2. Ensure that the port 2 coupler is reversed. Learn how.

  3. Press Measure.

  4. The PNA will make a series of measurements which can take several minutes:

    1. First, the match of the phase reference is measured.

    2. Then, a sweep is made at multiples of 10 MHz across the entire frequency span. Each measurement is repeated 100 times and averaged to reduce the noise in the measurement.

    3. An error is reported if the approximate power level is not detected.

5. When finished, press Next>

Power sensor A

Connect the power sensor to port 1 test port.

SOLT cals are performed at the test ports.

  • The THRU standard always performs an Unknown Thru.

  • Follow the prompts to complete the Phase Reference Cal.

Phase Ref Cal Finished

Enter a name for the Phase Ref Cal, then click Finished.

During subsequent SMC calibrations, select the Phase Reference Cal Set at the SMC Cal setup dialog. Learn more.

Specify Delay

If you know the delay value of the Unknown Thru connection, enter it here.

Otherwise, click OK to accept the calculated value.