Phase Reference Calibration (NA520xA)

A Phase Reference calibration is performed to simplify the SMC+Phase or MODX (Modulation Distortion Converters) calibration process.

• Features, Requirements, and Limitations

• How it Works

• Hardware Setup

• Phase Reference Calibration Summary

• How to Perform the Phase Reference Calibration

Other FCA Topics

Features

• Greatly simplified phase measurements on mixers.

• A Reference Mixer or Calibration Mixer is NOT required.

• Works well with segmented sweeps, and mixers with 2-stage LOs.

Requirements

• One of the following Keysight Comb Generators (Phase Reference):

  • U9391C (26.5 GHz)

  • U9391F (50 GHz)

  • NOT Supported: U9391G (67 GHz)

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.

• Power meter or USB power sensor.

• S-parameter Cal Kit (mechanical or ECal module).

• For measurements below 300 MHz, an additional 'Unknown mixer' is required. Learn more.

Limitations

• Available with opt. S93083A/B (NOT with opt. S93082A/B) Opt S9x083A/B

• Swept LO measurements are NOT allowed.

• Lowest frequency for Phase Reference Cal is 10 MHz.

How it Works

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

A power calibration is also performed on port 1 as part of the Phase Reference Cal.

Unknown Mixer Calibration (extends the phase reference calibration below 300 MHz

By itself, the phase reference can only be used to calibrate down to 300 MHz. With one additional unknown mixer connection, the phase reference calibration can be extended to the lower limit frequency which is 10 Mhz and maximum frequency range of the VNA.

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 VNA receivers from 10 MHz to 300 MHz.

Unknown Mixer Properties

• The mixer must be able to output a signal from DC to 1GHz.

• Over these frequencies, the delay response of the mixer should be constant. Therefore, the unknown mixer must be passive, with no filtering or amplification.

• Add a 10 dB attenuator on the mixer input and output to minimize mismatch effects.

Hardware Setup

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 Phase Reference to a DC power supply by one of the following cables. See the U9391 Technical Overview for more information.

   • The black wire is “ground”.

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

2. Connect a cable from the VNA Reference signal output to the U9391 input. The cable should be as short as possible.

3. Connect the U9391 output directly to the VNA test port 2.

4. Insert the U9391 USB connector into any of the USB ports.

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)

• The input and output match of the unknown mixer is measured. Then SC21 is measured using 100 averages.

Phase Reference measurement above 650 MHz and S-Parameter calibration below 50 GHz.

• S-parameter cal is NOT performed when Omit Coupler Measurements is checked.

Full band power sensor measurement.

• Connect Power Sensor to Port 1.

• Port 2 coupler in normal position.

Full band  S-parameter calibration.

• Connect standards to Port 1.

• Port 2 coupler in normal position.