Phase Reference Calibration

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

In this topic:

• 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)

  • U9391G (67 GHz)

Note: Do not use the U9391-60009 sine-square wave converter.

• Power meter or USB power sensor.

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

• For measurements below 650 MHz, an additional 'Unknown mixer' is required. Learn more.

• For measurements between above 50 GHz, an additional high-pass filter is required for power calibration. Two back-to-back Keysight V281A waveguide-to-coax adapters is recommended.

Note: The 67 GHz comb generator creates a large pulse for generating useful harmonics up to 67 GHz. receiver attenuation must be used to prevent overloading the VNA receiver due to the large signal.

Limitations

• Available with S96083B

• Swept LO measurements are NOT allowed.

• Lowest frequency for Phase Reference Cal is 10 MHz.

• If the frequency range contains 50 GHz (like 20 GHz to 53 GHz), due to the hardware characteristic, consider dividing the channel like Ch1: 20~50 GHz, Ch2: 44~53 GHz. In this case, use a high-pass filter for Port 1 for accurate power measurement and subsequent S-parameter measurement for Ch2 Phase Reference cal.

• The RF IF path configuration for the port 1 should be the same when the reference calibration is executed. If the path setting is different, the calibration would not be correct. 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.

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 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 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 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. It does NOT have to be performed again during the SMC Cal.

Unknown Mixer Calibration (extends the phase reference calibration below 650 MHz

By itself, the phase reference can only be used to calibrate down to 650 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. The unknown mixer is also used to improve the quality of the phase reference calibration below 650 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 900 MHz. By using the data acquired using the phase reference, the system is able to make produce calibrated measurements of the mixer above 650 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 650 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.

• If the frequency range contains 50 GHz (like 20 GHz to 53 GHz), due to the hardware characteristic, consider dividing the channel like Ch1: 20~50 GHz, Ch2: 44~53 GHz.  In this case, use a high-pass filter for Port 1 for accurate power measurement and subsequent S-parameter measurement for Ch2 Phase Reference cal.

Hardware Setup

1. Connect the U9391 Phase Reference to a DC power supply by one of the following cables.
   For U9391-20013 cable (2 banana plug, see the U9391 Technical Overview for more information)

• The black wire is “ground”.

• The red wire is + 15Vdc.  

For U939b. For 87405-20006 cable (3 banana plugs, see the 87405C Technical Overview for more information),

• The green wire is “ground”.

• The red wire is + 15Vdc.

• The black wire is not used.

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

Note: Do not use the U9391-60009 sine-square wave converter.

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

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

Phase Reference measurement above 650 MHz .

• It takes some time (~ 25 minutes, depending on frequency range).

•  If the frequency range includes 45 GHz, the sweep is divided below and above 45 GHz

Full band power sensor measurement.

• Connect Power Sensor to Port 1.For frequency over 50 GHz, use a high-pass filter.

Full band  S-parameter Port 1 one-port calibration.

• Connect standards to Port 1.

• For frequency over 50 GHz, use a high-pass filter.

Full band Port 2 one-port calibration.

• Connect standards to Port 2.

• Do not use a high-pass filter.

Full band Port1-2 thru calibration.

• Connect standards to Port 2.

• Do not use a high-pass filter.