Before proceeding to the calibrations and some of the alternatives available, there are certain instrument setup issues that must be discussed first since they will affect the performance of all calibrations. In almost all cases, the current VNA settings will be used during the calibration so setting up the VNA as desired beforehand will help.
Frequency Start, Stop, and Number of Points
The Start Frequency, Stop Frequency, and number of points should be decided and set in the VNA before performing a calibration. Segmented sweeps should also be set up in advance if a more custom frequency list is desired.
IF Bandwidth, Averaging and Power
These parameters control the digital filtering and post-processing that determine the effective noise floor, the amount of trace noise and, in some special cases, the immunity to interfering signals. The trade-off for improved noise performance is slower sweep speed.
• IF Bandwidth (IFBW)
Settings of 1 Hz to 1 MHz are allowed with the RMS trace noise ranging from <1 mdB at the low end to a few hundred mdB at the high end (for high level signals, more for lower level signals). Sweep time will be roughly proportional with the reciprocal of IFBW once below 100 kHz IFBW.
• Point-by-Point vs. Sweep-by-Sweep Averaging
• Point-by-Point averaging incurs additional measurements at each given frequency point and will increase sweep time roughly proportionally. Because the additional measurements are taken at once, the effect is similar to proportional change in IFBW.
• Sweep-by-sweep averaging acquires additional measurements on subsequent sweeps and is better at removing lower frequency variations than point-by-point averaging or IFBW reduction. Sweep-by-sweep averaging is a rolling average so the time to fully stabilize from a sudden DUT change is roughly proportional to the average count.
• Power
Port power is somewhat less critical because of the excellent linearity of the MS4640A Series VNA receivers, but any step attenuator settings must be selected before the calibration. Because changing the step attenuator settings alters the RF match in the measurement paths as well as insertion loss, changing them will invalidate the calibration