Self Cal is the way the Cassini platform adapts measurements for thermal variation over time and is designed to have a minimal impact to the production test time. This allows unprecedented stability of measurements by periodically adjusting the Receiver's gain to account for thermal variation that occurs throughout testing.
How does temperature variation affect measurements?
Temperature affects measurements by affecting the electrical conductors, cables and wires, in the instruments.
"Aside from the geometry of the wire, temperature also has a significant effect on the efficacy of conductors. Temperature affects conductors in two main ways, the first is that materials may expand under the application of heat. The amount that the material will expand is governed by the thermal expansion coefficient specific to the material. Such an expansion (or contraction) will change the geometry of the conductor and therefore its characteristic resistance. However, this effect is generally small, on the order of 10−6. An increase in temperature will also increase the number of phonons generated within the material. A phonon is essentially a lattice vibration, or rather a small, harmonic kinetic movement of the atoms of the material. Much like the shaking of a pinball machine, phonons serve to disrupt the path of electrons, causing them to scatter. This electron scattering will decrease the number of electron collisions and therefore will decrease the total amount of current transferred." source: https://en.wikipedia.org/wiki/Electrical_conductor
What is a Self Cal?
Self Cal applies to any Instruments with measurement buttons and allows them to compare a previously known thermally stable measurement and adjust for thermal variation by subtracting the difference and adding the value to the actual results. The Self Cal is built in to the Instrument's software drivers and can not be managed by the user. The RI8587 Receiver TIM, for example, is switched to a dedicated Noise Source located in the RI8545 in that is compared to the previous measurement and used to zero the detector's results. The RI8567 12 GHz Receiver uses a self contained Noise Source to self cal.
When is it performed?
Self cal is design to have minimal impact to the production test. The self cal is performed immediately after Start is pressed at the Test Exec UI and in between parts when tester is waiting for the handler's response. It is always performed before the test plan is executed, never during testing.
How does it affect DUT throughput and efficiency timing?
Self Cal occurs more frequently when a Test Exec is first run versus after 20 minutes. Accurate operational efficiency timing exercises should start timing after 20 minutes of operation and average across 60 minutes of testing. The interval of self cal cycle is also varies, starting at 15 second intervals then doubles in time until once performed every 20 minutes. Not all instruments require a self cal at every cycle.
How long does a Self Cal take?
The time for each self cal could be from as short as 100 milliseconds to around 5 seconds, depending on the tester configuration.
To see what is the longest time for a self cal, usually also the first one performed at the beginning of the Test Exec run, open a Workspace window from Cassini system and type "Time millisecondsToRun:[ RiEquipmentPool activeSystem forceSelfCalibrate ]" highlight the text and choose "Show It" from the right mouse button menu. It will show you number of milliseconds it takes to run a full self cal. The Self Cal function is managed directly by the Cassini software (Short Cut) and may change behavior with a software update.
Where is the Cal value stored?
The thermal variation is stored in memory while the Cassini software is run and is reset when the Start button is pressed in a Test Exec or in the Test plan editor.
What instruments have Self Cals performed?
Only measurement TIMs can be configured to operate a Self Cal, but not all have them implemented. Instruments that source or initiate signals do not have Self Cals. Some Self Cals are functionally disabled and will not take any time nor affect measurements. Inspect the Cassini Log window (System > Messages) at Startup (or perform System > Equip > Startup) to see which instruments perform a Self Cal.
Common Instruments with Self Cal: RfMeasure1, FetPulser, Receiver