THETA/DVF TIMING CHECK
This procedure checks the heating pulse width (TH) and measurement delay (TMD) for the THETA or DVF tests.
It is assumed that you will use a Digital Storage Oscilloscope (DSO) to make the measurement. The settings described below are typical and serve to illustrate the procedure. Of course you may change the times, currents, test number, etc. to suit your needs and to check a range of values.
You will need to remove the front panel covering the tester’s card cage to get access to the SYNC jacks. You will use the SYNC2 BNC connector both to sync your DSO and as one of the 2 input channels that you will view. The best way to sync the scope is to use “External” AC coupled sync connected with a direct BNC to BNC cable from SYNC2 to the external sync DSO input. A “T” connector will allow you to view this pulse also on one scope channel.
Here is a typical test setup.....TEST 1 THETA 1MA 1A <10/W T10 K2000 D100
The DSO should be connected as above with the remaining vertical input channel connected to the Cathode of the diode under test using a 10X probe. The probe ground should tie to the “ground” connection on the test fixture.
If the test station being used is #1, the cursor on the MAXX screen should be on station 1 also. If you use a different station, change the cursor to that station. We will assume station 1 here.
Enter the following to setup the sync pulse:
This makes a negative going pulse on the SYNC2 jack at the exact time that the final “Hot” VF reading is locked into the TRACK/HOLD circuit for later conversion to Digital. The pulse is derived from a TTL level circuit.
The probe on the Cathode terminal will see the VF across the device plus some additional voltage due to the drop in the wiring, relays, test fixture, etc. This drop will have an inductive component, which will cause no problem with this procedure.
You can use a horizontal scale of 2mS/div in this instance to check the 10mS TH pulse, which will occur BEFORE the sync pulse. In this instance setting the scope for 75% before the sync pulse is helpful. There will be some VF both before and after the main 10mS pulse due to the 1mA IM current. You will have no trouble however finding both the leading and trailing edges of the main pulse. There will typically be a disturbance on the VF pulse at the exact time that the current starts to fall due to the inductance of the wiring.
After viewing the main 10mS pulse you can change the horizontal scale to about 20µS/div in order to check the TMD. The TMD is the time between the above mentioned trailing edge of the main pulse, and the beginning of the sync pulse which you are viewing on another channel. In the example above this delay should be 100’S.
Please note that the waveforms you see in this test are not a reliable indication of amplitude. This test is intended to check time delays only. The FEC100/200 tester uses a good differential input signal conditioner to sense the amplitude of the VF for this and other VF type tests.