Introduction to the working principle of embedded control and measurement fixture

The detachable oscilloscope probe will disturb the circuit under test during the test. When the probe is removed, the working conditions of the circuit will change. Imagine how a 10PF load from the probe will affect the high-speed line. The embedded detection device can use the circuit in the same working conditions at any time.

In addition, the parasitic capacitance of the embedded detection device described below is approximately 1PF, which is much smaller than those 10PF probes.

The embedded fixture shown in Figure 3.16 implements the function of a 21:1 probe , providing a convenient method of grounding the test point through a 50 ohm resistor when connecting the test to an oscilloscope.

There are a variety of test point to test point connectors with 0.1IN centers and 0.025IN square pins. This part is cheap and can be used with a variety of connectors. I prefer to use a MOLEX/WALDOM KK series plug. The RG-174 coaxial cable is pressed directly into the jack of the MOLEX/WALDOM KK plug, and then the plug is inserted into the pins on the circuit board. The sensing loop inductance of this connection is expected to be 10NH. When a 50 ohm cable is connected in series, the resulting T10~90% is 0.22NS. If the MOLEX pin is placed next to the 1K resistor, the effective resistance in series with the connector's loop inductance is 1K, and the resulting T10~90% is 0.025NS.

Regardless of the connection method chosen, a method for terminating the traces should be provided when the test traces are not used or the sense resistors are not removed, or a method for disconnecting the traces should be provided. In Figure 3.16, when the test signal line is not used, it can be connected to a 50 ohm termination resistor through a jumper.

Even if the oscilloscope probe is removed, the termination of the test signal leads will still provide a fixed 1050 ohm resistive load.