Terminated HF Probes
At higher frequencies, the input capacitance has much less impedance (Xe) than the 10- or 20-MO input resistance of the probe. For the circuit under test, this means that if the internal source impedance is high, low input capacitance of the probe is important indeed. But in hf techniques very often low source impedances of 50 to 75 n are met and a normal 50-0 coaxial cable can be used as the probe, provided that the cable is terminated with its characteristic impedance at the oscilloscope end. For an oscilloscope with an input impedance of 1 MO in parallel with 20 pF this means that a 50-0 termination resistor is to be connected to its input terminals. Special hf oscilloscopes already have a 50-0 input impedance.
If the cable load is too heavy, a 10:1 probe is available as a “terminated HF probe.” The 10:1 attenuation is obtained by mounting a 450-0 series resistor in the probe tip. The Philips 10:1 probe, PM 9342, for example, offers a 500-0 loading in parallel with 0.7 pF and possesses a bandwidth of 3.5 GHz. The Philips 100:1 probe, PM 9343, has a 4950-0 series resistor in the probe tip, thus showing an input resistance of 5000 n. The parallel capacitance, in this case, is 0.6 pF and the bandwidth is 1.7 GHz. DC loading of the terminated hf probes can be eliminated by means of a blocking capacitor, normally supplied as an accessory to the probes.
ACTIVE-VOLTAGE PROBES (FET PROBES)
With normal passive-voltage probes, low-capacitive loading can only be obtained by means of high attenuation. The terminated hf probes offer better characteristics, but because of the heavy resistance loading, they can only be used for low impedance sources. The active-voltage probe (FET) was developed to overcome both disadvantages. The Philips probe PM 9353 has a load at its tip of only 3.5 pF in parallel with IMO at 1:1 attenuation. An even further reduction in capacitive loading can be active circuitry to achieve their favorable characteristics.
Active-voltage probes obtain their power either from the oscilloscope probe power supply (available at the oscilloscope front panel) or from a separate power unit (probe accessory).
To keep the probe small and easy to handle, a miniaturized amplifier is built into the tip of the probe. The input stage comprises a FET, which provides a high input impedance. Miniaturization results also in very small stray capacitances on the order of 3 to 4 pF. The amplification of the FET amplifier is adjusted exactly to 1, so no reduction in sensitivity occurs.
When applying a FET probe, it should never be forgotten that the justification for using an active probe is to obtain a high input impedance, that is, high resistance and low capacitance. This makes it possible to measure high frequencies without a reduction in sensitivity, in other words, to measure millivolts above 25 MHz.
Slip-on attenuators can be attached to the probe tip. These attenuators contain a resistor divider of 10:1 or 100:1. Therefore, the input resistance remains IMO, but the capacitances become 2.5 and 1.5 pF, respectively. Thus, the capacitive loading is even less compared with the normal 1:1 FET probe. However, this is achieved at the cost of 10 X and 100 X attenuation.