HP HP-Bench-Briefs-1974-07-08 free download

File name HP-Bench-Briefs-1974-07-08.pdf

SI i l l SERVICE INFORMATION FROM HEWLETT-PACKARD JULY-AUGUST 1974 OSCILLOSCOPE Divider Probes PROBES & MEASURE- Let's talk about how to make mea- MENT TECHNIQUES surements with a typical general purpose scope-probe combina- by Chuck Donaldson tion. Initially, we'll restrict the discussion to passive probes work- ing into a high-impedance oscillo- scope. In order to appreciate the Historically, the oscilloscope has problems involved, we might want been used as a tool to make mea- to take a look at just what a probe surements of amplitude versus is, and what it is intended to do. time over a rather broad frequency The input circuit of an oscilloscope range. Since the display is com- has traditionally been a parallel pletely visual, the capability of RC circuit as shown in Figure 1. deriving a great deal of qualitative The advantage of this type of cir- information, e.g. waveform shapes, cuit is that Rin can be .made perturbations, etc. as well as the high enough to present an insig- quantitive values .of amplitude and nificant load to many circuits for those cases where the signal time, has caused the oscilloscopeto (1 Megohm) while Cin can be he!d voltage is sufficient to drive the become the engineer's "screw- to a low enough value to maintain oscilloscope after the 1O:l division. driver". When a problem exists, or the desired system bandwidth. A simplified circuit of this type of gross circuit characteristics are How does the probe affect the probe is shown in Figure 3.Looking required, he has become accus- measurement? The simplest case into the probe tip, the circuit sees tomed to reaching for his scope, is that of a 1:l transfer probe where resistance Of R probe + Rinput. hooking it up to his circuit, and the probe simply looks like a small Typically, this would be 9M ohms gathering information, without resistance (200 to 300 ohms) in + 1M ohm. More importantly, note regard for such considerations as series with the scope input, and that the capacitors are in series input and output impedances, rise a fairly large capacitance (approxi- and the effective capacitance is times, etc. If a probe was required, mately 37 to 55 pf) in shunt with (Ccomd (CinDut). the one which was mechanically the input of the oscilloscope. See Ccomp + Cinput most convenient was typically the Figure 2. The total magnitude of R