R2009D oscilloscope input impedance

The Motorola 2009D incorporates a basic oscilloscope function, specified with bandwidth of 0.5MHz and input impedance of 1MΩ. The input impedance specification is naively incomplete, there will be some parallel equivalent capacitance that is very important to selecting a compatible probe.

Above is the schematic of a test circuit to find that input capacitance Cin.

We need to know the Thevenin equivalent source impedance of the source and 1MΩ voltage divider.



Next is a check of the rise time of the raw input and 50Ω source (ie R2=0) to be sure it is much faster than with R2=10kΩ

Above is the rise time with R2=10kΩ, divisions are 1µs. The yellow box denotes the voltages and times that will be used to calculate the time constant T of the RC circuit. In this case, a single RC time constant dominates the response and the calculator results will be quite good.

T is 8.647e-7, and R'=9950, so we can calculate the value of C.

\(C=\frac{T}{R^{\prime}}=87 \text{ pF}\)

Above is the equivalent input circuit, 1MΩ in parallel with 87pF.

This can be used to find a compatible 1x,10x probe… but it will be challenging, this is a very high input capacitance and remember that it appears in shunt with the probe cable, reasonably approximated at around 100-120pF at low frequencies. This would lead to the input capacitance of a compatible 10x probe being 20-25pF… double what is common. Another way that this is expressed is a compensation range for a probe, and it is commonly up to 35pF, maybe 45pF and this input is 87pF… way out of range of common probes.

BWD probe

With some old BWD x10 probes compensated for best square wave response, we get a 0.1-0.9 rise time of 200ns, not stunning at all, but an acceptable audio oscilloscope.