
RF circuits and techniques 243
The 50 mV pp of Figure 8.18(c) corresponds to 177 mV rms at the input
of the 20 dB pad, which seems unlikely at best, not least because the rated
bandwidth of the scope is only 250 MHz. However, there are other factors
to take into account: principally the input impedance of the scope. At low
frequencies this can be represented as a lumped impedance of 1 MQ, in
parallel with 20 pF, and 20 pF at 418 MHz has an impedance of -jl9 £2.
However, the input circuitry is anything but lumped and at 418 MHz its
input impedance could be anything - quite possibly approaching an open
circuit. In this case, the output of an