
126 Analog Circuits Cookbook
Figure 4.27(b) shows the same response with the original detector time
constant, showing the distorted response caused by using an excessive post-
detection filter time constant - a point which will not be lost upon anyone
who has used early spectrum analysers which did not incorporate inter-
locking of the sweep speed, span, IF bandwidth and post detector filter
settings. The measurement could of course have been taken without error
using the original detector by reversing the polarity of the ramp to give a
falling frequency test signal - at the expense of having a back-to-front
frequency base. Conversely, there