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9.2. B
&
T PROGRAMMES
9.2.1. BandT variable
The two programmes (positions "3" and"4" on the B
&
T PROGRAM
selector) allow the product of bandwidth (B) and averaging time (T) to be
kept constant while progressively increasing the bandwidth with frequency.
This enables the ratio B2/f to be the same at the different transition fre-
quencies. For a given bandwidth the maximum frequency to be considered
in equation 6 is the first transition, or switchover, frequency. This enables
the value of the frequency f in equation (6) to be reduced. Therefore the
measuring time can be reduced considerably without any sacrifice of the
relative resolution.
Reference should be made to Tables 5.3 and 5.4 for choice of pro-
gramme for a particular application. Reference should also be made to
section 4.6 on remote control of these programmes.
9.2.2. T variable
This programme (position 2 on the B
&
T program selector) enables the
product T x f to be kept practically constant at the switchover frequencies.
The bandwidth B is kept constant at any selected value throughout the
measurement. See Table 5.2.
This programme is of particular use in the study of deterministic signals
with discrete frequency components only (the study of harmonic distortion
for example). The same relative error on all the components is obtained, as
shown in the following calculation:
For a logarithmic sweep, the minimum time, t, taken to sweep through a
frequency f
0
,
with a filter bandwidth B, is:
t
I
t+~
0
2
=
K df/:
t - -
o
2
where K is a constant with dimensions of time.
86
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