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ponent ( t
~
K 8/f) can now be kept constant, because the ratio 8/f can be
kept constant.
Also, the time, t, taken for a filter of bandwidth 8 to pass through a
frequency band
b.
f is given by
J
f
0
+~f+~
t
1
K df/f
b.
f
8
f - - - -
0
2
2
b.t
8
f
+ - + -
0
2
2
b.
f
8
f
- - - -
0
2
2
~
b.t
8 ]
1
+ - - + -
2 f
0
2 f
0
. ·
b.
f
8
1 - - - -
2 f
0
2 f
0
If the case of two resonances with the same
Q
factor is considered, where
Q
= f
0
/t~
f, it can be seen from the equation that the time spent on sweeping
each resonance will be the same. Also, as the Effective Averaging Time is
kept constant, the recordings of the resonances will be identical. The
relative resolution will also be the same.
9.3. LOWER FREQUENCY LIMIT
In a selective analysis, the analysis bandwidth used imposes a lower limit
to the frequencies measurable without introducing inconsistent measuring
errors. In practice, it is not recommended to analyse frequencies lower than
the bandwidth.
The first mixer receives two signals, the input signal at a frequency f
5
,
and the signal from the V.C.O., f
0
-f
5
•
Assuming the mixer is balanced, two
frequencies are obtained at the output: f
0
and f
0
-2 f
5
•
When f
5
is sufficient-
ly large, the signal f
0
-2 f
5
,
is filtered and does not appear in the reading.
Conversely, if f
5
approaches zero, f
0
-2 f
5
is not filtered and a 3 dB error
can be introduced into the reading.
88
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