Tone Detector
FX105A
1.6.2.2 Calculate Minimum Usable Bandwidth (%)
Minimum Usable Bandwidth (MUBW) is the TOTAL (%) bandwidth required for the following:
(a)
(b)
(c)
Input signal frequency tolerance (
∆
f
0
)
FX105A VCO temperature coefficient (T
C
=
−
100ppm/ºC)
FX105A VCO supply voltage coefficient (V
C
= 2330ppm/V)
Add (a), (b) and (c) and express as TOTAL (%) bandwidth,
not
as a ± (%) value.
MUBW =
Df
0
+ |T
c
|DTEMP + V
c
DV
DD
MUBW = 0.5 + 0.01 x 100 + 0.233 x 1
»
2%
1.6.2.3 Calculate the Recommended Operating Bandwidth
Note again that this is the TOTAL (%) bandwidth:
BW = �½ [10 + MUBW] = �½ (10 + 2) = 6%
1.6.2.4 Select R
2
for Operating BW
R
2
= 4.8 BW/ [10.35
-BW]
= 4.8 x 6/ (10.35
-6) »
6.8kW
The exact bandwidth given by any value of R
2
will vary slightly. In applications where an exact
bandwidth is required, R
2
should be a variable resistor to permit adjustment.
1.6.2.5 Calculate R
V
C
2A
(C
2A
= C
2B
)
Use nearest preferred values
R
V
C
2A
»
100/ [3 f
0
BW]
»
100/ (3 x 2800 x 6)
»
2msec
Therefore
R
V
»
200kW for C
2A
= C
2B
= 10nF
1.6.2.6 Define the Maximum Allowed Response Time
The maximum response time (T
ON
) is the sum of the VCO lock time (T
LOCK
) and the Word integration
time (T
WORD
). The FX105A’s T
ON
must not exceed the maximum time allowed for the application, but
a value lying near the maximum gives the best S/N performance.
(a)
Calculate T
LOCK
T
LOCK
= 150/ [f
0
BW] = 150/ (2800 x 6)
»
9msec
Note: T
LOCK
may vary from near zero to the value given, causing corresponding variations in actual
T
ON
.
(b)
Calculate Maximum Allowable T
WORD
T
WORD
= T
ON
MAX
-T
LOCK
= 50 - 9 = 41msec
Note: Since the maximum allowed response time (T
ON
) is 50msec, a maximum Word integration time
of 41msec is available.
©
1999 Consumer Microcircuits Limited
8
D/105A/5
CMLMICRO [ CML MICROCIRCUITS ]
