Tone Detector
FX105A
1.6.2.2 Calculate Minimum Usable Bandwidth (%)
Minimum Usable Bandwidth (MUBW) is the TOTAL (%) bandwidth required for the following:
(a) Input signal frequency tolerance (Df0)
(b) FX105A VCO temperature coefficient (TC = - 100ppm/ºC)
(c)
FX105A VCO supply voltage coefficient (VC = 2330ppm/V)
Add (a), (b) and (c) and express as TOTAL (%) bandwidth, not as a ± (%) value.
MUBW = Df0 + |Tc|DTEMP + VcDVDD
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 R2 for Operating BW
R2 = 4.8 BW/ [10.35 -BW] = 4.8 x 6/ (10.35 -6) » 6.8kW
The exact bandwidth given by any value of R2 will vary slightly. In applications where an exact
bandwidth is required, R2 should be a variable resistor to permit adjustment.
1.6.2.5 Calculate RVC2A (C2A = C2B) Use nearest preferred values
RVC2A » 100/ [3 f0 BW] » 100/ (3 x 2800 x 6) » 2msec
Therefore
RV » 200kW for C2A = C2B = 10nF
1.6.2.6 Define the Maximum Allowed Response Time
The maximum response time (TON) is the sum of the VCO lock time (TLOCK) and the Word integration
time (TWORD). The FX105A’s TON 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 TLOCK
TLOCK = 150/ [f0 BW] = 150/ (2800 x 6) » 9msec
Note: TLOCK may vary from near zero to the value given, causing corresponding variations in actual
TON
(b) Calculate Maximum Allowable TWORD
TWORD = TONMAX -TLOCK = 50 - 9 = 41msec
.
Note: Since the maximum allowed response time (TON) is 50msec, a maximum Word integration time
of 41msec is available.
ã 1999 Consumer Microcircuits Limited
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D/105A/5
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