Microcyber HT1200M, Руководство пользователя

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Figure 7 OTXA Waveform (2200 Hz)
CARRIER DETECT CIRCUITRY
The Carrier Detect Comparator shown in Figure 8 below generates logic low output if
the IRXAC voltage is below ICDREF. The comparator output is fed into a carrier
detect block. The carrier detect block drives the carrier detect output pin OCD high if
INRTS is high and four consecutive pulses out of the comparator have arrived. OCD
stays high as long as INRTS is high and the next comparator pulse is received in less
than 2.5 ms. Once OCD goes inactive, it takes four consecutive pulses out of the
comparator to assert OCD again. Four consecutive pulses amount to 3.33 ms when the
received signal is 1200 Hz and to 1.82 ms when the received signal is 2200 Hz.
ANALOG RECEIVER CIRCUITRY
Voltage References
The HT1200M requires two voltage references: IAREF and ICDREF. IAREF sets the
do operating point of the internal operational amplifiers and comparators. A 1.235 V
DC
reference(Analog Devices AD589) is suitable as IAREF. The level at which OCD
(carrier detect) becomes active is determined by the do voltage difference (ICDREF-
IAREF). Selecting a voltage difference of 0.08 V
DC
will set the carrier detect to a
nominal 100 mVp-p.
Bias Current Resistor
The HT1200M requires a bias current resistor to be connected between OCBIAS and
Vss. The bias current controls the operating parameters of the internal operational
amplifiers and comparators. The value of the bias current resistor is determined by
the reference voltage IAREF and the following formula:
⎛ IAREF ⎞
R
BIAS

⎜
⎝
⎟
2.5

A
⎠