5–4
5.4
Internal Oscillator Operation for Transmit and Receive Modes Sampling
Frequency
The internal oscillator of the transmitter or receiver can be externally sampled at OSCC and OSCR. The
waveform at OSCC is triangular and the waveform at OSCR is square. The amplitude of these waveforms
depends on the capacitor and resistor values used.
5.5
External Oscillator Operation for Transmit and Receive Modes
Instead of using the internal oscillator (with an external resistor and capacitor) in the transmit or receive
modes, it is possible to externally drive the device by applying a logic level clock to OSCC. When an
externally driven oscillator is used, OSCR must be held to GND. To avoid entering the test/program modes,
ensure that the external clock applied to OSCC does not exceed V
CC
(for more information see
Section 5.12).
5.6
Internal Amplifier/Comparator, Description and Gain Setting
The TMS3637 has an internal amplifier that is designed to amplify received signals up to logic levels. In
addition, a comparator is cascaded with the amplifier to provide wave shaping of received signals. The
comparator also inverts the signal. The minimum received signal strength must be at least 3 mV
peak-to-peak (see Figure 5–3 for a schematic of the amplifier/comparator section). The amplifier is enabled
only when the TMS3637 is configured as an analog receiver. When the amplifier is not configured as an
analog receiver, it is disabled and bypassed to reduce power consumption in any of the three logic receiver
modes. A capacitor connected between CEX to GND determines the gain of the amplifier stage. When no
capacitor is connected from CEX to GND, the amplifier assumes unity gain and the comparator still functions
to shape the received signal. When the internal amplifier is used, it is usually run at the maximum gain of
200. The maximum gain is set by resistances internal to the device as shown in the equation 2. However,
to achieve this maximum gain, a low impedance from CEX to GND must exist. Equation 2 defines the
capacitance necessary at CEX for maximum gain at different oscillator frequencies (f
osc
):
CEX > 1 / (6.28
×
f
osc
×
R1)
(2)
where:
CEX = capacitance required for maximum gain
R1 = 178
Ω
(set internally)
With a low impedance between GND and CEX, note that the maximum gain is derived from the noninverting
operational amplifier gain equation, (see Figure 5–3):
Gv = 1 + R2/R1 = 200
(3)
where:
R1 = 178
Ω
(set internally)
R2 = 35.5 k
Ω
(set internally)
If a capacitor is used at CEX, but maximum gain is not desired, equation 4 can determine the gain for any
value of CEX:
Gv
+
ǒ
1
)
4
p
2fosc2CT2(R1
)
R2)2
1
)
4
p
2f
2
osc CT
2R12
Ǔ
Ǹ
(4)
where:
f
osc
= oscillator frequency of transmitter (it is the transmitted frequency that is being amplified)
C
T
= CEX + 0.15 nF (there is an internal capacitance of 0.15 nF at CEX)
R1 = 178
Ω
(set internally)
R2 = 35.5 k
Ω
(set internally)
Summary of Contents for TMS3637P
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