Datasheet
LMV821-N, LMV822-N, LMV822-N-Q1
LMV824-N, LMV824-N-Q1
www.ti.com
SNOS032G –AUGUST 1999–REVISED NOVEMBER 2013
TELEPHONE-LINE TRANSCEIVER
The telephone-line transceiver of Figure 38 provides a full-duplexed connection through a PCMCIA, miniature
transformer. The differential configuration of receiver portion (UR), cancels reception from the transmitter portion
(UT). Note that the input signals for the differential configuration of UR, are the transmit voltage (V
T
) and V
T
/2.
This is because R
match
is chosen to match the coupled telephone-line impedance; therefore dividing V
T
by two
(assuming R1 >> R
match
). The differential configuration of UR has its resistors chosen to cancel the V
T
and V
T
/2
inputs according to the following equation:
(1)
Figure 38. Telephone-line Transceiver for a PCMCIA Modem Card
Note that Cr is included for canceling out the inadequacies of the lossy, miniature transformer. Refer to
application note AN-397 for detailed explanation.
“SIMPLE” MIXER (AMPLITUDE MODULATOR)
The mixer of Figure 39 is simple and provides a unique form of amplitude modulation. Vi is the modulation
frequency (F
M
), while a +3V square-wave at the gate of Q1, induces a carrier frequency (F
C
). Q1 switches
(toggles) U1 between inverting and non-inverting unity gain configurations. Offsetting a sine wave above ground
at Vi results in the oscilloscope photo of Figure 40.
The simple mixer can be applied to applications that utilize the Doppler Effect to measure the velocity of an
object. The difference frequency is one of its output frequency components. This difference frequency magnitude
(/F
M
-F
C
/) is the key factor for determining an object's velocity per the Doppler Effect. If a signal is transmitted to a
moving object, the reflected frequency will be a different frequency. This difference in transmit and receive
frequency is directly proportional to an object's velocity.
Figure 39. Amplitude Modulator Circuit
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