Application Note (Continued)
3. Dual Amplifier Active Filters (DAAFs)
•
•
a. Low-Pass Filter (LPF)
b. High-Pass Filter (HPF)
4. Tri-level Voltage Detector
TELEPHONE-LINE TRANSCEIVER
The telephone-line transceiver of Figure 7 provides a full-
duplexed connection through a PCMCIA, miniature trans-
former. 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 (VT) and VT/2. This is because
Rmatch is chosen to match the coupled telephone-line imped-
10012839
>>
ance; therefore dividing VT by two (assuming R1
Rmatch). The differential configuration of UR has its resistors
chosen to cancel the VT and VT/2 inputs according to the
following equation:
FIGURE 8. Amplitude Modulator Circuit
f
mod
f
carrier
10012840
FIGURE 9. Output signal per the Circuit of Figure 8
DUAL AMPLIFIER ACTIVE FILTERS (DAAFs)
10012833
The LMV822/24 bring economy and performance to DAAFs.
The low-pass and the high-pass filters of Figure 10 and
Figure 11 (respectively), offer one key feature: excellent
sensitivity performance. Good sensitivity is when deviations
in component values cause relatively small deviations in a
filter’s parameter such as cutoff frequency (Fc). Single am-
plifier active filters like the Sallen-Key provide relatively poor
sensitivity performance that sometimes cause problems for
high production runs; their parameters are much more likely
to deviate out of specification than a DAAF would. The
DAAFs of Figure 10 and Figure 11 are well suited for high
volume production.
FIGURE 7. 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 8 is simple and provides a unique form
of amplitude modulation. Vi is the modulation frequency
(FM), while a +3V square-wave at the gate of Q1, induces a
carrier frequency (FC). Q1 switches (toggles) U1 between
inverting and non-inverting unity gain configurations. Offset-
ting a sine wave above ground at Vi results in the oscillo-
scope photo of Figure 9.
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 compo-
nents. This difference frequency magnitude (/FM-FC/) is the
key factor for determining an object’s velocity per the Dop-
pler Effect. If a signal is transmitted to a moving object, the
reflected frequency will be a different frequency. This differ-
ence in transmit and receive frequency is directly propor-
tional to an object’s velocity.
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