AD6402
VCO
VCOGND
1.2kΩ
8pF
47pF
V
TUNE
39nH
AD6402
1nF
SMV
1204-37
SMV
1204-36
4.7nF
330Ω
330Ω
1nF
1nF
FMMOD2
FMMOD1
TXOUT
TXOUTB
330Ω
TXMOD
TXIF+
100pF
240Ω
180pF
TXIF–
MODOUT
150pF
VCCI
Figure 1.
O VERVIEW
equal to the sum of the IF frequency plus the frequency of the
PLL demodulator input as defined by the reference clock
divider ratios.
T he AD6402 forms the basis of a highly integrated RF trans-
ceiver with the benefits of increased sensitivity and wide dy-
namic range that a dual-conversion architecture provides. T he
IC contains a low dropout voltage regulator to isolate the IF and
demodulator VCOs from variation in the battery voltage, such
as power-supply transients caused by the PA. T he AD6402 also
provides control circuitry that allows subcircuits to be turned off
and on as necessary to minimize power consumption.
T he transmit IF VCO uses an external tank circuit. T his signal
is upconverted to the transmit frequency in the RF mixer sec-
tion of the radio. Using a transmit IF VCO prevents two prob-
lems: feedback from the PA at the RF frequency does not cause
distortion in the modulating circuit because the frequencies are
widely separated and the IF tank circuit can be optimized for
modulation linearity.
O per ation D ur ing Receive
T he AD6402 contains the second mixer, integrated second-IF
bandpass filter, logarithmic-limiting amplifier, and PLL de-
modulator. A SAW IF bandpass filter is usually required at the
IF input in order to provide channel selectivity.
T he output of the transmit VCO passes through buffer amplifier
and leaves the AD6402 via an optional LC filter between the RF
and IF ICs. T he output of the LC filter may then be fed to a
transmit upconversion mixer for conversion to the final RF
frequency.
T he placement of the SAW filter in the signal path between
the AD6402 and the RF section and the partitioning of the
receiver’s RF and IF receive circuits minimizes the leakage
around the SAW filter and maximizes the RF to IF isolation.
O nboar d Voltage Regulation
T he AD6402 contains a low dropout voltage regulator to spe-
cifically isolate the VCOs and synthesizer from the voltage
“kick” that occurs when a power amplifier switches on and the
battery voltage abruptly drops. T he AD6402 uses an integral
vertical PNP pass transistor.
T he output of the SAW filter enters the AD6402 via the second
downconversion mixer. T his mixer is a high gain, doubly-
balanced Gilbert-cell type. T he mixer downconverts the signal
to the second IF, which is 1.5 × or 2.5 × the reference frequency.
T his multiple is determined by the state of the REFSEL pin. An
on-chip two section bandpass filter provides additional selectiv-
ity to provide attenuation of adjacent channels. T he VCO con-
trol voltage output of the PLL demodulator tunes this filter to
the second IF.
T he regulator in the AD6402 IF IC supplies the voltage for the
VCOs on both the RF section and AD6402. T he other sections
of the AD6402 should be powered from an independently regu-
lated source at 2.85 V. Since the VCOs are isolated from this
source, possible problems due to VCO supply pushing are con-
siderably reduced.
T he bandpass filter’s output enters a successive-detection loga-
rithmic-limiting IF amplifier. T he RSSI detectors are distrib-
uted across the entire IF strip, including the mixer, and provide
80 dB RSSI range. T he IF strip’s limiting gain also exceeds 80
dB. T he RSSI signal is low-pass filtered and proceeds off-chip
to the baseband subsystem. T he limited output of the logarith-
mic amplifier enters a PLL demodulator, which provides de-
modulation of the received signal. T he PLL uses an integrated
VCO with no external components.
Fr equency Contr ol
T he AD6402 requires an external synthesizer to provide the
control voltages for the tank circuit of the IF VCO. Normally
this will be the IF section of a dual synthesizer controlling both
IF and RF frequency generation.
It is recommended that the VCO on the RF section implement
the channel selection on transmit and receive; the VCO on the
AD6402 may therefore operate at a fixed frequency. T his ac-
complishes two goals: first, the IF VCO being modulated can be
optimized for modulation linearity and the RF VCO can be
optimized for tuning range, and second, feedback from the PA
at will not couple into the modulating circuit to cause spurious
responses.
O per ation D ur ing Tr ansm it
T he transmit signal path consists of a low-pass filter that can be
user configured for antialiasing of a baseband transmit signal.
An IF VCO, which should be tuned to a frequency equal to the
receive IF frequency plus the desired demodulator input fre-
quency, may be open-loop modulated by the transmit signal for
FM and FSK schemes. T he receive IF mixer uses high side
mixing and therefore the IF VCO should be set to a frequency
All key sections of the AD6402 may be powered up or down as
necessary to minimize power consumption and maximize
battery life.
–4–
REV. 0
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