CN-0245 Datasheet, PDF (2/5 Pages) Analog Devices – Wideband LO PLL Synthesizer with Simple Interface to Quadrature Demodulators

CN-0245
Circuit Note
CIRCUIT DESCRIPTION
The ADF4350 is a wideband fractional-N and integer-N phase-
locked loop frequency synthesizer covering the frequency range
of 137.5 MHz to 4400 MHz. The ADF4350 has an integrated
voltage controlled oscillator (VCO) with a fundamental
frequency range of 2200 MHz to 4400 MHz. The ADF4350
offers high quality synthesizer performance. However,
depending on the demodulator architecture, LO filtering may
be required to minimize the effects of harmonics from the PLL
on the quadrature accuracy of the I/Q demodulator.
Analog Devices offers quadrature demodulators that cover a
wide frequency range. The ADL5387 frequency range spans
from 50 MHz to 2 GHz, and the ADL5380 covers the higher
frequency range from 400 MHz to 6 GHz. The ADL5387 and
ADL5380 utilize two different architectures to generate the 90°
phase shift between the I and Q paths. The ADL5387 utilizes a
2 × LO architecture where the local oscillator is at twice the RF
frequency, while the ADL5380 uses a polyphase filter-based
phase splitter. The polyphase architecture has a narrower
fractional bandwidth (i.e., operates across less octaves) and is
more sensitive to PLL harmonics compared to a 2 × LO-based
phase splitter. As a result, the ADL5380 requires harmonic
filtering of the LO to maintain the quadrature accuracy of
the I/Q demodulator, while filtering is only required for the
2 × LO-based ADL5387 at the top end of its frequency range.
LO_IN
DQ
CK Q
LO_I (0°)
DQ
CK Q
LO_Q (90°)
Figure 2. Simplified 2 × LO-Based Phase Splitter
Figure 2 shows a simplified 2 × LO phase splitter as
implemented in the ADL5387. The 90° phase split of the LO
path is achieved via digital circuitry that uses D-type flip-flops
and an inverter. This architecture requires an external LO
operating at twice the frequency of the desired LO.
LO_I (0°)
LO_IN
LO_Q (90°)
Figure 3. Simplified First Order Polyphase Filter
Figure 3 shows a simplified first order polyphase circuit, as
implemented in the ADL5380. The polyphase circuit consists of
complementary RC subcircuits that create a low-pass transfer
function from input to one output, and a high-pass transfer
function to the other output. If the R and C values of the two
polyphased paths are matched, then both paths have the same
corner frequency and, more importantly, the phase of one
output tracks the other with a 90° phase shift.
Interfacing the ADF4350 PLL with the ADL5387 I/Q
Demodulator
The ADL5387 and ADL5380 I/Q demodulators utilize different
architectures to achieve the ultimate goal of generating precise
quadrature signals. When interfacing with an LO synthesizer
like the ADF4350, it is important to consider how the
architectures respond to the LO signal and its harmonics.
This will determine the requirement for LO filtering. Figure 4
shows the basic interface between the ADF4350 and ADL5387.
Depending on the frequency of operation, an LO harmonic
filter may or may not be required between the ADF4350
and ADL5387.
3.3V
RF_OUTA+ ₁₂
ADF4350
RF_OUTA– ₁₃
Z_BIAS
Z_BIAS
3 LOIP
4 LOIN
ADL5387
WIDEBAND
SYNTHESIZER
QUADRATURE
DEMODULATOR
Figure 4. ADF4350 PLL Interface to the 2 × LO-Based Phase Splitter of the
ADL5387 Demodulator
In a 2 × LO-based phase splitter, the quadrature accuracy is
dependent on the duty cycle accuracy of the incoming LO.
The matching of the internal divider flip-flops also affects
quadrature accuracy but to a much lesser extent. So a 50% duty
cycle of the externally applied LO is critical for minimizing
quadrature errors. Additionally, any imbalance in the rise and
fall times causes even order harmonics to appear. When
driving the demodulator LO inputs differentially, even order
cancellation of the harmonics is achieved and results in
improved overall quadrature generation.
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