ANALOG DEVICES CN-0245 Service Manual

Circuit Note

CN-0245

Wideband Synthesizer with
Integrated VCO

400 MHz to 6 GHz Quadrature
Demodulator

0

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0°

90°

LOIP

LOIN

ADF4350

ADL5380/ADL5387

QUADRATURE DEMODULATOR

WIDEBAND

SYNTHESIZER

RF

OUT

A+

RF

OUT

A–

LPF

3.3V

RF+ RF–

Q+

Q–

I+

I–

Z

BIAS

Z

BIAS

10224-001

Devices Connected/Referenced

Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0245.

Wideband LO PLL Synthesizer with Simple Interface to Quadrature Demodulators

ADF4350

ADL5387 50 MHz to 2 GHz Quadrature Demodulator

ADL5380

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

ADL5387 Evaluation Board (ADL5387-EVALZ)
ADL5380 Evaluation Board (ADL5380-30A-EVALZ)
CN0134 Evaluation Platform (CFTL-CN0134-EVALZ)

Design and Integration Files
Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit, shown in Figure 1, highlights the ease of interfacing
the ADF4350 wideband synthesizer with integrated VCO with
the ADL5380 and ADL5387 wideband I/Q demodulators. In
this circuit, the ADF4350 provides the high frequency, low
phase noise local oscillator (LO) signal to the wideband I/Q
demodulator.

This circuit configuration offers quite a few benefits that make
it an attractive solution in applications requiring quadrature
mixing down to baseband or to an intermediate frequency.

The ADF4350 offers RF differential outputs and, likewise, the

ADL5380/ADL5387 accept differential inputs. This interface

offers both ease of use and performance advantages. The
differential signal configuration provides common-mode noise
reduction and even order cancellation of the LO harmonics,
which maintains the quadrature accuracy of the I/Q
demodulators. Additionally, the output power level of the

ADF4350 matches the input power requirements of the

quadrature demodulators very well. As a result, an LO buffer is
not necessary.

The ADF4350 outputs cover a wide frequency range from

137.5 MHz to 4400 MHz. 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. Between the

ADL5380 and ADL5387 the RF input range can span from

50 MHz to 6 GHz. Therefore, the two chip circuit configuration
as shown in Figure 1 offers coverage of a wide frequency range
from 50 MHz to 4400 GHz.

Figure 1. Simple Interface Between the ADF4350 PLL Synthesizer and the ADL5380 or ADL5387 Quadrature Demodulator

Rev.

Circuits from the Lab™ circu

each circuit, and their function and performance have been tested and verified in a lab environment at

be liable f

(Simplified Schematic: All Connections and Decoupling Not Shown)

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]

www.analog.com

CN-0245 Circuit Note

D Q

Q

LO_IN

CK

D Q

QCK

LO_I (0°)

LO_Q (90°)

10224-002

LO_I (0°)

LO_Q (90°)

LO_IN

10224-003

ADF4350

WIDEBAND

SYNTHESIZER

RF

OUT

A+

RF

OUT

A–

3.3V

Z

BIAS

Z

BIAS

12

13

ADL5387

QUADRATURE

DEMODULATOR

LOIP

LOIN

3

4

10224-004

CIRCUIT DESCRIPTION

The ADF4350 is a wideband fractional-N and integer-N phase­locked 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.

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.

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.

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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