CN-0245

Circuit Note

Rev. 0 | Page 2 of 5

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.

D

Q

Q

LO_IN

CK

D

Q

Q

CK

LO_I (0°)

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_Q (90°)

LO_IN

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.

ADF4350

WIDEBAND

SYNTHESIZER

3.3V

12

13

ADL5387

QUADRATURE

DEMODULATOR

LOIP

LOIN

3

4

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.