Circuit Note

CN-0245

Rev. 0 | Page 3 of 5

With a target image suppression of −40 dBc, Figure 5 shows the

performance of the ADL5387 with the ADF4350 providing the

differential LO source with and without filtering. The blue

signal trace representing the “Signal Generator” is the ideal case

where the LO is generated using a Rhode & Schwarz signal

generator with a sinusoidal output and much lower harmonic

levels compared to the ADF4350. This is the ideal case and the

target comparison point. From Figure 5, it can be seen that

filtering is not required at frequencies below 1 GHz. However,

above 1 GHz small errors due to harmonics of the LO become a

larger percentage of the input period. In this case, filtering

should be used to further attenuate the even order harmonics of

the LO and so that the I/Q demodulator’s specified quadrature

accuracy can be achieved.

–20

–70

425

1225

RF FREQUENCY (MHz)

–25

–30

–35

–40

–45

–50

–55

–60

–65

525

625

725

825

925

1025

1125

ADF4350, NO FILTER

SIGNAL GENERATOR

ADF4350 + FILTER

Figure 5. ADL5387 Image Rejection vs. RF Frequency

ADF4350

WIDEBAND

SYNTHESIZER

LPF

3.3V

12

13

ADL5380

QUADRATURE

DEMODULATOR

LOIP

LOIN

3

4

Figure 6. ADF4350 Interface to the Polyphase Filter Architecture of the

ADL5380 Demodulator

Interfacing the ADF4350 PLL with the ADL5380

Quadrature Demodulator

Unlike the ADL5387, the polyphase architecture of the

ADL5380's phase splitter requires filtering of the ADF4350

outputs, as shown in Figure 6. Filtering is required to attenuate

the odd order harmonics of the LO to minimize errors in the

quadrature generation block of the ADL5380. From

measurement and simulation as explained in CN-0134, the odd

order harmonics contribute more than even order harmonics to

quadrature errors. Figure 7 shows the measurement results

when the ADF4350 outputs are filtered before they are applied

to the differential LO inputs of the ADL5380. After filtering, the

resulting image rejection is comparable to what is achievable

from a low harmonic signal generator.

–10

–70

850

–20

–30

–40

–50

–60

1350

1850

2350

RF FREQUENCY (MHz)

ADF4350, NO FILTER

SIGNAL GENERATOR

ADF4350 + FILTER

Figure 7. ADFL5380 Image Rejection vs. Frequency.

Filtering Requirements

In summary, LO filtering the ADF4350 outputs to suppress the

harmonics of the fundamental helps to maintain the phase

accuracy of the quadrature signals of the demodulator. In the

case of the ADL5380, which uses a polyphase architecture,

filtering is a requirement. The ADL5387 architecture consists of

digital circuitry which is more immune to the harmonics of the

LO signal. Therefore filtering may not be required, depending

on the frequency of operation.

In the case where filtering is necessary, Figure 8, shows an

example LO output filter schematic, and Table 1, summarizes

the filter component values. This circuit is flexible and provides

four different filter options to cover four different bands The

filters were designed for a 100 Ω differential input and 50 Ω

differential output to match the LO input requirements of the

demodulator. A Chebyshev response was used for optimal filter

roll-off at the expense of increased pass-band ripple. Please

refer to CN-0134 for a more detailed discussion on the filtering

of the ADF4350 outputs.