CN-0245 Datasheet, PDF (3/5 Pages) Analog Devices – Wideband LO PLL Synthesizer with Simple Interface to Quadrature Demodulators

Circuit Note
CN-0245
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
–25
ADF4350, NO FILTER
–30
SIGNAL GENERATOR
–35
ADF4350 + FILTER
–40
–45
–50
–55
–60
–65
–70
425 525 625 725 825 925 1025 1125 1225
RF FREQUENCY (MHz)
Figure 5. ADL5387 Image Rejection vs. RF Frequency
3.3V
RF_OUTA+ ₁₂
ADF4350
RF_OUTA– ₁₃
Z_BIAS
Z_BIAS
LPF
WIDEBAND
SYNTHESIZER
3 LOIP
4 LOIN
ADL5380
QUADRATURE
DEMODULATOR
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
–20
ADF4350, NO FILTER
–30
–40
SIGNAL GENERATOR
–50
ADF4350 + FILTER
–60
–70
850
1350
1850
RF FREQUENCY (MHz)
2350
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.
Rev. 0 | Page 3 of 5