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ADF4350 데이터시트(PDF) 2 Page - Analog Devices

부품명 ADF4350
상세설명  Broadband Low Error Vector Magnitude Direct Conversion Transmitter Using LO Divide-by-2 Modulator
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CN-0144
Circuit Note
Rev. C | Page 2 of 4
To achieve optimum performance, the only requirement is that
the LO inputs of the modulator be driven differentially. The
ADF4350 provides differential RF outputs and is, therefore, an
excellent match. This PLL-to-modulator interface is applicable
to all I/Q modulators and I/Q demodulators that contain a
2XLO-based phase splitter. Low noise LDOs ensure that the
power management scheme has no adverse impact on phase
noise and error vector magnitude (EVM). This combination of
components represents industry-leading direct conversion
transmitter performance over a frequency range of 68.75 MHz to
2.2 GHz. For frequencies above 2.2 GHz, it is recommended to
use a divide-by-1 modulator, as described in CN-0134.
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 utilizes the ADF4350, a fully
integrated fractional-N PLL IC, and the ADL5385 wideband
transmit modulator. The ADF4350 provides the local oscillator
(the LO is twice the modulator RF output frequency) signal for
the ADL5385 transmit quadrature modulator, which upconverts
analog I/Q signals to RF. Taken together, the two devices
provide a wideband baseband I/Q-to-RF transmit solution.
The ADF4350 is powered off the ultralow noise 3.3 V ADP150
regulator for optimal LO phase noise performance. The ADL5385
is powered off a 5 V ADP3334 LDO. The ADP150 LDO has an
output voltage noise of only 9 µV rms, integrated from 10 Hz to
100 kHz, and helps to optimize VCO phase noise and reduce
the impact of VCO pushing (equivalent to power supply
rejection). See CN-0147 for more details on powering the
ADF4350 with the ADP150 LDO.
The ADL5385 uses a divide-by-2 block to generate the quadrature
LO signals. The quadrature accuracy is, thus, dependent on the
duty cycle accuracy of the incoming LO signal (as well as the
matching of the internal divider flip-flops). Any imbalance in
the rise and fall times causes even order harmonics to appear, as
evident on the ADF4350 RF outputs. When driving the modulator
LO inputs differentially, even-order cancellation of harmonics
is achieved, improving the overall quadrature generation. (See
“Wideband A/D Converter Front-End Design Considerations:
When to Use a Double Transformer Configuration.” Rob
Reeder and Ramya Ramachandran. Analog Dialogue, 40-07.)
Because sideband suppression performance is dependent on the
modulator quadrature accuracy, better sideband suppression is
achievable when driving the LO input ports differentially vs.
single-ended. The ADF4350 has differential RF outputs
compared to a single-ended output available on most
competitor PLL devices with integrated VCO.
The ADF4350 output match consists of the ZBIAS pull-up and, to
a lesser extent, the decoupling capacitors on the supply node. To
get a broadband match, it is recommended to use either a resistive
load (ZBIAS = 50 Ω) or a resistive in parallel with a reactive
load for ZBIAS. The latter gives slightly higher output power,
depending on the inductor chosen. Use an inductor value of 19 nH
or greater for LO operation below 1 GHz. The measured results
in this circuit were performed using ZBIAS = 50 Ω and an
output power setting of 5 dBm. When using the 50 Ω resistor,
this setting gives approximately 0 dBm on each output across
the full band, or 3 dBm differentially. The ADL5385 LO input drive
level specification is −10 dBm to +5 dBm; therefore, it should be
possible to reduce the ADF4350 output power to save current.
A sweep of sideband suppression versus RF output frequency
is shown in Figure 2. In this sweep, the test conditions were as
follows: baseband I/Q amplitude = 1.4 V p-p differential sine
waves in quadrature with a 500 mV dc bias; baseband I/Q
frequency (fBB) = 1 MHz; LO = 2 × RFOUT. A simplified
diagram of the test setup is shown in Figure 3. A modified
ADL5385 evaluation board was used because the standard
ADL5385 board does not allow a differential LO input drive.
0
–70
–60
–50
–40
–30
–20
–10
0
500
1000
1500
2000
2500
FREQUENCY (MHz)
ADF4350 AS LO SOURCE
DIFFERENTIAL CONNECTION
DATA SHEET SPECIFICATION
Figure 2. Sideband Suppression, RFOUT Swept from 68.75 MHz to 2200 MHz
This circuit achieves comparable or improved sideband
suppression performance when compared to driving the
ADL5385 with a low noise RF signal generator, as used in the
data sheet measurement. Using the differential RF outputs of
the ADF4350 provides even-order harmonic cancellation and
improves modulator quadrature accuracy. This impacts sideband
suppression performance and EVM (error vector magnitude).
A single carrier W-CDMA composite EVM of better than 2%
was measured with the circuit shown in Figure 1. The solution
thus provides a low EVM broadband solution for frequencies
from 68.75 MHz to 2.2 GHz. For frequencies above 2.2 GHz,
a divide-by-1 modulator block should be used, as described in
CN-0134.
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0144-DesignSupport.


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