CN-0285 Datasheet, PDF (2/5 Pages) Analog Devices – Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter

CN-0285
Circuit Note
Figure 2. Evaluation Board for CN-0285 Direct Conversion Transmitter
Low noise, low dropout regulators (LDOs) ensure that the power
management scheme has no adverse impact on phase noise and
EVM. This combination of components represents industry-
leading direct conversion transmitter performance over a
frequency range of 500 MHz to 4.4 GHz
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 uses the ADF4351, a fully integrated
fractional-N PLL IC, and the ADL5375 wideband transmit
modulator. The ADF4351 provides the LO signal for the ADL5375
transmit quadrature modulator, which upconverts analog I/Q
signals to RF. Taken together, the two devices provide a wideband,
baseband IQ-to-RF transmit solution. The ADF4351 is powered
off the ultralow noise 3.3 V ADP150 regulator for optimal LO
phase noise performance. The ADL5375 is powered off a 5 V
ADP3334 LDO. The ADP150 LDO has an output voltage noise of
only 9 µV rms and helps to optimize VCO phase noise and reduce
the impact of VCO pushing (equivalent to power supply rejection).
Filtering is required on the ADF4351 RF outputs to attenuate
harmonic levels to minimize errors in the quadrature generation
block of the ADL5375. From measurement and simulation, the
odd-order harmonics contribute more than even-order harmonics
to quadrature error and, if attenuated to below −30 dBc, results
in sideband suppression performance of −40 dBc or better. The
second harmonic (2H) and third harmonic (3H) levels of the
ADF4351 are as given in the data sheet and shown in Table 1.
To get the third harmonic below −30 dBc, approximately 20 dB of
attenuation is required.
Table 1. ADF4351 RF Output Harmonic Levels Unfiltered
Harmonic Content Value (dBc) Description
Second
−19
Fundamental VCO output
Third
−13
Fundamental VCO output
Second
−20
Divided VCO output
Third
−10
Divided VCO output
This circuit gives four different filter options to cover four different
bands. The filters were designed with a 100 Ω differential input
(ADF4351 RF outputs with appropriate matching) and a 50 Ω
differential output (ADL5375 LOIN differential impedance). A
Chebyshev response was used for optimal filter roll-off at the
expense of increased pass-band ripple.
The filter schematic is shown in Figure 3. This topology allows
the use of either a fully differential filter to minimize component
count, a single-ended filter for each output, or a combination of
the two. It was determined that for higher frequencies (>2 GHz)
two single-ended filters gave the best performance because the
series inductor values are twice the value compared to a fully
differential filter and, hence, the impact of component parasitics is
reduced. For lower frequencies (<2 GHz), a fully differential
filter provides adequate results.
Table 2. ADF4351 RF Output Filter Component Values (DNI = Do Not Insert)
Frequency Range (MHz)
ZBIAS
L1
L2
C1a C1c
(nH) (nH) (pF) (pF)
C2a C2c C3a C3c
(pF) (pF) (pF) (pF)
500 to 1300 (Filter Type A)
27 nH||50 Ω
3.9 3.9 DNI 4.7
DNI 5.6 DNI 3.3
850 to 2450 (Filter Type B)
19 nH||(100 Ω in Position C1c)
2.7 2.7 3.3 100 Ω 4.7 DNI 3.3 DNI
1250 to 2800 (Filter Type C) 50 Ω
0 Ω 3.6 DNI DNI
2.2 DNI 1.5 DNI
2800 to 4400 (Filter Type D) 3.9 nH
0 Ω 0 Ω DNI DNI
DNI DNI DNI DNI
Rev. 0 | Page 2 of 5