CN-0134 Datasheet, PDF (2/5 Pages) Analog Devices – Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter

CN-0134
Circuit Note
Figure 2. Evaluation Board for CN-0134 Direct Conversion Transmitter
Low noise 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 utilizes the ADF4350, a fully
integrated fractional-N PLL IC, and the ADL5375 wideband
transmit modulator. The ADF4350 provides the local oscillator
(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 ADF4350 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 ADF4350 RF outputs to attenuate
harmonic levels so as 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 ADF4350’s 2nd harmonic (2H) and 3rd harmonic
(3H) levels are as given in the data sheet and shown in Table 1.
To get the 3rd harmonic below -30 dBc, approximately 20 dB of
attenuation is required.
Table 1. ADF4350 RF Output Harmonic Levels Unfiltered
Harmonic Content
(Second)
Harmonic Content
(Third)
Harmonic Content
(Second)
Harmonic Content
(Third)
−19 dBc
−13 dBc
−20 dBc
−10 dBc
Fundamental VCO
output
Fundamental VCO
output
Divided VCO output
Divided VCO output
This circuit gives four different filter options to cover four
different bands. The filters were designed for a 100 Ω differen-
tial input (ADF4350 RF outputs with appropriate matching)
and 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.
Rev. B | Page 2 of 5