CN-0134

Circuit Note

Rev. B | Page 2 of 5

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)

−19 dBc

Fundamental VCO

output

Harmonic Content

(Third)

−13 dBc

Fundamental VCO

output

Harmonic Content

(Second)

−20 dBc

Divided VCO output

Harmonic Content

(Third)

−10 dBc

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.