CN-0285

Circuit Note

Rev. 0 | Page 2 of 5

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)

L1

(nH)

L2

(nH)

C1a

(pF)

C1c

(pF)

C2a

(pF)

C2c

(pF)

C3a

(pF)

C3c

(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