LMX2430 datasheet(32/49 Pages) NSC

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LMX243x FinRF Input Impedance Test Setup

20053591

The block diagram above illustrates the setup required to

measure the LMX243x device’s RF input impedance. The

same setup is used for the LMX2430TM Evaluation Board.

Measuring the device’s input impedance facilitates the de-

sign of appropriate matching networks to match the PLL to

the VCO, or in more critical situations, to the characteristic

impedance of the printed circuit board (PCB) trace, to pre-

vent undesired transmission line effects. The FinIF input

impedance is evaluated in the same way.

Before the actual measurements are taken, the Network

Analyzer needs to be calibrated, i.e. the error coefficients

need to be calculated. The Network Analyzer’s calibration

standard is used to calculate these coefficients. The calibra-

tion standard includes an open, short and a matched load. A

1-port calibration is implemented here.

To calculate the coefficients, the PLL chip is first removed

from the PCB. A piece of semi-rigid coaxial cable is then

soldered to the pad on the PCB which is equivalent to the

FinRF pin on the PLL chip. Proper grounding near the ex-

posed tip of the semi-rigid coaxial cable is required for

accurate results. Note that the DC blocking capacitor is

removed for this test. The Network Analyzer port is then

connected to the other end of the semi-rigid coaxial cable. In

this way, the semi-rigid coaxial cable acts as a transmission

line. This transmission line adds electrical length and pro-

duces an offset from the reference plane of the Network

Analyzer; therefore, it must be included in the calibration.

The desired operating frequency is then set. The typical

frequency range selected for the LMX243x device’s RF syn-

thesizer is from 100 MHz to 6000 MHz.

The Network Analyzer calculates the calibration coefficients

based on the measured S

11 parameters. With this all done,

calibration is now complete.

The PLL chip is then placed on the PCB. A power supply is

then connected to Vcc. The EN, ENosc, and OSCin pins are

all tied to Vcc. Alternatively, the OSCin pin can be tied to

ground. In this setup, the complementary input (FinRF*)is

AC coupled to ground. With the Network Analyzer still con-

nected to the semi-rigid coaxial cable, the measured FinRF

impedance is displayed.

The OSCin input impedance is measured in the same way.

The impedance is measured when the oscillator buffer is

powered up (ENosc is set HIGH) and when the oscillator

buffer is powered down (ENosc pin is set LOW).

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