LMX2430 datasheet(29/49 Pages) NSC

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LMX243x Charge Pump Test Setup

20053588

The block diagram above illustrates the setup required to

measure the LMX243x device’s RF charge pump sink cur-

rent. The same setup is used for the LMX2430TM Evaluation

Board. The purpose of this test is to assess the functionality

of the RF charge pump. The IF charge pump is evaluated in

the same way.

This setup uses an open loop configuration. A power supply

is connected to Vcc. By means of a signal generator, a 10

MHz signal is typically applied to the FinRF pin. The signal is

one of two inputs to the phase/ frequency detector (PFD).

The 3 dB pad provides a 50

Ω match between the PLL and

the signal generator. The OSCin pin is tied to Vcc. This

establishes the other input to the PFD. Alternatively, this

input can be tied directly to the ground plane. The EN and

ENosc pins are also both tied to Vcc. A Semiconductor

Parameter Analyzer is connected to the CPoutRF pin and

used to measure the sink, source, and TRI-STATE leakage

currents.

Let F

r represent the frequency of the signal applied to the

OSCin pin, which is simply zero in this case (DC), and let F

represent the frequency of the signal applied to the FinRF

pin. The PFD is sensitive to the rising edges of F

r and Fp.

Assuming positive VCO characteristics (RF_CPP bit = 1);

the charge pump turns ON, and sinks current when the first

rising edge of F

p is detected. Since Fr has no rising edge, the

charge pump continues to sink current indefinitely. In order

to measure the RF charge pump source current, the

RF_CPP bit is simply set to 0 (negative VCO characteristics)

in CodeLoader. Similarly, in order to measure the TRI-STATE

leakage current, the RF_CPT bit is set to 1.

The measurements are typically taken over supply voltage

and temperature. The measurements are also typically taken

at the HIGH and LOW charge pump current gains. The

charge pump current gain can be controlled by the RF_CPG

bit in CodeLoader. Once the charge pump currents are

determined, the (i) charge pump output current magnitude

variation versus charge pump output voltage, (ii) charge

pump output sink current versus charge pump output source

current mismatch, and (iii) charge pump output current mag-

nitude versus tempeature, can be calculated. Refer to the

Charge Pump Current Specifications Definition for more

details.

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