Datasheet

Frequency

Device

Under

Test

Evaluation Board

Power Supply

Network Analyzer

Calibrate for Open,

Short, Load Here

LMX2541SQ2060E, LMX2541SQ2380E

LMX2541SQ2690E, LMX2541SQ3030E

LMX2541SQ3320E, LMX2541SQ3740E

SNOSB31I –JULY 2009–REVISED FEBRUARY 2013

www.ti.com

Input and Output Impedance Test Setup

A network analyzer can be used to measure the input impedance of the OSCin and ExtVCOin pin as well as the

output impedance of the RFout pin. The general technique is to connect the desired pin with no DC blocking

capacitor to a network analyzer and measure the impedance directly. The part needs to be programmed to

ensure that it is in a known state. There are some special considerations that should be taken for different

measurements of the three different impedances.

OSCin Input Impedance Measurement

For this pin, the provided calibration standards are typically good enough for a decent measurement. A single-

ended measurement at the OSCin or OSCin* pins can be made For a differential measurement, this needs to be

treated by the instrument as a two port network.

ExtVCOin Input Impedance Measurement

Because this pin goes higher in frequency, it is often difficult get a good measurement at higher frequencies

because of the effects of the board and SMA connector. One technique that can be used is instead of using the

provided calibration standards that come with the equipment, solder resistors directly to the board in order to

calibrate out the effects of the board as well. A 0 ohm resistor functions as a short, no resistor functions as an

open, and two parallel 100 ohm resistors serve as a load. These should be soldered as close to the part as

possible. Once this calibration is done, the measurement can be done as normal.

RFout Output Impedance Measurement

Although output and input impedance are not the same thing, they can be measured in a similar way. Because

this pin is a higher frequency, it is better to use the same method for calibration as used for the ExtVCOin pin.

The other consideration for the RFout pin is that there are many different settings that impact this input

impedance. When in bypass mode (VCO_DIV=1), the VCOGAIN and OUTTERM words can change the

impedance. When in divided mode (VCO_DIV>1), the DIVGAIN and OUTTERM words can impact the

impedance.

Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E

LMX2541SQ3320E LMX2541SQ3740E