LMX2364 National Semiconductor Corporation, LMX2364 Datasheet - Page 22

• Manufacturer Part Number: LMX2364
• Description: 2.6 Ghz Pllatinum Fractional Rf Frequency Synthesizer With 850 Mhz Integer If Frequency Synthesizer
• Manufacturer: National Semiconductor Corporation

www.national.com
Input Impedance
The above block diagram shows the test procedure measur-
ing the input impedance for the LMX2364. This applies to the
FinRF, FinIF, OSCinRF, and OSCinIF pins. The input imped-
ance of the CSP and the TSSOP package should always be
assumed to be different, until proven otherwise. The basic
test procedure is to calibrate the network analyzer, ensure
that the part is powered up, and then measure the input
impedance.
The network analyzer can be calibrated by using either
calibration standards or by soldering resistors directly to the
evaluation board. An open can be implemented by putting no
resistor, a short can be implemented by using a 0 ohm
resistor, and a short can be implemented by using two 100
ohm resistors in parallel. Note that no DC blocking capacitor
is used for this test procedure. This is done with the PLL
removed from the PCB. This requires the use of a clamp
down fixture that may not always be generally available. If no
clamp down fixture is available, then this procedure can be
done by calibrating up to the point where the DC blocking
22
capacitor usually is, and then adding a 0 ohm resistor back
for the actual measurement.
Once that the network analyzer is calibrated, it is necessary
to ensure that the PLL is powered up. This can be done by
toggling the power down bits (RF_PD and IF_PD) and ob-
serving that the current consumption indeed increases when
the bit is disabled. Sometimes it may be necessary to apply
a signal to the OSCinIF pin in order to program the part. If
this is necessary, disconnect the signal once it is established
that the part is powered up.
It is useful to know the input impedance of the PLL for the
purposes of debugging RF problems and designing match-
ing networks. Another use of knowing this parameter is make
the trace width on the PCB such that the input impedance of
this trace matches the real part of the input impedance of the
PLL frequency of operation. In general, it is good practice to
keep trace lengths short and make designs that are relatively
resistant to variations in the input impedance of the PLL.
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