Download 1. Introduction 2. Oscillator Center Frequency and Inductor Choice 3

AN41
E XTE NDE D F REQUE NCY O PE RAT ION
R A D I O F R E Q U E NC Y S YNTHESIZERS
1. Introduction
The frequency range of selected Silicon Laboratories
RF synthesizers may be extended using a short in place
of the external tuning inductor. Some applications
require
frequency
ranges
that
cannot
be
accommodated by the standard Silicon Laboratories
offerings. This application note details how to extend
the upper frequency limits of the external-inductor
synthesizers such as the Si4133.
Only specific synthesizers may be used with an
extended frequency range. The data sheets of
applicable synthesizers will reference this application
note. These data sheets will provide two frequency
ranges: a center frequency range and an extended
center frequency range.
OF
S I L I C O N L ABORATORIES
tuning range must also be reserved to compensate for a
±10% inductor tolerance. When the external inductor is
zeroed, however, variation in LEXT is negligible and
more of this tuning range is available for the channel
frequency range. Refer to the synthesizer’s data sheet
for details.
3. Printed Circuit Board Layout
The MLP is required for extended frequency range
operation because it has a smaller package inductance.
Figure 1 illustrates the recommended PCB pattern for
LEXT = 0 nH. This figure illustrates shorts on the RF1,
RF2, and IF oscillators. Refer to the data sheet for
details about which oscillators may be operated with
LEXT = 0 nH.
2. Oscillator Center Frequency and
Inductor Choice
The oscillator is tuned using a parallel capacitorinductor tank circuit. The capacitor is internal to the
synthesizer and the inductor is formed by package and
external inductances. The center frequency is
determined by adjusting the inductance and is given by
the following expression:
1
f CEN = --------------------------------------------------------------2  L
PKG + L EXT C NOM
The application designer selects the frequency
operating range by selecting LEXT. LPKG and CNOM are
fixed by the design of the synthesizer and packaging.
The smallest LC product maximizes the output
frequency. Zero LEXT minimizes the LC product.
Figure 1. Example PCB MLP Layout with RF1,
RF2, and IF External-Inductor Tuning
Pins Set to 0 nH.
Internal-inductor synthesizers, such as the Si4136,
utilize an internal bond wire inductor. All inductance is
from LPKG, and the frequency range is fixed. This series
of synthesizers is typically centered between 2 GHz and
2.5 GHz.
4. Operating Condition
Requirements
External-inductor synthesizers, such as the Si4133, are
designed to be tuned by an external inductor. When
LEXT = 0 nH, the oscillator will oscillate at a higher
frequency tuned by LPKG, approximately 1.8 GHz to
2 GHz.
Extended frequency operation requires a higher bias
voltage for certain analog stages within the synthesizer.
The following steps implement this change:
Silicon Laboratories specifies an allowable channel
frequency range of ±5% for the external-inductor
synthesizers. This range is limited because some of the
Rev. 0.1 10/06
Some changes are required in the operating
specifications to support extended frequency operation.
1. Set VDDR, VDDI, and VDDD to between 3.0 V and
3.6 V.
2. Set bit D1 of the Main Configuration Register (see
Copyright © 2006 by Silicon Laboratories
AN40
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Table 1) to 1.
Table 1. Main Configuration Register
Bit
D17
D5
D4
D3
D2
D1
D0
LPWR
0
AUTOPDB
AUTOKP
1
0
...
Name
0
Key synthesizer specifications, such as phase noise
and output power, may also change with extended
frequency operation. Refer to the synthesizer’s data
sheet for more detail.
5. Conclusion
The output frequency range can be extended for
selected Silicon Laboratories synthesizers by
performing the following steps:
1. Review the revised specifications for Extended
Frequency Operation in the synthesizer’s data
sheet.
2. Design the PCB for a short under the RFLA-RFLB or
RFLC-RFLD tuning pins of a designated MLP
synthesizer.
3. Ensure that a minimum 3.0 V supply voltage is
furnished to the synthesizer.
4. Set bit D1 of Main Configuration Register to 1.
2
Rev. 0.1
AN41
NOTES:
Rev. 0.1
3
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