Download

PDF Abstract Text:

Capacitance Selection Guide and Cross Reference

Silicon Chip RF Capacitors

Capacitance Selection Guide and Cross Reference
Vishay
Silicon Chip RF Capacitors
CAPACITANCE VS. FREQUENCY SELECTION GUIDE AND CROSS REFERENCE
CAPACITANCE vs. FREQUENCY 10 pF 22 20
It can be seen in Figure 1 that, when these LTCC and TF capacitors are used above 2 GHz, any small drift in frequency will cause a large change in capacitance. In a typical application, such as impedance matching between the power amplifier (PA) and the antenna in a mobile phone, this could be very important. The large change in capacitance will cause a mismatch in impedance, resulting in much of the signal output of the PA being reflected back into the phone instead of being transmitted by the antenna. The phone will then attempt to boost the output signal, draining the battery while trying to increase the transmitted signal. The capacitance stability of the HPC0402 silicon capacitor shown in Figure 1, which demonstrates much better frequency stability than the other technologies, eliminates the problem. Figure 2 illustrates the data with which a capacitor may be selected for a specific operating frequency.
CAPACITANCE vs. FREQUENCY
easured on Agilent 4287 A + 16197 A
FREQUENCY (MHz)
Figure 2: Capacitance vs. Frequency Data for Capacitor Selection Consider, for example, the case in which a user requires a 13-pF capacitor at 2.7 GHz. Capacitors are supplied with the capacitance value measured at 1 MHz. The user consults the characterization data and finds that a value of 13 pF at 2.7 GHz demands an 8.2 pF LTCC or TF capacitor. The drawback to working with these technologies is two-fold. The capacitor is working in a region where the LTCC and TF capacitance changes quickly with a small change of frequency, and they each exhibit curves which diverge with wider tolerance drift as frequency increases. As a result, the user must specify and order a tighter tolerance at 1 MHz to remain within the allowable error envelope at the operating frequency. By comparison, HPC silicon capacitors provide a much more uniform capacitance curve over a wide frequency range. In contrast to the capacitance increase of the LTCC and TF devices with a rise in frequency, the HPC silicon
www.vishay.com 25
CAPACITANCE (pF)
18 Thin Film Technology 16 14 LTCC Technology 12 10 Vishay-HPC 8 0 1000 FREQUENCY (MHz) MEASURED ON ANGILENT 4287 A + 16197 A 2000 3000
Document Number: 10134 Revision: 12-Oct-04
For technical questions, contact: siliconRFcap@vishay.com
Capacitance Selection Guide and Cross Reference
Vishay
Silicon Chip RF Capacitors
When the designer is using the HPC directly, he can put in the value and tolerance required at the desired operating frequency and the conversion will yield the effective capacitance of the HPC and its part number. When the designer wants to show multiple sources on the Bill of Materials (BOM) he simply includes the appropriate part number for each technology. For example:
ITEM C115 MANUFACTURER VISHAY AVX MANUFACTURERS PART NUMBER HPC0402A160FXYT 0402J100FBWTR
Note that each item on the BOM may show different values for the different technologies in the same application. There is no other way to take advantage of the superior stability of the HPC precision capacitor while still holding positions on the BOM for competing products. Note: A technical paper describing these effects in greater detail titled: "0.1 pF - 180 pF 0402 RF Capacitor Capacitance Stability From 1 MHz to several GHz." is available at www.vishay.com
www.vishay.com 26
For technical questions, contact: siliconRFcap@vishay.com
Document Number: 10134 Revision: 12-Oct-04