变容二极管模型

Application Note

Varactor SPICE Models for RF VCO Applications

APN1004

Varactor Equivalent Circuit ModelDefinitions

A simplified equivalent circuit of varactor is shown in Figure 1.

This varactor model is useful for RF VCO applicationsalthough it neglects some parasitic components oftenneeded for higher frequency microwave applications, suchas the distributed line package model and somecapacitance due to ground proximity.For most RF VCOapplications, to about 2.5 GHz, these parasitic

components would not be important unless higherharmonics generated by the varactor affects performanceof the VCO.In this case, a more detailed equivalent circuitmodel is needed.The technique used should be basedon the varactor model extraction procedure from S-parameter data.

A SPICE model, defined for the Libra IV environment, isshown in Figure 2, with the description of the parametersemployed.It neglects the package capacitance, CP, itstypical 0.10 pF value is absorbed within the junctioncapacitance CV.

Parallel Capacitance

Figure 1.Simplified Equivalent Circuit of Varactor

Figure 2.Libra IV SPICE Model

Alpha Industries, Inc.[781] 935-5150 •Fax[617]824-4579•[email protected]•www.alphaind.com

Specifications subject to change without notice.6/99A

1

Varactor SPICE Models for RF VCO ApplicationsAPN1004

Parameter

ISRSNTTCJOVJMEGXTIKFAFFCBVIBVISRNRIKFNBVIBVLNBVLTNOMFFE

Series resistance

Description

Saturation current (with N, determine the DC characteristics of the diode)Emission coefficient (with IS, determines the DC characteristics of the diode)Transit time

Zero-bias junction capacitance (with VJand M define nonlinear junction capacitance of the diode)Junction potential (with VJand M define nonlinear junction capacitance of the diode)Grading coefficient (with VJand M define nonlinear junction capacitance of the diode)Energy gap (with XTI, helps define the dependence of IS on temperature)

Saturation current temperature exponent (with EG, helps define the dependence of IS on temperature)Flicker noise coefficientFlicker noise exponent

Forward-bias depletion capacitance coefficientReverse breakdown voltage

Current at reverse breakdown voltageRecombination current parameterEmission coefficient for ISRHigh-injection knee currentReverse breakdown ideality factorLow-level reverse breakdown knee currentLow-level reverse breakdown ideality factor

Nominal ambient temperature at which these model parameters were derivedFlicker noise frequency exponent

UnitAΩ-SFV-EV----VAA-A-A-°C-

Default1e-14010010.51.113010.5Infinity1e-302Infinity101271

Table 1.SPICE Model Parameters

Table 1 describes the model parameters.It shows defaultvalues appropriate for silicon varactor diodes, which maybe used by the Libra IV simulator unless others arespecifically defined.

The effect of the diode junction is ignored in this model.This simplification ignores the rectifying effect of diodeduring a positive voltage swing.However, for most RF VCOapplications, the lowest practical DC control voltage valueis 0.5 V and the magnitude of RF voltage rarely exceeds0.2 V peak.Therefore, the varactor is maintained in itsreverse bias state.However, in a large signal applicationwhere it is necessary to consider the diode’s rectifyingproperties, it may be done by entering the additional diodeparameters in the SPICE model defined for the LIBRA IVenvironment.

According to the SPICE model in Table 1, the varactorcapacitance, CV, is a function of the applied reverse DCvoltage, VR, and may be expressed as follows:

CV=

CJO

+ CP

This equation is a mathematical simulation of thecapacitance characteristic.The model is accurate forabrupt junction varactors (SMV1400 Series);forhyperabrupt junction varactors the model is less accuratebut very reliable.The form is similar to the traditionalvaractor equation but uses values for VJ, M and CP, thatwere extracted individually from measured CV(VR) datafor each varactor part number.

Series resistance, RS, is a function of applied voltage andoperating frequency and may be considered constant.Thevalue used should be taken from the specified maximumvalue or derived from its Q specification.

Series inductance, LS, is also considered constant at avalue of 1.7 nH.This incorporates the 1.5 nH packageinductance with some insertion inductance typical for PCboards in RF wireless applications.

(1 +VR)

VJ

M

2

Alpha Industries, Inc.[781] 935-5150 •Fax[617]824-4579•[email protected]•www.alphaind.com

Specifications subject to change without notice.6/99A

Varactor SPICE Models for RF VCO ApplicationsAPN1004

Table 2 gives values for Alpha’s plastic packagedvaractors that may be used for SPICE model simulationequation.It may be employed for each varactor junctionin the SOD-323 and SOT-23 package.It also givescalculated values for the capacitance ratio between 0.5–2.5 V for each diode that is a typical voltage rangefor battery operated wireless VCO circuits.

Note:The values listed for VJ, M and CPin the table wereempirically determined and do not represent the precisephysical or electronic properties of the semiconductor orthe package.

Part NumberSMV1127SMV1129SMV1139SMV1140SMV1141SMV1142SMV1143SMV1144SMV1145SMV1146SMV1147SMV1148SMV1175SMV1206SMV1207SMV1212SMV1213SMV1214SMV1215SMV1223SMV1224SMV1225SMV1227SMV1228SMV1229SMV1232SMV1233SMV1234SMV1235SMV1236SMV1237SMV1245SMV1247SMV1248SMV1249SMV1250SMV1251

CJO(pF)23.927.5870.447.3213.3818.9924.0141.861.1389.52104.713.4326.1159.472.4728.922.7414.3625.1925.1917.4652.46130.1271.724.24.128.7516.1321.6366.166.99.2221.54394760

VJ(V)2.22.81.23.52.22.22.22.22.52.52.52.25346.[***********]01105551.71.72.388103.[1**********]7

M11.10.651.411111.11.11.11.11.151.452.[***********]1.81.81.80.90.91.144.25.31.710010.5141414

CP(pF)[1**********]000.324.52.21.51.12.52.51.600000.71.223.290.470.550000

RS(Ω)0.50.40.60.30.70.70.650.650.60.60.550.51.00.70.40.450.80.71.01.51.51.80.550.320.251.51.20.80.60.50.13221.81.51.51.3

LS(nH)1.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.71.7

C0.5/C2.51.681.731.681.681.661.671.671.671.681.681.681.71.681.691.732.822.532.602.732.102.102.051.751.751.751.871.711.821.841.862.051.822.156.26.755.415.86

Table 2.Plastic Packaged Varactor Values for SPICE Model Simulation Equation

Alpha Industries, Inc.[781] 935-5150 •Fax[617]824-4579•[email protected]•www.alphaind.com

Specifications subject to change without notice.6/99A

3

Varactor SPICE Models for RF VCO ApplicationsAPN1004

Part NumberSMV1253SMV1255SMV1299SMV1405SMV1408SMV1409SMV1410SMV1411SMV1413SMV1414SMV1415SMV1416SMV1417SMV1419SMV1420SMV1421SMV1493SMV2022SMV2023

CJO(pF)708213.732.923.75.25.547.5759.211.212.816.0419.221.430.236.1297.0825.79

VJ(V)17171900.680.80.80.80.80.790.780.780.840.840.870.80.80.637250

M14141100.410.430.450.450.450.450.460.460.480.480.540.470.470.472.3110

CP(pF)001.10.050.130.130.130.130.130.130.130.130.130.130.130.1300.42.4

RS(Ω)1.212.50.80.60.50.450.400.350.30.270.240.22

0.20.190.180.252.11.3

LS(nH)1.71.71.71.71.71.71.71.71.71.71.71.71.71.71.7

1.71.71.71.7

C0.5/C2.55.884.422.611.411.51.511.521.521.521.541.551.541.541.611.591.571.631.652.09

Table 2.Plastic Packaged Varactor Values for SPICE Model Simulation Equation (Continued)

Examples

Figure 3shows the SPICE model calculated capacitanceAlpha abrupt junction varactor SMV1493-011 with measuredcapacitance values.

Figure 4 shows the SPICE model calculated capacitancefor Alpha hyperabrupt junction varactor SMV1235-011 withmeasured capacitance values.

20

Capacitance (pF)

15

10

5

00

2

4

6

8

10

12

Varactor Voltage

Figure 4.SMV1235 7.575/(1-VV/0.8)^0.45

4

Alpha Industries, Inc.[781] 935-5150 •Fax[617]824-4579•[email protected]•www.alphaind.com

Specifications subject to change without notice.6/99A

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