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[[File:RFSpice_Screen.png|thumb|400px|]]

[[RF.Spice A/D]] is an enhanced superset of EMAG Technologies' older [[B2.Spice A/D]] application with an extensive library of RF devices that include S-parameter-based [[Multiport Networks|multiport networks]] and a variety of generic and physical transmission line types. You can use [[RF.Spice]] to simulate or design distributed analog and mixed-mode circuits at high frequencies.

* AC Frequency Sweep Test

* Network Analysis Test

From a simulation point of view, an RF circuit is made up of a collection of [[Multiport Networks|multiport networks]] that are interconnected via RF [[Transmission Lines|transmission lines]]. If the input of your circuit is connected to a source and its output is connected to a load, then you can compute all the voltages and currents at all the external or internal ports of the circuit (i.e. at the various circuit nodes). Or you can calculate the port characteristics of the overall network by designating input and output ports to your RF circuit.

The RF circuit analysis performed by [[RF.Spice]] is based on the assumption that your distributed RF circuit can be modeled as an interconnected network of multiport devices and transmission line segments and components. This means that all the coupling or crosstalk effects must have been captured by the S-parameter-based models of devices or by the transmission line and discontinuity models used by [[RF.Spice]]. Most of these models work satisfactorily at lower frequencies up to several Gigahertz. At these frequencies, a quasi-static regime may be able to represent the physics of your RF circuit to a good level of accuracy. In the quasi-static regime, the different parts of your circuits can be treated as multiport devices or components that are governed by Kirchhoff circuit laws.

* Complex Impedance (a two-pin device)

* One-port (a two-pin device)

* Two-port (a four-pin device)

* Three-port (a six-pin device)

* Four-port (an eight-pin device)

{{Note | Multipart Network devices do not work with “Live Simulation” or Transient Test as their models normally contain S-[[parameters]] at high frequencies only.}}

Each multiport network device has a property dialog where you can specify its S-[[parameters]] as a function of frequency. Each row of the S-parameter table in the property dialog represents a frequency sample. The complex-valued elements of the scattering matrix are listed column by column in each row of the table after the frequency value. For example, for a two-port, the format is as follows:

* Real / Imaginary

* Magnitude /Phase

* dB / Phase

Note that the default reference impedance of the Complex Impedance is zero and must always stay zero to function properly. In order to have a fixed impedance element, define the same Real(z11) and Imag(z11) values for the minimum and maximum frequencies of your circuit. Due to the interpolation between these two values, you will always get the same impedance value at all the frequencies in between those two limits.

The standard SPICE provides two types of general-purpose transmission line models: lossless (TRA) and lossy (LTRA). These models are primarily intended for transient analysis. The lossless transmission line model is characterized by either its delay in seconds or by its normalized length at a given frequency. On the other hand, the lossy transmission line model is characterized by distributed RLCG [[parameters]]: resistance per unit length (R), inductance per unit length (L), capacitance per unit length (C), and conductance per unit length (G). Both [[B2.Spice A/D]] and [[RF.Spice]] offer the native SPICE transmission line models TRA and LTRA as passive devices.

However, [[RF.Spice]] also offers a number of other transmission line models specifically intended for use in RF circuit analysis. These include the generic T-Line, the generic coupled T-lines, and a variety of physical transmission line models.

[[File:tline.png|thumb|250px| The schematic symbol of the Generic T-Line device.]]

* Z0: Characteristic Impedance in Ohms

* eeff: Effective Permittivity

* alpha: Attenuation Constant in dB/m

* len: Physical Length in meters

[[File:tline2.png|thumb|350px| The schematic symbols of the Generic Open Stub (left) and Generic Short Stub (right) devices.]]

The Generic Open Stub and Generic Short Stub are two one-port devices based on the Generic T-Line device. They represent terminated generic transmission line segments. In the open stub case, the termination load is Z_L → ∞, and in the short stub case, the termination load is Z_L = 0. The [[parameters]] of both open and short stub devices are the same as those of the generic T-line device.

[[File:tline3.png|thumb|250px| The schematic symbol of the Generic Coupled T-Lines device.]]

Many passive RF devices such as directional couplers, hybrids and some filter designs involve segments of parallel coupled [[Transmission Lines|transmission lines]]. According to the coupled mode theory, one can define even and odd mode impedances for such [[Transmission Lines|transmission lines]]. The resulting RF structure can be modeled as a four-port network device as shown in the opposite figure. Note that the four-port device has eight pins. Ports 1 and 2 correspond to the input and output of the first transmission line segment, while Ports 3 and 4 correspond to the input and output of the second (coupled) line segment. It is very important to connect and ground the negative pins at the input and output of the two transmission line segments.

[[File:tline4.png|thumb|450px| The schematic symbols of the Generic T-Line Discontinuity devices: (a) Open End, (b) Bend, (c) Step Junction, (d) Tee Junction and (e) Cross Junction.]]

In real practical RF circuits, you often need to transition from one transmission line to another or connect two or more [[Transmission Lines|transmission lines]] together. These transitions can be modeled as [[Multiport Networks|multiport networks]]. [[RF.Spice]] currently offers five generic T-Line discontinuity devices as follows:

For more information about generic transmission line discontinuity devices, please refer to [[Glossary_of_Generic_RF_Devices | Glossary of Generic RF Devices]].

In addition to the Generic T-Line, [[RF.Spice]] also offers a number of physical [[Transmission Lines|transmission lines]] as follows:

[[File:tline5.png|thumb|450px| The schematic symbols of the some Physical Transmission Line Discontinuity devices. (Top Row) microstrip components: right-angled bend, mitered bend, tee and cross junctions, (Bottom Row) CPW components: open end, short end, gap and step junction.]]

Besides the Generic T-Line Discontinuity devices described earlier, [[RF.Spice]] also offers a large number of physical transmission line discontinuity devices based on some of the transmission line types listed above. Among these, the microstrip components are more widely used and have more variety. Unlike the generic T-line components which require that you supply the S-parameter data, the physical components are parameterized based on their geometrical and material properties. As a result, you simply enter the physical dimensions and other [[parameters]] and their S-[[parameters]] are automatically calculated by [[RF.Spice]] during the circuit simulation. In other words, the property dialog of these components contains their physical [[parameters]] only and does not show the values of their S-[[parameters]] as a function of frequency.

For more information about physical transmission line discontinuity devices, please refer to [[Glossary_of_Physical_Transmission_Lines_and_Components|Glossary of Physical Transmission Lines and Components]].

When you place a generic T-line part in your circuit, you have to specify its characteristic impedance (Z0), effective permittivity (eeff) and attenuation constant (alpha). Or you may accept the default values Z0 = 50, eeff = 1, and alpha = 0. In the case of physical transmission line parts like mircostrip, coaxial line or CPW, you specify the physical parameters of the line such as various dimensions and material properties. In that case, RF.Spice automatically calculates the necessary transmission line parameters based on your physical data at the time of simulation. Oftentimes, you may want to design a 50Ω transmission line of a certain type, or calculate and compare the characteristics of several transmission line types. Another practical need in RF design is to quarter-wavelength line segments. In this case, you must calculate the guide wavelength of the transmission line as defined by λ_g = λ₀ / √ε_eff, where λ₀ = c/f is the free space wavelength at the operational frequency.

The Device Editor of [[RF.Spice]] provides ten line calculators and ten designer tools for all the transmission line types listed in the above table. These tools are accessible form the RF Menu of the Device Editor. The line calculators take the substrate properties and the physical dimensions of a line types and calculate its characteristic impedance (Z0) and effective permittivity (eeff). The Line Calculator dialog also has an "Operational Frequency" input with a default frequency of 1GHz, which is used to calculate the guide wavelength of the transmission line at that frequency. Some of the line types, microstrip, stripline, coaxial line, twin-lead line and twisted-pair line, have loss [[parameters]]: dielectric loss tangent and metal conductivity. For these lines, the line calculator calculates the conductor attenuation constant (α_c) and dielectric attenuation constant (α_d), both in Neper per meter (Np/m). Note that the total attenuation constant is the sum of these two: α = α_c + α_d. Also, you can convert these values from Np/m to dB/m using the relationship: 1Np = 8.6859dB.

</table>

For every physical transmission line type, [[RF.Spice]] also provides a designer tool. The designer ignores conductor and dielectric losses and calculates the physical dimensions of the line for a given value of the characteristic impedance Z0. For example, given a substrate with thickness h and relative permittivity &epsilon;_r, the "Microstrip Designer" calculates the microstrip width in mm for a given value of Z0 (50 Ohms by default). Some line type like CPW and coaxial line have more than one dimensional parameter that can be varied. For example, CPW has slot width (w) and center strip width (s), while coaxial line has inner and outer conductor radii. In such cases, the Line Designer dialog provides radio button options to fix one parameter and vary the other.

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In the case of coupled microstrips and coupled striplines, it is the even and odd mode characteristic impedances (Z0e and Z0o) that really matter. The Coupled Microstrips Calculator and Coupled Striplines Calculator find these two impedances for the given strip width and strip spacing. They also calculate the even and odd mode effective permittivities, which are typically different. The system characteristic impedance of the coupled line is calculated from the formula: Z_0s = &radic;( Z_0e . Z_0o ). To calculate the guide wavelength, the average of the two effective permittivities is used. In addition, the coupling coefficient of the coupled line is calculated in dB from the formula: C = ( Z_0e - Z_0o ) / ( Z_0e + Z_0o ).