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===Generating Characteristic v-i Curves for Active Devices===

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===DC Sensitivity Test===

{{Note | In order to view the node numbers in your circuit, check the "Show Node Numbers" option in the View Menu or use the keyboard shortcut "Ctrl+Alt+N".}}

The results of a DC sensitivity test are displayed in a table window. The left column of the table lists the part parameters and the right columns shows the derivative of the output voltage with respect to the respective parameters. For example, consider the simple voltage divider circuit of Tutorial lesson No. 1, which consists of a 1V DC voltage source in series with a 1k resistor and another 2k resistor. The output voltage is designated as the voltage across the 2k resistor. The results of the DC sensitivity test for this circuit indicate that the output voltage is increased by 666.66mV for every 1-volt increase in the input source voltage, it is decreased by -1.111 mV for every 1 Ohm change in the first resistor's value, and it is increased by 555.55 microvolts for every 1 Ohm change in the second resistor's value. Note that these are all small-signal operating point results.

===Device Output Parameters Test===

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== AC Test Types ==

===AC Frequency Sweep Test===

{{Note | Before running an AC analysis, you have to designate one or more AC sources for your circuit and set their AC amplitudes (and phases). During an AC analysis, [[RF.Spice A/D]] shorts out all of the non-constant voltage sources that have a zero AC magnitude and opens all the non-constant current sources that have a zero AC magnitude. Make sure that the voltage and current sources used as inputs have non-zero AC voltage/current values.}}

All the sources with non-zero AC values are treated as sinusoidal sources with the specified peak voltage/current amplitudes and phases and default zero offsets. You can view or edit the AC values of voltage and current sources by double clicking on the parts and selecting the "Small-Signal AC and Distortion" tab of their property dialog. Make sure to check the "Use" checkbox in the section titled "AC Properties for Small-Signal AC Properties only". Given the specified voltage and current source values, the DC operating point of your circuit is calculated, and all nonlinear elements are replaced with their small-signal circuit models. If the DC operating point of your circuit with the sources shorted out is not where you want it, you may have to place some constant (DC) voltage sources in the circuit to compensate. Alternatively, you can set a nonzero value for the "Small-Signal Offset" parameter of an AC source as shown in the figure below.

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===AC Sensitivity Test===

The AC Sensitivity Test calculates the small-signal sensitivity of an output port to all device values and model [[parameters]] over a range of frequencies. The output port for sensitivity calculation is specified by a pair of positive and negative (reference) nodes. You can also prespecify whether the plots use decibels or magnitude, or degrees or radians.

===Distortion Test===

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===Small-Signal Transfer Function Test===

The Transfer Function Test calculates the small-signal transfer function of your circuit by linearizing it around its DC operating point. It generates the input impedance, output impedance and voltage gain seen across your circuit from the input port to the output port. All you need to do is specify the the input source and the output port.

For the simple voltage divider circuit of Tutorial Lesson 1 consisting of two resistors R1 = 1k and R2 = 2k, the transfer function is requested with the output port set between node 2 to ground (across R2), and the voltage source set as the input port. The test results for this example are shown in the opposite figure. The input impedance seen through the voltage source is R1 + R2 = 3k. The output impedance seen from the output port is the parallel combination R1 || R2 = 666.67. The transfer function is V2/V1 = 0.667.

===Pole-Zero Test===

The Pole-Zero Test generates a list of small-signal poles and zeros of the transfer function of your circuit given the input and output nodes.

Specify the input and output ports by the Positive and Negative (Reference) node numbers. To define the transfer function, you have two options: voltage gain by choosing "(output voltage)/(input voltage)" or trans-impedance by choosing "(output voltage)/(input current)". You can instruct the program to compute "Poles Only", or "Zeros Only", or "Pole & Zero".

The results of a Pole-Zero test is displayed as a table. As an example, consider the RLC circuit of Tutorial Lesson 2. Using the voltage gain definition for the transfer function, the Pole-Zero test reports two poles listed below:

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===Noise Test===

The Noise Test analyzes the device-generated noise over a range of frequencies. For this test you provide an input source, an output port and a range of frequencies. [[RF.Spice A/D]] calculates the noise contributions of each device (and each noise generator within each device) to the output port voltage. The program also calculates the equivalent to the output noise referred back to the specified input source. This is what is meant by the "Input Noise". The calculated value of the noise over the specified range of frequencies corresponds to the spectral density of the signal viewed as the square root of a stationary Gaussian stochastic process. After calculating the spectral densities, the simulator integrates these values over the frequency range to arrive at the total noise voltage/current over this frequency range. This calculated value corresponds to the variance of the circuit variable viewed as the square root of a stationary gaussian process.

As for the specific noise sources, they include the "shot" noise associated with the DC currents in semiconductor devices and the "thermal" noise associated with resistance. Semiconductors also display "flicker" (1/f) noise. However, due to the lack of a unified model, [[RF.Spice A/D]] handles this type of noise on a "case by case" basis. There are flicker noise parameters available for transistors in the list of model parameters. The program can also provide the noise gain associated with the 1/f source.

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Just as in low frequency circuits, RF circuits must be excited using a voltage or current source. However, in order to ensure a successful RF circuit simulation, your source must be a single-frequency (sinusoidal) AC source. Remember that in [[RF.Spice A/D]] circuits, you must specifically designate a source to operate as an AC source for all tests of the AC type. The RF Menu of [[RF.Spice A/D]] provides two additional types sources: the AC voltage source with the keyboard shortcut "Alt+V" and the AC current source with the keyboard shortcut "Alt+I". These sources are identical to the regular voltage and current sources of [[RF.Spice A/D]], whose "Use" checkboxes in the AC section of their source property dialog are automatically checked.

{{Note | For AC-type RF circuit analysis, you can only use AC voltage or current sources with a single common frequency.}}

In most RF circuits, the sources are modeled to have an internal source impedance typically denoted by Z_s. This source impedance is usually real-valued and typically has a value of 50 Ohms. To model the source impedance, you can simply use a resistor in series with the AC voltage source or a resistor in parallel with the AC current source as shown in the opposite figure. If you need a complex-valued source impedance, you can use a "Complex Impedance" and connected it either in series or in parallel with the AC voltage or current source, respectively.

Similarly, most RF circuits have a load impedance typically denoted by Z_L. This load impedance, too, is usually real-valued and typically has a value of 50 Ohms. To model the load impedance, you can simply use a resistor at the output port of your RF circuit. The opposite figure shows a simple RF circuit consisting of a two-port network N1 connected to an AC voltage source with a 50&Omega; internal resistance and terminated at a 100&Omega; resistive load. Note how the negative input and output pins of the two-port device have been grounded.

===Using Transmission Lines for Connecting Parts===

You can use Generic T-Line segments or physical transmission line types to connect the various RF parts and devices in your circuit. The opposite figure shows the same simple RF circuit of the previous figure, but containing two T-line segments, one connecting the source to Port 1 of two-port N1 and the other connecting Port 2 of N1 to the resistive load. Note how the negative input and output pins of both T-line segments have been grounded. Lossless transmission line segments cause a phase shift of the propagating signal, while lossy transmission lines also cause additional signal attenuation.

===Defining RF Circuit Observables===

{{Note | Typically the input and output voltage, currents and powers are of primary interest. These are measured at the input port (between the source impedance and input transmission line segment) and output port (between the output transmission line segment and the termination load).}}

In the previous RF circuit, the input port has node index 2 and the output port has node index 5. Therefore, the voltage v(2) and v(5) are designated as the simulation output data. The figure below shows a plot of the computed input and output voltages over the frequency range 1-10 GHz. A frequency step size of 10MHz has been set for the frequency sweep.

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== Network Analysis ==

{{Note | Network analysis is an AC circuit analysis that is typically performed for high frequency circuits.}}

For amplitude graphs, you have the option to plot them in Decibels (dB) scale. For phase graphs, you have the option to express them in degrees. You also need to specify which parameter set to calculate as the output of network analysis. The four options are Z, Y, S or H.

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