RF Design Software Combines Synthesis and Simulation

by Dale Henkes, Applied Computational Sciences


Perhaps the most efficient way to create a new circuit design would be to let a circuit synthesis program create the initial design from a set of specifications. The circuit thus created would serve as the initial or “approximate” solution which could then be entered into a circuit simulator for verification. During the simulation and verification process it might be necessary to modify the circuit to include parasitic elements and/or the conversion of ideal components to physical or practical ones.

There are a number of popular RF CAE/CAD software packages on the market today. Most provide circuit simulation in the frequency or time domain or both. Some provide modules that are capable of synthesizing a specific circuit or sub-circuit. These are usually provided as upgrades or add-on modules for an additional cost. The LINC2 RF CAE program from Applied Computational Sciences offers an integrated design environment for the design (synthesis) and simulation of RF and microwave circuits. With LINC2 a project can flow smoothly from design to verification because the program couples schematic capture and a suite of RF design tools to a powerful simulator engine. LINC2 is a high performance, low cost, RF design solution which includes schematic capture and all synthesis tools for under $500.00.

When the software provides simulation only, the design becomes a process of trial and error. Some simulation packages provide circuit templates or example schematics of common circuits. However, there is no guarantee that the circuit topology provided will satisfy a new set of specifications, even after employing optimization. Consider, for example, the output matching network for an RF amplifier. One of the most economical circuits for narrow-band matching is the two-element “L” network. There is a form of “L” network that will match any complex source to any other complex load. However, if one of these “L” configurations is borrowed from an existing design or template and applied to a new device and/or load it is likely to fail to provide an impedance match, even after lengthy attempts to use an optimizer. The designer may not realize the futility of the attempt until a great deal of time has been expended.

Starting a design with synthesis is not only a more direct approach, but the well-designed synthesis program will only provide circuit topologies that will meet the specifications. Some times the component values generated may not be practical, but at least they are returned almost immediately for review. Often, several alternative circuit topologies are returned by the synthesis program so that the user can choose the one with the most realizable component values.



The synthesis module may be linked to a simulator so that the newly created circuit can be analyzed against a much broader set of performance criteria using a larger set of analysis tools. For example, the circuit simulator can check if a design is manufacturable by looking at its sensitivity to component tolerances using Monte Carlo analysis. Also, parasitic elements can be modeled and added to the schematic and ideal components can be replaced by physical ones. The simulator can then be run and these effects can be reduced or eliminated by tuning or optimization. The process flow, starting with synthesis, is shown in Figure 1.

LNA Design Example

This following example illustrates the design process outlined in Figure 1. Proposed is the design of an LNA (low noise amplifier) operating at 2400 MHz with the following specifications:

1. Use the NEC NE76038 GaAs MESFET biased at Vds = 3V and Id = 10 mA.
2. Operate with a source impedance of 75 ohms and a load impedance of 50 ohms.
3. The amplifier must be uncondi- tionally stable at the operating frequency.
4. Design for a gain greater than 13 dB at 2400 MHz.
5. The noise figure (NF) should be less than 1 dB.
6. Use distributed (microstrip) matching networks.
7. Provide a conjugate output match (|S22| > 20 dB).

The LINC2 Circles Utility is an amplifier design module that automatically synthesizes input and output networks for a transistor or FET based on interactive displays of gain, noise figure, and stability circles overlaid on the Smith Chart. The circles and other related printed data provide the user with a great deal of pre-synthesis information designed to guide the user to the best choice of matching networks for a particular design goal. The program then provides the user with a menu list of matching circuit topologies (including L, PI, T and various transmission line/stub forms). The use of this tool to achieve the above specifications will be demonstrated next.




Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Select the FET type and operating conditions (specification 1):
The first step is to load the S-parameter file for the NE76038 FET. When the S-parameter file is read from the disk the noise parameters are automatically extracted from the file and used to calculate the noise data. The operating frequency is then set to 2400 MHz via the “Frequency” menu.

Design for a given source and load impedance (specification 2):

Setting the source (generator) impedance to 75 ohms is accomplished by selecting “Target Z0in” from the “Options” menu and entering the new value (Figure 2). The default is 50 ohms, so it is not necessary to change the “Target Z0out”.

Design the amplifier for unconditional stability (specification 3):
Selecting “Stability” from the “View” menu generates input and output stability circles on Smith Charts in their respective planes. Initially the stability circles cut into the Smith Chart and the program reports that the stability factor, K, is less than 1, a condition for potential instability. It is not always necessary to correct this condition if the terminating impedances can be maintained at a safe distance from the unstable regions defined by the circles. However, rendering the device unconditionally stable has other advantages that include making it possible to match both input and output ports simultaneously or at least achieving a more desirable match if matching both ports is not intended.

LINC2 provides several ways to automatically stabilize a device. As shown in the “Options > Stabilize Device” menu in Figure 2, the device can be loaded with a series or shunt resistor or an inductance can be applied to the common (FET source) lead. For this example, inserting a small amount of inductance between the source lead and ground was chosen. The program automatically determined that about 1 nH was the minimum amount of inductance required. Applying 1.2 nH produced some additional margin (K>1 and (³<1) as shown in the data box at the bottom of the window. Also, the stability circles no longer cut into the Smith Chart. They have been pushed out past the outer edge of the chart as shown in Figure 2. In addition to meeting the goal of unconditional stability at the operating frequency, it will be necessary to ensure out-of-band stability as well. This can be accomplished by designing bias and DC supply feeds that have little effect in-band but provide stable terminations out-of-band.

Design for a gain greater than 13 dB (specification 4):
Selection of a target for gain cannot be made independent of noise considerations. Typically, a tradeoff exists between producing more gain and the noise figure that can be achieved at that value of gain. The LINC2 program facilitates making gain-noise tradeoffs by providing a slider control that allows continuous adjustments (tradeoffs) between maximum gain and minimum noise figure. Selecting “Noise and Ga > Tradeoffs” from the “View” menu produces simultaneous displays of gain and noise data as shown in Figure 3. Moving the gain-noise control from max gain to min noise produces the data in Table 1.

Design for a noise figure (NF) less than 1 dB (specification 5):
As mentioned above, noise figure can usually be traded for gain and vice versa. Table 1 indicates that about 1 dB of gain can be traded for 1 dB of noise figure. Sliding the gain-noise control to its midpoint position generated the data on line 5 of Table 1. The corresponding input match is plotted as a point located between the points labeled Max Gain and Min Noise on the input Smith Chart (Figure 3). This was the position taken in this example to complete the design and achieve the required specifications.

Use distributed (microstrip) matching networks (specifications 6 and 7):
At this point the program has all of the information it needs to complete the design. A menu of matching topologies are available that includes distributed (transmission line) networks. Selecting “Transmission Line > Stub and TRL Cascade” from the “Match” menu produces the schematic shown in Figure 4 with matching networks applied to the device. When the user selects a particular topology the program automatically calculates all component values necessary to realize the match points plotted on the input and output planes (Figure 3). The program links the input and output planes through a mapping process that generates a conjugate output match for any input match selected by the user.

Clicking “OK” confirms the selection of this matching topology and automatically generates the circuit netlist shown in Figure 5. The corresponding LINC2 schematic shown in Figure 6 represents the initial synthesized circuit using “ideal” components.



Figure 8
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Figure 9
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Figure 10
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Figure 11
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Figure 12
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Figure 13
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The next step is to replace the ideal synthesized components with physical ones. But first, we can run a simulation on the schematic to generate a performance baseline from which to compare the final circuit with the synthesized one. Figure 7 indicates that the gain is about 14 dB at 2400 MHz, agreeing closely with the value predicted by the Circles Utility (from Table 1 and Figure 3, Ga = 14.285 dB). At this point any slight differences are due entirely to the number of significant digits used for the component values in the simulation. The output return loss (M22) reported in Figure 7 is nearly ideal at 32.94 dB. We will now see how this performance holds up as the ideal components are replaced by physical ones.

Synthesize Physical Components
The next step of the process, outlined in Figure 1, is to convert the ideal components to physically realizable components (microstrip transmission lines printed on a circuit board substrate). The LINC2 transmission line tool does this automatically as shown in Figure 8 and 9. Figure 8 shows how the physical dimensions of the 75ž shorted stub (input matching element T1 in Figure 6) are generated. Figure 9 shows how the 1.2 nH source inductor, L1, is converted to physical printed trace dimensions for the circuit board material indicated.

After converting the rest of the transmission line elements from electrical parameters (based on line impedance Z0, degrees of electrical length, and frequency) to simple physical descriptions of length, L, and width, W, the schematic is updated as shown in Figure 10. Since the performance of the amplifier is very sensitive to the way it is grounded, a ground via (V1 in Figure 10) has also been added to the end of the source trace to model the effects of a non-ideal ground. Figure 11 shows the results of rerunning the simulation from the schematic representation of the “physical” circuit (Figure 10).

Figure 11 indicates that losses in the physical components have reduced the gain by just over 0.5 dB while the output match has degraded by almost 10 dB. However, the output match is still very respectable at 23 dB and the gain remains above our 13 dB goal. Stability factors K and ³ in Figure 12 report that the LNA is unconditionally stable over at least a 1.5 GHz band around the operating frequency.

Since the input match determines the amplifier’s noise figure, a simulation of just the input network was run to determine if the required source impedance of 49.8 + J121.8 (from Figure 3) has been preserved in the final circuit. Marker 3, at 2400 MHz in Figure 13, indicates that the source impedance remains at 50 + J121 ohms, thus preserving the original 0.7 dB noise figure.


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Performance Summary

Table 2 lists the design goals (synthesis objectives) and the stimulation results of the final circuit. All objectives have been met after replacing the ideal components with the microstrip lines based on the physical descriptions and taking into account the board and strip losses.

Summary and Conclusions

This article demonstrates how synthesis and simulation can be used together to speed up the design process. LINC2 enhances the efficiency of this process by providing a set of synthesis and analysis tools (including simulation) from within a common design environment. The design flows smoothly from synthesis to verification because all the data input, calculations, and displayed output are performed within a single integrated program. The entire LNA design project presented here, for example, was performed in only a matter of minutes by the LINC2 computer program.

LINC2 is a high performance, low cost, RF and microwave design solution from Applied Computational Sciences, Escondidi, CA. More information on LINC2 can be found on the Web at