Thank you Carmina. Welcome all to our presentation of Direct Filter Synthesis for Customized Response 1
This is just a brief review of our agenda, first we will review the Functions and types of filters and filter selection. The we will discuss conventional filter design and synthesis techniques. This is in preparation to discuss the advantages of direct filter synthesis over the conventional techniques. Then we will discuss the Sfilter Direct Filter Synthesis tool and follow this with the filter design flow within the Agilent Genesys integrated design platform using the MomentumGXF Electromagnetic simulator and the Spectrasys System Architecture design tool. Finally, we will summarize what we have learned today. 2
Filters are basic to all RF networks. The allow us to limit the signals and/or noise that Are Received And Transmitted. They help to suppress harmonics and reduce unwanted energy to keep from overdriving the amplifier. They Help To Select The Information That Will Be Processed And Signals That Are Developed and they Direct Frequency Dependant Energy such as in a Diplexer 3
Filters have a common function based upon the frequency at which they are required to operate. Different physical realizations are used based upon these frequencies. Passive filters made up of inductors and capacitors make up the bulk of filters up to about 1-2GHz. They are well understood and direct synthesis methods are available. The hybrid filter offers superior Q characteristics in the VHF-UHF range useable to 3GHz. Above 1GHz distributed filters make up the bulk of applications to near light frequencies. Active filters are supported by amplifiers which are limited to lower frequencies, however they provide very compact solutions at the low end of the spectrum. Finally, digital filtering techniques are widely used at baseband frequencies. They provide nearly perfect brick wall filtering in a very compact size and can be integrated into the basic signal processing chips that are used for decoding digital data. 4
Each of the physical types discussed in the previous slide offer tradeoffs between the considerations. For example, for high power transmitter applications; distributed, hybrid, or at low frequencies passive filters are producible for thousands of watts. At the same time, the cost increases due to the size and materials that are needed. 5
First, we are going to discuss the conventional filter synthesis process as opposed to the Direct filter synthesis process to be discussed later. Conventional LC Filter Design starts with lowpass filters normalized to 1 ohm and 1 radian cutoff and have series inductors in henries and shunt capacitors in farads. These values are referred to as prototype or g-values. This lowpass serves as a prototype for designing specific lowpass filters with other cutoff frequencies and termination resistances. These specific lowpass filters are easily found by simply scaling the g-values by the desired resistance and cutoff frequency. Bandpass filters are designed by transformation and scaling the lowpass prototype. With the conventional bandpass transform, each lowpass prototype series inductor transforms into a series inductor and a series capacitor. Each prototype shunt capacitor transforms into a shunt inductor and a shunt capacitor. Therefore, the bandpass has twice the number of reactive elements as the prototype. 6
Agilent Eesof has many different tools for conventional filter synthesis. Passive discrete synthesis, Distributed synthesis for transmission line and cavity filters and Active Op Amp Synthesis are available in the Agilent Genesys Suite of tools. Digital Filter Synthesis is available in the Agilent SystemVue Suite. 7
First we will examine the process for synthesizing a conventional LC filter. The procedure for the 3 conventional analog synthesis tools is almost the same. All have 5 tabs, Topology, Settings, Defaults or Options, G Values and Summary. Now we will explore the design process looking at each tab in the passive filter tool. 8
The first step in the conventional process is to select the type of Filter to be designed. Filter type specifies whether the filter is lowpass, highpass, bandpass or bandstop. Shape specifies elliptic or all-pole. All-pole filters have monotonically increasing attenuation in the stopbands with increasing frequency from the passband. All zeros of transmission occur at DC or infinite frequencies. The elliptic transfer function has one or more zeros at finite frequencies. For a given number of inductors, this type of filter has superior performance in some respects to all-pole filters. Selectivity is improved at the expense of ultimate attenuation and complexity. Each of these generic types may have alternate forms. For example, the lowpass type, can have a series inductor first (minimum capacitor) or a shunt capacitor first (minimum inductor). For each type, you can specify whether you want a single-ended or balanced (differential) topology. 9
Filter synthesizes over 20 types suitable for a wide range of applications and each topology can be synthesized as either single-ended or balanced. 10
The settings tab varies slightly depending on the selections on the topology tab, but basically it is where the cutoff frequencies, input and output impedances, order, attenuation and, for a chebychev filter, ripple are entered. This tab also has an Estimate Order button which allows the engineer to estimate the required filter order by entering attenuation at various frequencies in the stop band. Filter then displayes the required order to achieve this attenuation and the 3db frequencies. 11
The defaults tab allows the specification of a finite Q for the inductors and capacitors for a realistic insertion loss. The default Q's entered on the Defaults tab are remembered and used until they are changed again. FILTER initially assigns the same Q value (from the defaults) to all inductors and all capacitors. These can be changed later by entering the desired Q values directly into the components on the schematic. 12
FILTER directly computes the lowpass prototype values or "G values" for popular response shapes and does not use tables, so any passband ripple greater than zero and less than 3 db may be chosen. This is true even for elliptic filters. The G values calculated for the currently selected filter design are shown on the G Values tab. The Summary tab shows the summation of the G values of the filter. This quantity may be used to estimate the filter insertion loss and group delay of the filter at frequencies well removed from the cutoff. 13
As we have seen, the conventional filter synthesis technique involves fitting a preexisting schematic to the user s design criteria. Over the years, hundreds of designs have been calculated, and these designs can be frequency-and impedance-scaled to fit many different scenarios. This works well for many applications. However, this puts a limitation on the shape of the filter response to conventional shapes such as Butterworth, Chebyshev and Elliptic. Here we have a 3 rd order 70MHz IF Filter designed using the conventional technique. We will compare this to the direct synthesis technique. 14
The S/FILTER synthesis tool, included in Agilent Genesys, is the fifth synthesis tool. It uses a technique called direct synthesis that allows for nonconventional designs. Contrary to the look-up approach described previously, direct synthesis takes the user s design criteria, forms a purely mathematical representation a transfer function and extracts a schematic. More economic and effective filters can be designed by controlling the location of transmission zeros. What do we mean by transmission zeros? Transmission zeros occur at frequencies of infinite attenuation or no signal transmission. All-pole filters such as Butterworth and Chebyshev have transmission zeros: At infinite frequency for lowpass filters At DC for highpass filters At both DC and infinity for bandpass filters Elliptic filters have finite frequency zeros in the stopband 15
DC zeros can be represented by a series capacitor or a shunt inductor. The capacitor will block DC and the inductor will short DC to ground. A transmission zero at infinity is represented by a series inductor and a shunt capacitor. And finite frequency transmission zeros can be either series or shunt LC circuits. 16
Consider the 3 rd order filter in Figure A at the top right. At DC, figure B, the series inductors and the shunt capacitor have no effect they vanish. These are the three Transmission zeros at DC. At infinity, figure C, the series capacitors and the shunt inductor vanish, and there are three Transmission zeros at infinity. The conventional all-pole band-pass filter has an equal number of transmission zeros at DC and infinite frequency. The 3 rd order band-pass shown has 3 transmission zeros at DC and 3 at infinity. 17
The conventional approach usually works well unless an engineer needs a slightly different topology than the existing ones, such as a symmetric response or to notch-out an unwanted frequency. Increasing the order increases attenuation both above and below the pass-band. In the past, engineers would often take a preexisting design, add some customizing elements to it and launch a linear optimizer to recover the desired network response. This approach requires more of the engineer s time, and the engineer must often settle for a non-optimal response. 18
LC/Filter offers a richer set of topologies than most filter design programs, thus helping satisfy many design objectives. However, the procedures used by conventional synthesis tools do not find all available topologies. In fact there are 226 unique realizations for these six transmission zeros, and many more if the six zeros are selected differently such as 1 at DC, 3 at infinity and 2 at finite frequency. But how do you unlock these other topologies? 19
The S/FILTER program synthesizes all available topologies by extracting the zeros in every possible sequence. It also allows the user to specify DC, infinite and finite zeros completely independently. This has many benefits. It may improve component value ratios, Allows independent control of rejection above and below the passband, Allows placement of notches at desired frequency and Offers designs with unequal terminations. Now lets examine the direct synthesis process as contrasted to the conventional process earlier. 20
The first tab in S/Filter is the Start tab, however, it is not necessary to start your design with this tab. The previous design is automatically loaded when you open S/Filter and the engineer can start with the Specifications tab. The Start tab has four selections New Filter, Save Settings, Load Settings and Shape Wizard. The New Filter selection erases the previous design and no parameters are entered. All filter parameters are entered on the specifications tab. The second and third buttons are to save your existing filter or load a pre-existing filter. The Shape Wizard allows the engineer to start with a familiar filter such as a Butterworth, Chebyshev or Elliptic shape, in lowpass, highpass or bandpass responses. Again we will start with a 70MHz, 3 rd order, Chebychev filter for our example. 21
The Specifications tab is where the direct filter design begins. If you used the Filter Wizard on the Start tab, the filter parameters will be entered for you and the response will be shown. The top part of the Specifications tab is similar to the setting tab in the conventional synthesis tools. Source and Load can be entered separately. Then the passband requirements are entered. Select the type of filter, shape and the process first as this affects the other entry parameters. Capacitor and Inductor Q s can be non-ideal. Here we have specified a Cap Q of 1000 and inductor Q of 200. The last step is to tune the response by placing zeros in the stop band. This is where the direct synthesis technique differs from the conventional technique. Since we selected a third order filter in the wizard, S/Filter placed 3 zeros each at DC and Infinity. The engineer can now change the zeros at DC and infinity or add finite zeros, tuning a custom response to fit the exact requirements. 22
For example, Sfilter allows the engineer to place a different number of zeros at DC and Infinity. For example, if the number of transmission zeros at infinity is 3 times the number at DC, then the response can be symmetric equal rolloff above and below the passband. The conventional band-pass has too few zeros at infinity in relation to the number at DC and therefore too little selectivity above the pass-band to be symmetric. Here we have 6 zeros at infinity and 2 at DC resulting in a symmetric response in both S21 and group delay. 23
S/Filter allows placement of zeros at finite frequencies which can then be tuned to obtain the desired response. Finite zeros can be used to increase attenuation or to notch out spurs, interferers or other unwanted frequencies. As you can see, the S/Filter Specifications tab is used to directly synthesize the filter response to the exact requirements of the design. 24
A filter s transfer function can be represented as a ratio of sums of products, or as a ratio of products of sums. Using the products-of-sums representation, the equivalent filter network can be obtained by extracting one product at a time, where each successive product represents some combination of circuit elements. This results in multiple possible schematics with the exact same response. The previous design, shown here, results in 58 unique schematics. The schematics are placed in a table for easy sorting and selection of the optimum design. Here the table is sorted on the lowest inductor ratio to minimize inductor values and types. 25
The table is customizable for easy sorting and selection of the optimum design. Our table has four headers: number of caps, number of inductors, the inductor ratio and the permutation order or zero extraction order. Add or remove column headers and sort by selecting the header. Designs can be selected based on many criteria including the number of caps or inductors, the ratio of values or the maximum or minimum values. This allows the engineer to select the optimum design to minimize parts and ratios for cost and manufacturability. 26
The number of designs in the table can be further filtered by entering extraction goals. Limits can be placed on a number of design parameters. Here we are limiting inductance to less than 200 nh. 27
S/FILTER has an option to allow similar but inexact extractions. These filters often have fewer parts or more favorable topologies than the exact solutions have, and can be optimized or tuned to exhibit more exact behavior. In the example, the unique solutions went up from 58 total to 160 total solutions after selecting the Allow inexact button. In exact solutions are highlighted in red in the table. 28
S/FILTER includes more than 100 built-in circuit transformation tools to customize and/or further improve the realizability of filters. 29
S/FILTER circuit transformations include basic operations such as splitting elements., Lumped to distributed and distributed to lumped transforms for converting from discrete LC filters to microstrip, stripline and other distributed filters. Lumped and distributed processes or both can be selected and exact or approximate transforms or both can be selected. Approximate transforms are shown in red. A diagram of the transform before and after application is displayed at the bottom of the window. 30
Additional transforms are Compound operations such as finding the dual of a schematic and making all inductors or capacitors the same value. There are also Norton Transforms, Kuroda transforms. Coupled lines, Tlines and many more that enable the designer to achieve more economical and realizable filter designs. For more information on these transforms, a good starting reference is HF Filter Design and Computer Simulation By Randall W. Rhea which is included in pdf format with Genesys. 31
As an example of using a transform, we apply the Equate all L s transform to our previous design. Before the transform on the left, the two outer inductors are 298 nanohenries and the two inner inductors are 670 and 525 nanohenries which results in 3 inductor values. After applying the transform on the right, Sfilter automatically coverts the two inner inductors to the same value as the outer inductors by applying a series of norton transforms. This reduces the number of unique inductors to only 1 that must be designed, modeled, purchased, tested, stocked and picked for assembly. The resulting design uses two more capacitors, but the cost trade-offs may be acceptable. 32
Finally, the History tab keeps track of all the transforms applied. When you start down a path using transforms to modify a filter, you will need to occasionally back up a few steps. The history tab lets the designer refer back to a previous design. The history is also a way to automate applying a particular set of transforms for a filter type with different Cutoff frequency, Bandwidth, or other characteristic. 33
This completes the synthesis of the filter using Sfilter. The response has been tailored to the exact requirements. The best available schematic was selected and modified to reduce cost and improve manufacturability. The synthesis modules design circuits depending on your specifications. Automatically a schematic will be drawn and analysis results displayed. But... How accurate is this design representation? It may be accurate enough for many designs especially at low frequencies. However, we will now examine ways to improve the accuracy of our design using the other tools in the Agilent Genesys suite. After synthesis, the schematic is yours to modify, optimize, and to include within other designs. 34
The Genesys Integrated suite of tools includes many capabilities from system architecture design, to circuit design, layout generation, linear, non-linear circuit and electromagnetic simulation, test equipment interface and pcb import export. So far, we have only discussed the Synthesis block circled. We will now look at how we can improve our design using the other tools within Genesys. 35
Adding microstrip, stripline or other transmission lines and via holes to the schematic is easy in Genesys and will improve the accuracy of the circuit simulation. Substrate material information can be added manually or substrates can be picked from a library of substrate manufacturers including GE, Rogers and Taconic. 36
The final synthesized element values are often fractional sizes which are not readily available, therefore we use the Tune Window and set the variable to Standard and the desired component tolerance. Here we chose 5% tolerance. If we were to choose another standard value, then the incremental values for these components would change accordingly. Optimization might improve the response however the resolution of component values is dependent upon the tolerance used. 1% components would offer finer step sizes however they are much more costly. We were able to remain very close to the original symmetric response with 5% values. 37
Substituting vendor models, measured S-data, using the built in models in Genesys or creating custom models can improve the accuracy of the simulation. When using vendor or measured s-parameter models, the designer should ensure that the measurements were taken as used in the design. Substrate material and thickness, pad size and component orientation should match the actual design layout, otherwise, accuracy may suffer instead of improving! 38
Once a layout of our filter is created, pads are added for connecting our components. Pad capacitance and coupling between pads and interconnecting lines can add problems of their own. Depending on the component values, a few tenths of a picofarad may or may not influence the filter s response. The ability to view this effect is not possible without an accurate integrated EM engine. 39
Genesys MomentumGXF provides insight into the actual filter response including cover height and coupling between pads and traces as well as inherent parasitics. Momentum in Genesys has unique cosimulation capability that does not require placing ports on the layout for the lumped elements. As shown here in the 3D view, the ports are associated with the footprints and are automatically placed by Genesys. Nor does it require an EM file or look alike component to be placed in the schematic. The cosimulation is all done in the background without modifying the schematic or layout. Layout pads decreased the passband width by shifting the stop band frequency downwards by about 2MHz. This EM simulation provided us with a new design insight that layout pad parasitics will usually tend to decrease BPF bandwidth by lowering the upper stop band frequency. Our original response can be recovered by tuning our discrete parts to compensate for the layout effects. The ability to co-simulate our design with actual component values, enhanced models and capture parasitics and box effects using EM provides us a truly accurate simulation that removes any doubt that our filter will behave as predicted. 40
Before we commit our design to manufacturing we need to verify that a large percentage of the filters will pass test. To validate the design for manufacturing variables and tolerances, we employ statistical analysis provided by the Genesys environment. Monte Carlo and Yield Analysis are probabilistic ways to simulate component variations caused by the production process. These methods can be used to determine which components need to be low tolerance, usually more expensive components, or to help create designs that are able to accommodate parameter variation. Monte Carlo and Yield analysis can help reduce the final cost of the product. 41
Now that we have our filter designed to the specifications. How will our filter perform in our system? Spectrasys system Architecture tool is fully integrated into Genesys with S/Filter and Momentum. Typically, the System Architecture was created first and the filter parameters flow down from the system requirements. A behavior model is initially used to determine the system performance specifications. The filter design can be substituted for the system behavioral model to verify the system design still meets specs with the filter circuit in place. 42
Spectrasys has the capability to display full spectrums at any node in the circuit. Here the output of our system shows the 70MHz passband, but it also shows some leakage from our second LO. It shows the 175MHz signal at - 96dBm originating at LO3 and the path it took through the mixer, IF AMP, Switch 2, our BPF filter and Switch 3. The LO leakage may or may not be a problem, but at least Spectrasys made us aware of it and we may be able to save a board turn. 43
Spectrasys also includes more than 100 cascaded path measurements. These can be displayed in what we refer to as level diagrams. Any path, forward or reverse in the system can be displayed. The system path shown at the top left is displayed in this level diagram. The node numbers and the components in the path are shown across the bottom of the level diagram. We are displaying Channel Power and Noise Power on the left and Noise Figure on the right Y axis of this level diagram. 44
Path measurements can also be displayed in tabular format. The Genesys Instagraph feature allows the designer to right click on the end node of the path in the schematic to display the table or many other frequently used plots. This information can then be exported to a spreadsheet if desired. 45
So I would like to summarize what we have learned to day. With S/Filter, engineers are able to: Directly synthesize multiple solutions for LP, HP and BP filters Asymmetrically place DC and infinite zeros to increase high or low side selectivity or for response symmetry Place and tune finite zeros for increased selectivity where needed and to notch out unwanted frequencies Select and transform schematics to obtain the best possible design and improve manufacturability EM simulation combined with enhanced models will ensure first pass success Statistical and swept analysis ensures manufacturability And Integrated System simulation ensures your design will meet system specifications 46
Thank you for attending the Genesys direct filter synthesis seminar. Genesys offers the industry s widest coverage of eleven RF and microwave circuit synthesis capabilities. Test drive them in Genesys to personally experience how your design productivity can increase by orders of magnitude. The latest Genesys 2010 also comes with breakthrough X-parameters nonlinear simulation technology, 10x optimization speedup and significant reliability improvement. Just Google Agilent Genesys for more information or for your own evaluation. 47