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Documentation:   Genesys 2009.04   >  Synthesis   >  WhatIF Frequency Planner

This document contains references to Agilent Technologies. Agilent's former Test and Measurement business has become Keysight Technologies. For more information, go to www.keysight.com.


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WhatIF Frequency Planner

A unique technique has been developed to reduce weeks and days spent doing spurious searches to hours and minutes. This technique is so powerful that spurious performance of all Intermediate Frequencies (IF) can be seen on a single graph. Spurious free regions are identified including multiple frequency band conversions to a common IF frequency. Users can see performance trade offs between IFs and can identify all spurious offenders giving them complete control over mixer requirements and specifications. This technique determines spurious responses and their amplitudes based on the characteristics of the mixers to be used, system frequencies and bandwidths, and desired IF bandwidths. Simulation speed is fast since conventional sweep analysis has been eliminated.

The WhatIF synthesis consists of the dialog box, schematic representation of the conversion process, and output graph.

Note

The schematic is not used in the simulation process. No analysis is run on the mixer(s) in the schematic. They are present only as a visualization of the selected conversion scheme. The mixer in the schematic is generic and its parameters do not correspond to values in WhatIF. Only the mixer orientation and number of mixers are used in the schematic.

WhatIF Walkthrough

As an example let's suppose that we have a dual band receiver operating in the 869 to 894 and 1930 to 1990 MHz bands.  A single IF frequency is desired to minimize cost of downstream components. An IF should be selected to minimize the potential for self inflicted interference. Let's also assume that spurs 100 dB below the desired IF output level is our definition of spurious free. Other important design criteria are IF bandwidth of 1.25 MHz and maximum RF input mixer drive level of -10 dBm with a mixer LO level of +7 dBm for the 800 MHz mixer and 0 dBm input drive level with a mixer LO level of +10 dBm for the 1900 MHz mixer.  It will also be assumed that a double balance mixer will be used to predict spurious amplitudes. Default values for the double balanced mixer is assumed.

DESIGN STEPS

  1. Create a new frequency planner synthesis by clicking the 'New' button. Find the 'Synthesis' submenu and then select the 'Add Frequency Planner' submenu item.
  2. Change the number of parallel mixers to 2.

  1. Click the 'Inputs' tab to specify the parameters for the parameters for our conversion scheme.
  2. Click 'Mixer1'.

  1. Select a Difference mixer with Low Side LO.
  2. Enter the RF center frequency of 881.5 MHz.
  3. Enter the RF bandwidth of 25 MHz (869 - 894 MHz).
  4. Enter the IF bandwidth of 1.25 MHz.
  5. Enter the Input drive level as -10 dBm.
  6. Enter the LO drive level of +7 dBm.
  7. Click 'Mixer2'.

  1. Select a Difference mixer with High Side LO.
  2. Enter the RF center frequency of 1960 MHz.
  3. Enter the RF bandwidth of 60 MHz (1990 - 1930 MHz).
  4. Enter the IF bandwidth of 1.25 MHz.
  5. Enter the Input drive level of 0 dBm.
  6. Enter the LO drive level of +10 dBm.
  7. Click the 'Apply' button since defaults for all remaining parameters will be used.

The figure shows results in an easy to interpret format. The performance of every valid IF frequency that meets the requirements for both RF bands appears on a single graph. Mouse fly-over text is used to identify each spur and spur free region. For the first time in the industry, IF frequency selection is optimized since performance trade offs are quickly made despite complications from multiband operation. Notice in Figure 1 that the half-IF spur (2x2 from 0 to 119.375 MHz) is easily identified on the left of the graph and well as all spurious free regions shaded in green.

To demonstrate the analysis capability of WhatIF a spur free IF frequency of 328 MHz will be selected and analyzed. This spur free region can be identified in preceding figure.

On the 'Settings' tab:

  • Select Single Intermediate Frequency
  • Set IF Center Frequency to 328 MHz

When this IF frequency is specified an LO for each RF band is created internally. This LO along with the specified RF bands will be analyzed using the appropriate equation (i.e. FIF = │m x FRF ± n x FLO │). The mixer output is as shown.

This display is equivalent to looking at the down converted single IF spectrum output of the dual band conversion scheme on a spectrum analyzer. If the RF and LO frequencies where varied across their ranges the respective spurs would trace the given ranges.

Notice that the valid IF region is also shown for convenience.

Frequency Planning Fundamentals

Frequency planning is one the first steps performed during the RF architecture design phase.  During this phase several frequency schemes are considered to ensure performance reliability internally as well as externally. External performance is typically controlled by a regulatory agency (i.e. Federal Communications Commission FCC or European Telecommunications Standards Institute ETSI) and the designer must comply with requirements before products are shipped and revenue collected. Spurious integrity for RF design is an age-old problem. Spurious responses, which occur naturally in the frequency conversion process, can render a particular design completely useless and non-compliant.

Spur Analysis Basics

The frequency conversion equation is:

FIF = │m x FRF ± n x FLO │ or FRF = │m x FIF ± n x FLO │

m = 0, 1, 2, ...

n = 0, 1, 2, ...

FIF = IF frequency

FRF = any frequency in the RF band

FLO = any frequency in the LO band

From this equation all spurious products can be determined. It's easy to analyze frequencies that fall in a given IF band. Determining the amplitude of these spurious products is more challenging. But, selecting the best IF frequency can be difficult at best because of the time needed to create and analyze the hosts of data sets representing all of the possible spurious combinations for a single IF frequency. The process is further complicated for multiple band operation where a common IF is desired for all bands. Also, locating an input IF frequency while trying to maintain spectral purity across a wide output band is troublesome.

Spur (Intermod) Tables

In an ideal world on the sum and difference product would come out of a mixer. However, in the real world all the products governed by the following equation come out of the mixer.

FIF = │m x FRF ± n x FLO │ or FRF = │m x FIF ± n x FLO │

m = 0, 1, 2, …
n = 0, 1, 2, …

FIF = IF frequency

FRF = any frequency in the RF band

FLO = any frequency in the LO band

One way to characterize the spurious performance of a mixer is to use a mixer spur table. This table shows the amplitude relationships of each harmonic combination of mixer input (RF/IF) and LO frequencies to the desired mixer output reference level.

The power of the spurious products on the mixer output is a strong function of the power levels of the RF and LO signals. A spur table example is shown below.

Table Characterization Parameters:

  • FRF = 500 MHz at -2 dBm
  • FLO = 470 MHz at 10 dBm
  • FIF = 30 MHz, measured to be -10 dBm

 

 

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The table contains the m harmonics of the RF and n harmonics of the LO. All values in the table are relative to the desired output and are expressed in dBc. The desired output is the 1 x 1 entry in the table should always be 0 since this is the reference point. Even though no negative signs are shown all values are assumed to be below the desired output level of the 1 x 1 product. Some vendors may show a '+' sign next to the number indicating the value is above the reference level. The first column contains harmonics of RF (n = 0) and the first row contains harmonics of the LO (m = 0).

For example, the 1 x 3 product ( 1 x FRF + 3 x FLO ) would occur at two frequencies: the sum at 1910 MHz and the difference at 910 MHz. The 1 x 3 entry in the table is 12. This means that the absolute power level of these two frequencies is 12 dB below the desired output at 30 MHz. Since the table was characterized with an output at -10 dBm the spurious level of the 1 x 3 would be at -22 dBm.

The following figures show the setup use to measure the spur table shown above and the results for some of the lower orders.

Note

Theoretically, the 'm x FRF' or 'm x FIF' products will decrease (m-1) dB for each dB the input power (RF or IF) is decreased below the RF (IF) characterization level of the spur table.

The following table shows how the relative power changes at the mixer output when the input power is dropped by 1 dB.

 

 

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For example, all the 2 x FRF +/- n x FLO products shown in the prior graph will drop their relative values by 1 dB. All the 3 x FRF +/- n x FLO products will drop by 2 dB.

The following figure shows the output spectrum of the same mixer used in the prior graph when the input power level has dropped by 1 dB.

Remember

The desired about of the mixer also dropped by 1 dB. The new reference level must be used to determine spur table entries.

Harmonics of the LO are independent of the RF (IF) drive level. These spurious products will not change as the RF (IF) drive level changes.

Spur Table Limitations

Spur tables are limited in the following ways:

  • They do not distinguish between sum or difference frequencies. The frequency response is assumed to be flat and the part perfect so the amplitudes are the same regardless of whether the output was a sum or a difference.
  • They are only valid under the conditions the table was characterized under. Compression effects are only considered at the characterization state. If mixer input drive changes it is assumed the spur tables scales accordingly.

Traditional Approaches

Traditional tools include spreadsheets, spur charts, and many other types of custom tools.

The following weaknesses of these tools are readily apparent:

  1. Interpretation of the results can be complicated and confusing
  2. Assume the LO band is independent of the RF band
  3. Don't account for IF bandwidth
  4. Don't account for spurious amplitude

Interpretation of Results

Spurious searches typically involve the interpretation of large amounts of data.  Charting techniques typically require normalized data and become very complicated when bandwidths are taken into account. Custom tools are difficult to use for engineering groups since many RF engineers have their own sets of custom tools they like to use. This makes it very difficult to pass the design from one engineer to another.

Dependence of LO and RF Bandwidths

This is one of the most glaring problems with spur search tools. In order to truly determine the performance of a given IF the LO band and RF band are always dependent and cannot be separated. The relationship between the RF, LO, and IF bands are as follows:

FRFBW = FLOBW + FIFBW

As seen the LO bandwidth must always be smaller than the RF bandwidth by the bandwidth of the IF. Violation of this rule falsely predicts spurs appearing across wider frequency ranges. True characterization of every IF frequency can only be obtained by preserving this bandwidth relationship for every case. However, this becomes tedious and time consuming with traditional approaches and is generally ignored.

Don't Account for IF Bandwidth

IF bandwidths are constantly increasingly causing additional difficulties during IF selection. As IF frequencies become wider the chances increase that a spur will fall in-band. As the IF bandwidth increases, the required LO bandwidth decreases yielding spurious combination's that cover smaller frequency ranges. The designer must account for these differences to minimize design time and component over or under specification.

Don't Account for Spurious Amplitude

Most spur searches are tedious and time consuming. Accounting for amplitude is yet another layer of complexity. Depending on the spurious combination and mixer input drive level, legitimate IF frequencies can be selected even though spurs may appear in-band if their amplitude is low enough. Knowing spurious amplitudes is very helpful when making trade-offs during the frequency planning process.

WhatIF

WhatIF is unique because it is a synthesis technique that accounts for the bandwidth of the RF, LO, and IF signals. This synthesis technique is fast and shows the entire frequency performance for every possible IF on a single easy to read graph.

Why it is Different

WhatIF Setup

Configuration Equations

Mixer Configuration Equations

IF at the mixer output

Difference, Low Side LO: IF = RF - LO.

Difference, High Side LO: IF = LO - RF.

Sum: IF = RF + LO.

IF at the mixer input

Difference, IF < LO: RF = IF - LO.

Difference, LO > IF: RF = LO - IF.

Sum: RF = IF + LO.

Mixer Drive Levels

Mixer input (RF/IF) drive levels are used to determine the amplitude of the spurious responses. These levels have no effect on the widths (frequency ranges) of the spurious bands that are generated by WhatIF. The widths of these spurious bands are controlled by the RF Center Frequency, RF Bandwidth, IF Bandwidth, and mixer configuration (sum, difference low side LO, or difference high side LO). The LO signal is used to turn the mixer on and off. The mixer input (RF/IF) is mixed with its harmonics with the harmonics of the LO as shown by the following equation.

FIF = │m x FRF ± n x FLO │ or FRF = │m x FIF ± n x FLO │

m = 0, 1, 2, …
n = 0, 1, 2, …

FIF = IF frequency

FRF = any frequency in the RF band

FLO = any frequency in the LO band

Note

Theoretically, the 'm x FRF' or 'm x FIF' products will decrease (m-1) dB for each dB the input power (RF or IF) is decreased. Obviously, the lower the input drive level to the mixer the more linear it will be and the lower the spurious responses are.

How it works

WhatIF uses closed form equations to generate all spurious responses based on the setup.

Note

The schematic is not used in the simulation process. No analysis is run on the mixer(s) in the schematic. They are present only as a visualization of the selected conversion scheme. The mixer in the schematic is generic and its parameters do not correspond to values in WhatIF. Only the mixer orientation and number of mixers are used in the schematic.

WhatIF Output

Each frequency conversion (or mixer) is represented by a different color on the graph. The user can change these colors on the graphs properties page.

All Intermediate Frequencies - Spurious free and valid output regions across the given mixer dynamic range is also shown in their own color. Spurious free ranges are post processing artifacts of the spurious calculations. Regions with no spurious signals across the specified amplitude range are identified and shown as spur free regions in the graphs.

Single IF Frequency - The valid output region across the given mixer dynamic range is also shown in its own color. The graph shows a spectrum analyzer view showing the given mixer output frequency with its bandwidth along with any spurious frequencies that may appear in the output spectrum. One way to think of the spurious responses for this graph is that if the mixer input signal is swept across its input range and the LO is swept across its range the spurious signals would track the shown frequencies ranges. At the top of the graph a 1x1 spur can be seen. Remember, that the desired output will only occur at the single output frequency plus its bandwidth. However, if the input were to be swept across its entire range and the LO was swept across its entire range this would produce a 1x1 output, which covers a much larger range than the single IF frequency as shown on the graph.

Flyover Help - The user can place the mouse cursor over any object in the graph to get information about the given spurious band or region. For spurious free or valid IF regions the information is self explanatory. For spurious bands in the Performance of All IFs case the information is as follows. In the 1st line the given mixer is identified (this can also be determined by its trace color). Next the given spurious combination along with the corresponding input ports are given. In the following example the spurious band is -1 x RF and -2 x LO. The LO frequency is given on the 2nd line of the flyover help. The 3rd line contains the spurious band where this spurious combination would fall at any IF frequency selected in this range. The 4th line contains the amplitude of the spurious signal relative to the IF power output level of 0 dBc. In the following example any IF frequency between 281.542 and 306.125 MHz would have a -1x-2 spur at the given amplitude of -32.12 dBc.

Using the flyover help information the user can manually verify the analysis is giving the correct results. The flyover information must be used in conjunction with the mixer configuration to determine the spurious frequency. For example, given the above flyover help we can determine the IF frequency range by considering the mixer configuration for Mixer#1. Since this is a difference mixer with a low side LO then the IF equation is IF = RF - LO. Substituting in the flyover help information we would get an IF center frequency spurious of -1 x RF - ( -2 x LO ). Using actual values we get IF Center Frequency = -1 x 881.5 MHz (RF Center Frequency for Mixer#1) - ( -2 x 587.667 MHz ) = 293.834 MHz. This frequency is the center of the 281.542 and 306.125 MHz band. Once we account for the RF and IF bandwidths we can estimate the entire spur band. The exact calculations of the band ends are more complicated that this is proprietary information.

Valid IF Frequencies

Valid IF frequencies are based on the mixer configuration equations. Valid IF frequencies are those frequencies whose equations will give a positive frequency result. For example, if we were trying to find an IF for a sum mixer with the IF at the input (RF = IF + LO) then by definition the IF frequency must be below the RF band. No valid IF frequencies could exist for a sum output where the IF frequency was greater than the RF frequency. If the user wants an IF frequency greater than the RF band for this configuration then a difference mixer must be selected. In the above example (RF = IF + LO) the maximum IF could be equal to the RF frequency if the LO was 0 Hz. The minimum IF frequency would be 0 Hz. Consequently, valid IF frequencies for this configuration would be between 0 Hz and the lowest RF frequency.

For difference mixer configurations where the IF is at the mixer output then IF frequencies will always be below the RF band. For difference mixer configurations where the IF is at the mixer input then valid IF frequencies will be greater than the RF band.

In the parallel mixer case then the intersection of valid frequencies for all mixers will be shown on the graph. For example, one mixer could have a sum configuration and another could have a difference configuration. Only the IF frequencies that satisfy both of these conditions will be shown on the graph. Frequencies outside the valid region will be marked as an Invalid Region. Frequencies in this region may satisfy one or more of the parallel mixers configurations but not all of them.

Degenerate Spurs

A degenerate spurious product is a spur that will be produced regardless of the LO frequency due to the relationship with the RF carrier frequency and the RF and IF bandwidths.

Degenerate spurious products occur under the following conditions for the specified configurations:

Intermediate Frequency at the Mixer Output ( RF In IF Out )

Spurious products will be created for any LO frequency when the following equation is true and the harmonic of the LO = 1 :

FRF <= |( |M| x BWRF + BWLO + BWIF ) / ( 2 ( 1 - |M| ) )|Where:

FRF - RF Center Frequency

BWRF - RF Bandwidth

BWLO - LO Bandwidth

BWIF - IF Bandwidth

M - Harmonic of the RF

The following figures depict a degenerate case:

If a 10 GHz IF is picked the 2 x 1 degenerate spur will appear in the IF band as shown below.

Intermediate Frequency at the Mixer Input ( IF in RF Out )

Spurious products will be created for any LO frequency when the following equation is true and the harmonic of the LO = harmonic of the IF (M=N) :

FRF <= |( BWRF + |M| x BWLO + |M| x BWIF ) / ( 2 ( 1 - |M| ) )|Where:

FRF - RF Center Frequency

BWRF - RF Bandwidth

 BWLO - LO Bandwidth

BWIF - IF Bandwidth

M - Harmonic of the IF

The following figures depict a degenerate case:

If a 10 GHz IF is picked the 2 x 2 degenerate spur will appear in the IF band as shown below.

Limitations

Dialog Box Reference

General

There Reposition, Undo, and Apply buttons that are available on every tab.

  • Reposition - The Reposition Button ()will cause the graph and schematic windows to be resized to fit within the Genesys environment.
  • Undo - The Undo Button () will revert WhatIF to the state when the 'Apply' button was last clicked.
  • Apply - The Apply Button () causes the spurious performance to be calculated and displayed.

Settings Tab

This tab is used to specify the type of spurious analysis that will be performed.

Name - Name of the frequency planner (WhatIF) that appears in the workspace tree.

Dataset - Name of the dataset where the WhatIF data will be stored when the apply button is clicked.

Intermediate Frequency at

Mixer Input - In this configuration WhatIF will determine a spurious combination of the mixer IF input and LO that will cause spurious signals to appear in the given RF band.

Mixer Output - In this configuration WhatIF will determine a spurious combination of the mixer RF input and LO that will cause spurious signals to appear in the IF band.

Examine Worst Case Behavior of

All Intermediate Frequencies - Shows graphically the performance of every valid IF for the given configuration. Each frequency conversion (or mixer) is represented by a different color on the graph.

Single IF Frequency - Specify an IF frequency that will be used to determine the exact LO frequencies for each mixer. Shows graphically output spectrum like what would be seen on a spectrum analyzer.

Number of Parallel Mixers - This is the number of RF bands that will be converted to / from a common IF. The schematic shows this representation when the 'Apply' button has been clicked. With the IF at the mixer input parallel mixers represent the numbers of RF bands which a simulcast will occur from a common IF. When the IF is at the mixer output then parallel mixers represent the number of RF bands being converted to a common IF.

Spurious

Maximum Order - Maximum order used for calculating spurious products

Amplitude Range - Only spurious products falling within this range of the desired 1x1 IF signal will be retained in the dataset and shown on the graph. The spurious free range is based on this amplitude range.

Apply - Clicking the button will cause the WhatIF frequency plan to be evaluated, the schematic to be updated, and the data to be saved in the dataset and displayed on the graph.

Undo - Clicking this button will undo all changes up to the last 'Apply'.

Factory Defaults - When clicked will reset all parameters on all tabs to their factory default values.

Inputs Tab

This tab is used to specify the characteristics of each RF band convert to or from an IF.  Drive levels can also be specified for each of the mixers used in the conversion process.

The number of parallel mixers is shown in the list at the left. All other information on this page is based on the selected mixer from this list. Each mixer or conversion block can have its own parameters. However, only valid outputs will be shown on the graph. It is possible that the users can configure multiple mixers in such a way to prohibit a common IF.
Desired Intermediate Frequency

The user can select either whether they want to find an IF for either a sum or difference configuration. A difference configuration can be achieved through either a High Side LO (LO frequency greater than the RF/IF frequency) or a Low Side LO (LO frequency lower than the RF/IF frequency). For the sum configuration the user can specify how high in frequency they want to view the results.

RF Center Frequency Center frequency of the RF band.

RF Bandwidth - Bandwidth of the RF band.

IF Bandwidth - Bandwidth of the IF frequency or IF filter. The IF frequency is generally the bandwidth of the modulated signal. However, since brick wall filters are very difficult to build, this bandwidth can be increased to account for filtering imperfections. During the analysis, when examining all IF frequencies any spurious signals outside this bandwidth are ignored since brick wall filtering is assumed.

Input Drive Level - This is the input drive level to the mixer and has no bearing on the width of the spurious bands. This level is used in conjunction with the LO drive and the 'Mixer Type' to determine the amplitude of the spurious responses.

LO Drive Level - This is the LO drive level to the mixer and has no bearing on the width of the spurious bands. This level is used in conjunction with the input drive and 'Mixer Type' to determine the amplitude of the spurious responses.

Note

The RF Center Frequency, RF Bandwidth, and IF Bandwidth determine the width of the spurious bands. Internally, during the simulation process an LO center frequency and bandwidth is determined that is used for spurious calculations. Even though the mixer Input and LO drive levels don't affect the width of the spurious bands they can affect the spurious free bands since spurious free regions are determined within the specified amplitude dynamic range. To compensate for spurious amplitude inaccuracies the user can either specify a larger amplitude range or use the intermod table to specify more accurate mixer amplitude characteristics.

Type Tab

This tab is used to specify the amplitude performance of each mixer. The user has the choice of either a double balanced mixer or a user defined intermod table.

The number of parallel mixers is shown in the list at the left. All other information on this page is based on the selected mixer from this list. Each mixer or conversion block can have its own amplitude specifications. There are two types to choose from.

Double Balance - Parameters for the double balanced model can be changed by clicking the 'Advanced' button. This model is based on the work of Bert Henderson at Watkins Johnson. The name of the application note discussing this is, "Predicting Intermodulation Suppression in Double-Balanced Mixers".

Note

The materials, text and company images in this application note are provided for engineers and the public.  Any commercial use of publication of them without authorization is strictly prohibited.  All materials are copyrighted and are not in the public domain.  Copying of materials from these pages is not permitted.

Intermod Table - This is the name and library of the table mixer. Click the ellipsis button ( ) to bring up the part selector. Select the 'Library' and 'Category' of the location of table mixers. At noted, this field uses actual mixer intermod table parts. Not just an numerically defined intermod table. Drive levels and directions through the mixer are read from this model. If the IF is at the mixer output then the RF data table will be used. If the IF is at the mixer input then the IF data table will be used. The corresponding drive levels for those tables will also be used.

Note

The default table mixer in Genesys can be used here.

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