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Jul 11.




                      
                      
                      
                     Comparing Spectral Purity in 
                     Microwave Signal Generators 
                      
                      
                      
                      
                      

                                                                             Written by: Leonard Dickstein
                                                                                                                 Marketing Manager 

                                                                                                       Giga‐tronics Incorporated 

                                                                                                                                    

                                                                                                          Published:  March 2010 

                                                                                                                        Revision:  A




                                         AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                               ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
                                                www.gigatronics.com | inquiries@gigatronics.com 




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
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                  A Giga-tronics White Paper          AN-GT115A



                  Introduction 

                  An ideal signal has no phase noise, harmonics nor spurious frequency components associated with it. 
                  Unfortunately the ideal signal does not exist in the real world, and these factors degrade the spectral 
                  purity of all real world signals to some degree. High‐performance microwave signal generators are 
                  designed to maximize spectral purity, with very low levels of phase noise, harmonics and spurs, while 
                  maintaining general purpose flexibility. The levels of spectral purity performance are one of the key 
                  differences between high‐performance, mid‐range and low‐end or entry‐level signal generators. This 
                  paper discusses the factors that make up spectral purity specifications to help you make more informed 
                  comparisons of microwave signal generator performance for test and measurement applications. 

                  Phase Noise 

                  Phase noise refers to short‐term, random fluctuations in the phase of the signal, and is usually expressed 
                  as value normalized to a 1 Hz bandwidth and at some offset frequency away from the signal frequency 
                  and relative to the amplitude of the signal. It can be expressed as a number (such as ‐100 dBc/Hz at 10 
                  GHz and 10 kHz offset), but is also shown as a graph of phase noise versus offset frequency for one or 
                  more signal frequencies. While the actual spectral density is double sided around the signal or carrier 
                  frequency, as you would observe on a Spectrum Analyzer display, the phase noise of microwave signal 
                  generators is typically shown as a graph of only one side, called single side band (SSB) phase noise. See 
                  Figure 1. 




                                                                                                                   Figure 1. 

                  The vertical axis is the phase noise to carrier ratio in dBc/Hz where a smaller value is better. The 
                  horizontal axis is the offset from the carrier frequency in Hz where offsets from 1 Hz to 100 Hz away 
                                          AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                                   ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             2 
                                                      www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                 Making waves...
www.sematron.com
White Paper
Jul 11.




                  A Giga-tronics White Paper           AN-GT115A



                  from the carrier are referred to as “close‐in” and offsets from 100 kHz to 10 MHz away from the carrier 
                  are referred to as “far out”. Phase noise close in to the carrier presents a problem in applications where 
                  the signal is used as a local oscillator and could limit the receiver’s sensitivity or when the signal is used 
                  as a clock and could limit the bit error rate (BER) performance. Phase noise far out from the carrier 
                  presents a problem in wideband communications systems raising the noise floor and limiting the 
                  systems ability to recover weak signals. Close‐in phase noise is often critical for Aerospace and Defense 
                  applications such as high‐resolution radar, while the far‐out phase noise is often critical for broadband 
                  digital wireless communications. 

                  When comparing microwave signal generator phase noise performance, it is important to realize that 
                  datasheet specifications can be misleading when you are just comparing the phase noise numbers. For 
                  example, Figure 2 shows a comparison of measured data for three competing high‐performance 
                  microwave signal generators, taken at the same carrier frequency: 




                                                                                                                   Figure 2. 

                  Notice that the performance of the three different synthesizers overlap and that no one is significantly 
                  better everywhere. All three products show very good phase noise performance. It is important to note 
                  that while the 10 kHz offset from carrier is typically used for comparison, it may not be a critical 
                  difference for many applications, where either the close‐in or far‐out phase noise dominates. A signal 
                  generator manufacturer may optimize their phase noise performance for an impressive specification at 
                  10 kHz offset, while compromising the performance in both the close‐in and far‐out regions, as with 
                  “Synthesizer 2” in the example. Although all three companies in the example can claim the lowest phase 
                  noise at some point, what is most meaningful is to take into consideration the entire range and how it 
                  applies to your specific test scenarios.  


                                           AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                                    ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             3 
                                                       www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                    Making waves...
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                  A Giga-tronics White Paper           AN-GT115A



                  Some times you may see a vendor advertise frequency synthesizer phase noise at 100 kHz offset rather 
                  than 10 kHz offset just to make their phase noise performance appear better or they present phase 
                  noise without specifying the carrier frequency or the offset which is ambiguous and of dubious value. 

                  As indicated in Figure 1 with the five traces for the five different carrier frequencies, the phase noise in 
                  most microwave signal generators increases as the carrier frequency increases. This is attributed to the 
                  fact that almost all modern microwave signal generators use frequency multiplication to generate the 
                  higher frequency ranges, and the phase noise increases as 20 log N where N is the multiplication factor. 
                  What this means is that one signal generator that multiplies less than another to provide the same final 
                  frequency will probably have lower phase noise at the higher frequency. It is important to note that 
                  while 10 GHz (and 10 kHz offset) is typically used for comparison for microwave signal generators, better 
                  performance at 10 GHz does not necessarily mean better performance at 40 GHz or 50 GHz, because of 
                  possible differences in multiplication factors.  

                  The majority of microwave signal generators used for test and measurement applications are designed 
                  around YIG‐based microwave oscillators, rather than VCO based. There are several reasons for this fact 
                  and phase noise performance is one of the key reasons. First, YIG oscillators have a very wide tuning 
                  range. This means that the multiplication factors can be smaller, resulting in lower phase noise at the 
                  higher frequencies. And second, the phase noise of YIG‐based oscillators drops off more rapidly far out 
                  from carrier than it does in VCOs, so the total integrated phase noise with YIG‐based oscillators tends to 
                  be lower.  

                  Phase noise is a function of the internal oscillator and any external frequency reference used. Different 
                  microwave signal generators use different loop bandwidths when locking to an external frequency 
                  reference. A narrow bandwidth on the external frequency reference input can help minimize the effects 
                  of phase noise resulting from a noisy external frequency reference signal. A few companies use a wider 
                  bandwidth to allow more stable phase tracking of multiple signal generators, but that makes them more 
                  susceptible to phase noise from the external frequency reference. Other companies overcome this 
                  trade‐off limitation by providing the capability to phase track using an internal 100 MHz oscillator with 
                  wide loop bandwidth while keeping a narrow bandwidth on the 10 MHz external frequency reference 
                  input. 

                  Occasionally, in a microwave signal generator datasheet, you will find that phase noise plots are 
                  provided for the absolute phase noise (the phase noise performance of the signal generator) and 
                  residual phase noise (the phase noise performance with the contribution from the internal frequency 
                  reference removed). This residual phase noise information is only useful when an external frequency 
                  reference is used, and is added to the phase noise of the external frequency reference. The residual 
                  phase noise information is needed because of that specific synthesizer’s sensitivity to the phase noise of 
                  the external frequency reference, and the fact that its phase noise performance is potentially degraded 
                  by using a noisy external frequency reference signal. Typically the differences between absolute and 
                  residual show up only in the close‐in phase noise region. 

                                           AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                                    ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             4 
                                                       www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                 Making waves...
www.sematron.com
White Paper
Jul 11.




                  A Giga-tronics White Paper           AN-GT115A



                  Harmonics and Spurious 

                  When comparing harmonics and spurious performance note that all frequency synthesizers create 
                  harmonic and spurious signals. Depending on your application, these harmonics and spurious signals 
                  may or may not be a problem. It is important to note that different applications may be sensitive to 
                  spurious at different amplitudes and frequencies, and while one synthesizer may have a problematic 
                  spur in one specific application and another synthesizer does not, the opposite will be true in a different 
                  application or in even the same application at different frequencies. It is incorrect to generalize that any 
                  of the top performing microwave signal generators are consistently better or worse than any other, 
                  when it comes to harmonic and spurious performance. 

                  A microwave signal generator can meet and exceed its spurious specification and still exhibit a 
                  problematic spur if it just happens to fall in a sensitive region in any specific application. But with that 
                  caveat, it is true that the better the spurious specs, the less likely that you will encounter this issue. 
                  Microwave signal generators with poor spurious specifications are probably unusable for some 
                  applications, such as testing radar and ECM receivers and EW systems where the high level of spurious 
                  will appear as ghost (false) signals or threats. In wireless communications systems, high levels of 
                  spurious can be a problem when they fall inside the channel. 




                                                                                                                    Figure 3. 

                  It’s often difficult to understand spurious performance, because the spurious content is usually removed 
                  from the graphs of phase noise shown in most datasheets. This is not done to be devious, but rather to 
                  separate and distinguish the different phenomena. Figures 1 and 2 show phase noise with the spurs 
                  removed and for phase noise comparison purposes, that is useful.  


                                           AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                                    ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             5 
                                                       www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                         Making waves...
www.sematron.com
White Paper
Jul 11.




                  A Giga-tronics White Paper          AN-GT115A



                  If you visualize Figure 3 as if it were displayed on a spectrum analyzer, then you would recognize the 
                  spurs as sidebands around the carrier. In Figure 3, the phase noise is shown in blue, while the spurious 
                  signals are shown in red. If the spurious performance specification was ‐50 dBc or better, then that 
                  instrument at that frequency would be meeting its specification. The highest spur appears to be 
                  occurring at 120 Hz off the carrier, probably an artifact from the power supply. But the next largest spur 
                  appears to be 200 Hz off the carrier, and it may be very difficult or impossible to determine its origin. 




                                                                                                                  Figure 4. 

                  Figure 4 shows a set of phase noise measurements at 40 GHz. The phase noise is quite consistent, but 
                  the spurious signals vary. Figure 4 illustrates one of the most annoying properties of spurious signals and 
                  that is the fact that spurious signals vary as a function of both manufacturing variations and the external 
                  environment, and may also vary with the output power level and other settings. Slight variations on 
                  gasket sealing or device parameters will influence the level of spurious signals, and many spurious 
                  signals are a result of external electro‐magnetic interference, both conducted and radiated. Factors like 
                  heavy I/O bus activity may also have an effect, as noise can couple from the I/O bus into the analog and 
                  microwave hardware, so minimizing remote control communications during sensitive testing is a best 
                  practice.  

                  While spurious signals in microwave signal generators are not always a function of output power level, 
                  harmonics are. When comparing harmonic specifications, be sure to check what output power level 
                  applies for the specifications. Note that for most microwave signal generators with a high power option, 
                  the harmonic levels increase, sometimes substantially, with the addition of the option. There is a strong 
                  argument for using external microwave power amplifiers rather than high power options as increased 
                  harmonics is only one drawback of the microwave signal generator high power option. See Giga‐tronics’ 
                  application note AN‐GT111A “Boosting RF Output Power – Signal Generator High Power Options versus 
                  External Power Amplifiers” for a more detailed discussion of the advantages of using external microwave 
                  power amplifiers. 


                                          AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                   

                                                   ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             6 
                                                      www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                        Making waves...
www.sematron.com
White Paper
Jul 11.




                  A Giga-tronics White Paper          AN-GT115A



                  Summary 

                  Spectral purity is one of the most important specifications to consider when selecting a microwave 
                  signal generator and is one of the more difficult set of specifications to fully understand. When 
                  comparing phase noise performance, it is important to realize that datasheet specifications can be 
                  misleading when you are just comparing the numeric values and that you also need to look at the phase 
                  noise plots and consider how they would apply to your specific application. In addition, harmonics and 
                  spurious signals are problematic, and that while one microwave signal generator may have a spurious 
                  signal in any specific measurement scenario and another microwave signal generator does not, the 
                  opposite will be true in a different application or even in the same application at a different setting. 
                  Lastly, be aware that the harmonics specifications may degrade significantly with the addition of a high 
                  output power option, and the use of external power amplifiers is often a better solution when higher 
                  output power levels are required. 




                                          AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators 
                                                   ©2010 Giga‐tronics Incorporated.  All Rights Reserved. 
             7 
                                                      www.gigatronics.com | inquiries@gigatronics.com 
                   




Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com                                                Making waves...
www.sematron.com

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Giga-tronics - Signal Purity in Microwave Signal Generators

  • 1. White Paper Jul 11.       Comparing Spectral Purity in  Microwave Signal Generators            Written by: Leonard Dickstein Marketing Manager  Giga‐tronics Incorporated    Published:  March 2010  Revision:  A AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  www.gigatronics.com | inquiries@gigatronics.com  Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 2. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A Introduction  An ideal signal has no phase noise, harmonics nor spurious frequency components associated with it.  Unfortunately the ideal signal does not exist in the real world, and these factors degrade the spectral  purity of all real world signals to some degree. High‐performance microwave signal generators are  designed to maximize spectral purity, with very low levels of phase noise, harmonics and spurs, while  maintaining general purpose flexibility. The levels of spectral purity performance are one of the key  differences between high‐performance, mid‐range and low‐end or entry‐level signal generators. This  paper discusses the factors that make up spectral purity specifications to help you make more informed  comparisons of microwave signal generator performance for test and measurement applications.  Phase Noise  Phase noise refers to short‐term, random fluctuations in the phase of the signal, and is usually expressed  as value normalized to a 1 Hz bandwidth and at some offset frequency away from the signal frequency  and relative to the amplitude of the signal. It can be expressed as a number (such as ‐100 dBc/Hz at 10  GHz and 10 kHz offset), but is also shown as a graph of phase noise versus offset frequency for one or  more signal frequencies. While the actual spectral density is double sided around the signal or carrier  frequency, as you would observe on a Spectrum Analyzer display, the phase noise of microwave signal  generators is typically shown as a graph of only one side, called single side band (SSB) phase noise. See  Figure 1.              Figure 1.  The vertical axis is the phase noise to carrier ratio in dBc/Hz where a smaller value is better. The  horizontal axis is the offset from the carrier frequency in Hz where offsets from 1 Hz to 100 Hz away  AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  2      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 3. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A from the carrier are referred to as “close‐in” and offsets from 100 kHz to 10 MHz away from the carrier  are referred to as “far out”. Phase noise close in to the carrier presents a problem in applications where  the signal is used as a local oscillator and could limit the receiver’s sensitivity or when the signal is used  as a clock and could limit the bit error rate (BER) performance. Phase noise far out from the carrier  presents a problem in wideband communications systems raising the noise floor and limiting the  systems ability to recover weak signals. Close‐in phase noise is often critical for Aerospace and Defense  applications such as high‐resolution radar, while the far‐out phase noise is often critical for broadband  digital wireless communications.  When comparing microwave signal generator phase noise performance, it is important to realize that  datasheet specifications can be misleading when you are just comparing the phase noise numbers. For  example, Figure 2 shows a comparison of measured data for three competing high‐performance  microwave signal generators, taken at the same carrier frequency:                     Figure 2.  Notice that the performance of the three different synthesizers overlap and that no one is significantly  better everywhere. All three products show very good phase noise performance. It is important to note  that while the 10 kHz offset from carrier is typically used for comparison, it may not be a critical  difference for many applications, where either the close‐in or far‐out phase noise dominates. A signal  generator manufacturer may optimize their phase noise performance for an impressive specification at  10 kHz offset, while compromising the performance in both the close‐in and far‐out regions, as with  “Synthesizer 2” in the example. Although all three companies in the example can claim the lowest phase  noise at some point, what is most meaningful is to take into consideration the entire range and how it  applies to your specific test scenarios.   AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  3      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 4. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A Some times you may see a vendor advertise frequency synthesizer phase noise at 100 kHz offset rather  than 10 kHz offset just to make their phase noise performance appear better or they present phase  noise without specifying the carrier frequency or the offset which is ambiguous and of dubious value.  As indicated in Figure 1 with the five traces for the five different carrier frequencies, the phase noise in  most microwave signal generators increases as the carrier frequency increases. This is attributed to the  fact that almost all modern microwave signal generators use frequency multiplication to generate the  higher frequency ranges, and the phase noise increases as 20 log N where N is the multiplication factor.  What this means is that one signal generator that multiplies less than another to provide the same final  frequency will probably have lower phase noise at the higher frequency. It is important to note that  while 10 GHz (and 10 kHz offset) is typically used for comparison for microwave signal generators, better  performance at 10 GHz does not necessarily mean better performance at 40 GHz or 50 GHz, because of  possible differences in multiplication factors.   The majority of microwave signal generators used for test and measurement applications are designed  around YIG‐based microwave oscillators, rather than VCO based. There are several reasons for this fact  and phase noise performance is one of the key reasons. First, YIG oscillators have a very wide tuning  range. This means that the multiplication factors can be smaller, resulting in lower phase noise at the  higher frequencies. And second, the phase noise of YIG‐based oscillators drops off more rapidly far out  from carrier than it does in VCOs, so the total integrated phase noise with YIG‐based oscillators tends to  be lower.   Phase noise is a function of the internal oscillator and any external frequency reference used. Different  microwave signal generators use different loop bandwidths when locking to an external frequency  reference. A narrow bandwidth on the external frequency reference input can help minimize the effects  of phase noise resulting from a noisy external frequency reference signal. A few companies use a wider  bandwidth to allow more stable phase tracking of multiple signal generators, but that makes them more  susceptible to phase noise from the external frequency reference. Other companies overcome this  trade‐off limitation by providing the capability to phase track using an internal 100 MHz oscillator with  wide loop bandwidth while keeping a narrow bandwidth on the 10 MHz external frequency reference  input.  Occasionally, in a microwave signal generator datasheet, you will find that phase noise plots are  provided for the absolute phase noise (the phase noise performance of the signal generator) and  residual phase noise (the phase noise performance with the contribution from the internal frequency  reference removed). This residual phase noise information is only useful when an external frequency  reference is used, and is added to the phase noise of the external frequency reference. The residual  phase noise information is needed because of that specific synthesizer’s sensitivity to the phase noise of  the external frequency reference, and the fact that its phase noise performance is potentially degraded  by using a noisy external frequency reference signal. Typically the differences between absolute and  residual show up only in the close‐in phase noise region.  AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  4      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 5. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A Harmonics and Spurious  When comparing harmonics and spurious performance note that all frequency synthesizers create  harmonic and spurious signals. Depending on your application, these harmonics and spurious signals  may or may not be a problem. It is important to note that different applications may be sensitive to  spurious at different amplitudes and frequencies, and while one synthesizer may have a problematic  spur in one specific application and another synthesizer does not, the opposite will be true in a different  application or in even the same application at different frequencies. It is incorrect to generalize that any  of the top performing microwave signal generators are consistently better or worse than any other,  when it comes to harmonic and spurious performance.  A microwave signal generator can meet and exceed its spurious specification and still exhibit a  problematic spur if it just happens to fall in a sensitive region in any specific application. But with that  caveat, it is true that the better the spurious specs, the less likely that you will encounter this issue.  Microwave signal generators with poor spurious specifications are probably unusable for some  applications, such as testing radar and ECM receivers and EW systems where the high level of spurious  will appear as ghost (false) signals or threats. In wireless communications systems, high levels of  spurious can be a problem when they fall inside the channel.                  Figure 3.  It’s often difficult to understand spurious performance, because the spurious content is usually removed  from the graphs of phase noise shown in most datasheets. This is not done to be devious, but rather to  separate and distinguish the different phenomena. Figures 1 and 2 show phase noise with the spurs  removed and for phase noise comparison purposes, that is useful.   AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  5      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 6. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A If you visualize Figure 3 as if it were displayed on a spectrum analyzer, then you would recognize the  spurs as sidebands around the carrier. In Figure 3, the phase noise is shown in blue, while the spurious  signals are shown in red. If the spurious performance specification was ‐50 dBc or better, then that  instrument at that frequency would be meeting its specification. The highest spur appears to be  occurring at 120 Hz off the carrier, probably an artifact from the power supply. But the next largest spur  appears to be 200 Hz off the carrier, and it may be very difficult or impossible to determine its origin.                  Figure 4.  Figure 4 shows a set of phase noise measurements at 40 GHz. The phase noise is quite consistent, but  the spurious signals vary. Figure 4 illustrates one of the most annoying properties of spurious signals and  that is the fact that spurious signals vary as a function of both manufacturing variations and the external  environment, and may also vary with the output power level and other settings. Slight variations on  gasket sealing or device parameters will influence the level of spurious signals, and many spurious  signals are a result of external electro‐magnetic interference, both conducted and radiated. Factors like  heavy I/O bus activity may also have an effect, as noise can couple from the I/O bus into the analog and  microwave hardware, so minimizing remote control communications during sensitive testing is a best  practice.   While spurious signals in microwave signal generators are not always a function of output power level,  harmonics are. When comparing harmonic specifications, be sure to check what output power level  applies for the specifications. Note that for most microwave signal generators with a high power option,  the harmonic levels increase, sometimes substantially, with the addition of the option. There is a strong  argument for using external microwave power amplifiers rather than high power options as increased  harmonics is only one drawback of the microwave signal generator high power option. See Giga‐tronics’  application note AN‐GT111A “Boosting RF Output Power – Signal Generator High Power Options versus  External Power Amplifiers” for a more detailed discussion of the advantages of using external microwave  power amplifiers.  AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators    ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  6      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com
  • 7. White Paper Jul 11. A Giga-tronics White Paper AN-GT115A Summary  Spectral purity is one of the most important specifications to consider when selecting a microwave  signal generator and is one of the more difficult set of specifications to fully understand. When  comparing phase noise performance, it is important to realize that datasheet specifications can be  misleading when you are just comparing the numeric values and that you also need to look at the phase  noise plots and consider how they would apply to your specific application. In addition, harmonics and  spurious signals are problematic, and that while one microwave signal generator may have a spurious  signal in any specific measurement scenario and another microwave signal generator does not, the  opposite will be true in a different application or even in the same application at a different setting.  Lastly, be aware that the harmonics specifications may degrade significantly with the addition of a high  output power option, and the use of external power amplifiers is often a better solution when higher  output power levels are required.  AN‐GT115A – Comparing Spectral Purity in Microwave Signal Generators  ©2010 Giga‐tronics Incorporated.  All Rights Reserved.  7      www.gigatronics.com | inquiries@gigatronics.com    Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E sales@sematron.com Making waves... www.sematron.com