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Some basic questions abut signal generators (signal sources)

Question asked by wangyiliu99 on May 29, 2013
Latest reply on Jun 26, 2013 by tabbott

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Hello, all

this is my first time to post thread at this board, I really look forward to your help.

As for power amplifier, there is the 1dB-compression point specification, which defines where the PA saturates. It is recommended to use the PA in its linear region.

My question is, does signal generator has similiar parameters? I noticed that for our current available model, when the output level is close to its upper limit, unleveled message occurs.

In other words, for the generation of same signal at frequency f=400MHz, output power P=15dBm. Which of the following would be preferred?

(a) SG 400MHz, output=15dBm
(b) SG 400MHz, output=-25dBm, plus a 40dB power amplifier

I guess it would be strongly dependent on the PA specifications. What specs. would be most important when viewing the PA's spec. sheet? If no spec. sheet is available, are there any simple method to check the signal quality using the spectrum analyzer?

Sorry that my question is rather general and please forgive me for raising this naive question.

Your help would be highly appreciated!

Yours,
Yiliu

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wangyiliu99 Sep 24, 2013 3:15 AM

Hi, all,

I got some help from the Agilent application note 5980-3079EN.

Here the related page is attached.

Still I have further question, it is mentioned that "Class A Amplifiers at the output of the sources" (in the middel column of that page) is helpful in power combination.

Usually for signal generators, PA used would be which type? Class A or others? Even if it is class A, the ALC circuit is included, so adding a class A PA(without ALC) would be preferred?

Thank you for your great help!

Yours,
Yiliu

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ReduceSourceDistortion.JPG134.2 KB

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wangyiliu99 Sep 24, 2013 3:15 AM

Any comments or advice is welcome,

thank you very much

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wangyiliu99 @ wangyiliu99 on Sep 24, 2013 3:15 AM

still waiting for help.

thanks alot

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tabbott

@ wangyiliu99 on Jun 18, 2013 9:27 AM

Hi Yiliu -

Basically all amplifiers – including those found in signal generators - will have a non-linear portion in the upper (compression) range. Unleveled conditions occur when the automatic leveling loop cannot successfully keep the output leveled. This can happen if the output level setting is too high. If your unit is outputting a non-modulated signal and displaying unleveled, but not exceeding the specified power limit, then I would suspect that maybe there is malfunction somewhere inside.

From your example, it seems that you are looking at two methods in which to obtain a 15dBm output:
(a) SG 400MHz, output=15dBm
(b) SG 400MHz, output=-25dBm, plus a 40dB power amplifier

You are correct, the choice really depends on the performance that you are after.

If your signal generator has a power out maxinum of 15dBm, but the performance (spectral purity) is degraded sufficiently at high power, then you might want to go with an external amplifier – provided that the external amplifier performance meets your requirements.

Here are some specifications that might be important:
Frequency response
Input power requirement
Harmonic performance
IMD, etc…

These can all be checked with a spectrum analyzer.

Regards -

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wangyiliu99 @ tabbott on Sep 24, 2013 3:15 AM

thanks tabbott.

Can I summarize as the following:

For PA, as in the spec. sheet, there is the harmonics/overtone, P1dB.

For SG, the non-linear/distortion performance can be confirmed by the spectrum analyzer.

BR,
Yiliu

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tabbott

@ wangyiliu99 on Jun 25, 2013 8:52 AM

Hi Yiliu -

I’m not quite sure what you mean by “harmonics/overtone”.

But in summary non-linear performance can be confirmed by the spectrum analyzer in both cases (PA or SG).

You are correct in that for PA’s, P1dB can usually be found in the data sheet.

For the SG it gets a bit more complicated as most SG’s will have an ALC circuit that tries to ensure linear operation over the specified range.

In order to confirm the P1dB point you would need to disable the ALC and adjust the power toward maximum.

Regards -

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wangyiliu99 @ tabbott on Sep 24, 2013 3:15 AM

Hi, tabbott,

I am sorry for the typo.

It should be harmonics, not harmonics/overtones.

FYI, attached is one PA spec. sheet, which is by R&K

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Amplifier_A00110-4040-R.pdf151.0 KB

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tabbott

@ wangyiliu99 on Jun 26, 2013 1:31 PM

Hi Yiliu -

Thanks for clearing this up. Performance specifications for the SG can also be found in the data sheets. For example power accuracy, harmonic performance for the E8257D can be found in the following data sheet:
http://cp.literature.agilent.com/litweb/pdf/5989-0698EN.pdf

In addition, our service documentation provides procedures to verify (using spectrum analyzers, etc…) the specified performance.

Regards -

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8 Replies

wangyiliu99 Sep 24, 2013 3:15 AM
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Hi, all,

I got some help from the Agilent application note 5980-3079EN.

Here the related page is attached.

Still I have further question, it is mentioned that "Class A Amplifiers at the output of the sources" (in the middel column of that page) is helpful in power combination.

Usually for signal generators, PA used would be which type? Class A or others? Even if it is class A, the ALC circuit is included, so adding a class A PA(without ALC) would be preferred?

Thank you for your great help!

Yours,
Yiliu
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- ReduceSourceDistortion.JPG134.2 KB
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wangyiliu99 Sep 24, 2013 3:15 AM
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Any comments or advice is welcome,

thank you very much
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- wangyiliu99 @ wangyiliu99 on Sep 24, 2013 3:15 AM
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  still waiting for help.
  
  thanks alot
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  - tabbott @ wangyiliu99 on Jun 18, 2013 9:27 AM
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    Hi Yiliu -
    
    Basically all amplifiers – including those found in signal generators - will have a non-linear portion in the upper (compression) range. Unleveled conditions occur when the automatic leveling loop cannot successfully keep the output leveled. This can happen if the output level setting is too high. If your unit is outputting a non-modulated signal and displaying unleveled, but not exceeding the specified power limit, then I would suspect that maybe there is malfunction somewhere inside.
    
    From your example, it seems that you are looking at two methods in which to obtain a 15dBm output:
    (a) SG 400MHz, output=15dBm
    (b) SG 400MHz, output=-25dBm, plus a 40dB power amplifier
    
    You are correct, the choice really depends on the performance that you are after.
    
    If your signal generator has a power out maxinum of 15dBm, but the performance (spectral purity) is degraded sufficiently at high power, then you might want to go with an external amplifier – provided that the external amplifier performance meets your requirements.
    
    Here are some specifications that might be important:
    Frequency response
    Input power requirement
    Harmonic performance
    IMD, etc…
    
    These can all be checked with a spectrum analyzer.
    
    Regards -
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    - wangyiliu99 @ tabbott on Sep 24, 2013 3:15 AM
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      Correct Answer
      thanks tabbott.
      
      Can I summarize as the following:
      
      For PA, as in the spec. sheet, there is the harmonics/overtone, P1dB.
      
      For SG, the non-linear/distortion performance can be confirmed by the spectrum analyzer.
      
      BR,
      Yiliu
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      - tabbott @ wangyiliu99 on Jun 25, 2013 8:52 AM
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        Correct Answer
        Hi Yiliu -
        
        I’m not quite sure what you mean by “harmonics/overtone”.
        
        But in summary non-linear performance can be confirmed by the spectrum analyzer in both cases (PA or SG).
        
        You are correct in that for PA’s, P1dB can usually be found in the data sheet.
        
        For the SG it gets a bit more complicated as most SG’s will have an ALC circuit that tries to ensure linear operation over the specified range.
        
        In order to confirm the P1dB point you would need to disable the ALC and adjust the power toward maximum.
        
        Regards -
        Like • Show 0 Likes0
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        wangyiliu99 @ tabbott on Sep 24, 2013 3:15 AM
        Mark Correct
        Correct Answer
        Hi, tabbott,
        
        I am sorry for the typo.
        
        It should be harmonics, not harmonics/overtones.
        
        FYI, attached is one PA spec. sheet, which is by R&K
        Attachments
        Amplifier_A00110-4040-R.pdf151.0 KB
        Like • Show 0 Likes0
        Actions
        tabbott @ wangyiliu99 on Jun 26, 2013 1:31 PM
        Mark Correct
        Correct Answer
        Hi Yiliu -
        
        Thanks for clearing this up. Performance specifications for the SG can also be found in the data sheets. For example power accuracy, harmonic performance for the E8257D can be found in the following data sheet:
        http://cp.literature.agilent.com/litweb/pdf/5989-0698EN.pdf
        
        In addition, our service documentation provides procedures to verify (using spectrum analyzers, etc…) the specified performance.
        
        Regards -
        Like • Show 0 Likes0
        Actions

Some basic questions abut signal generators (signal sources)

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