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 Noise Figure of an upconverting mixer under input IF signal 

Last post Mon, Nov 25 2013 2:08 AM by Frank Wiedmann. 16 replies.
Started by bennys 29 Oct 2013 02:59 AM. Topic has 16 replies and 4762 views
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  • Tue, Oct 29 2013 2:59 AM

    • bennys
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    Noise Figure of an upconverting mixer under input IF signal Reply

    I'm trying to simulate the noise figure of a transmit mixer as a function of input IF power. 

    From the cadence tutorials it seems as if the HBnoise sim is performed under small signal conditions where the input IF port is set to DC

    (for example in http://www.cadence.com/Community/blogs/rf/archive/2011/03/10/tips-for-simulating-a-transmit-mixer-in-spectrerf.aspx, its stated "If you are running PAC or Pnoise, you set the IF sourcetype to dc so that the LO is the only sinusoid in the system (Edit Properties on your IF port in the schematic). Since the input for this example is sinusoidal, we will choose Harmonic Balance. " )

    How whould you recommend to perform a noise figure sim with IF signal applied.

     

    In addition when I look at the noise separation print form it shows that the noise of the mixer comes from a biasing circuit which is AC disconnected from the circuit..., this must be an artifact and can not be the real noise source of the mixer, not sure but maybe you have some input on this as well? 

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  • Sat, Nov 2 2013 1:13 PM

    • MicheleA
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Hi Bennys,

    just a couple of random thoughts. I guess that in a TX mixer the IF input is 'small signal' anyways, meaning that its amplitude should be well within the range of linear behavior of the IF port, as to avoid introducing distortion. Under this view, one can make full use of the NF concept, which is indeed a 'small signal' concept.

    You may still want to see whether the NF changes while the IF signal changes, but for this you could just analyze your circuit under different DC inputs. In the range foreseen for the IF signal, however, you should not see any appreciable difference...Well, I guess :)

    As for the noise contributions, I think it is entirely possible that the low frequency noise introduced by the bias circuitry gets upconverted and you find it in the contributors @TX frequency.

    I never tried them myself but for sure Spectre RF allows you to simulate multitone stimuli, either commensurate (HB) or not (QPSS), or also ENV, if you want to explore things further.

    My two cents,

    Michele 

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  • Sun, Nov 3 2013 1:20 AM

    • bennys
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Thanks for the reply Michele,

    I would also expect that the NF not be dependant on IF input power, but this is not the result I get from simulation (the NF makes sence at low input level but then gets unrealisticly high as the input power is raised).

    There is an interest in verifying at different IF power level as you need the Tx to have good linearity at high output power as well as low noise, you therefore need to verify your design when the IF power is set to some setback level from P1dB (depending on required linearity).

    As for the noise contribution, I agree that DC noise can be up converted by the mixer but I don't think this is the case here for two reasons:

    1. the noise depends heavaly and is very sensitive to the size of the bias device (a very small change in size may change the NF by 10dB) so it does not make any sense

    2. I ac block the bias circuit by attaching it to a separated power supply and placing a 1Farad shunt capacitor at its output with no effect on noise contribution or actual noise... 

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  • Sun, Nov 3 2013 2:56 AM

    • MicheleA
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply
    bennys:

    I would also expect that the NF not be dependant on IF input power, but this is not the result I get from simulation (the NF makes sence at low input level but then gets unrealisticly high as the input power is raised).

    Ok, but how are you simulating it then? Maybe it is an artifact of the simulation?

    bennys:
     

    There is an interest in verifying at different IF power level as you need the Tx to have good linearity at high output power as well as low noise, you therefore need to verify your design when the IF power is set to some setback level from P1dB (depending on required linearity).

    Understood. I would however mantain that unless you drive your IF port into compression (which you will draw from linearity analysis), you should still be 'small signal' for that port, meaning that you should not see appreciable differences in the NF. If you see different results, it may be IMVHO that either you are not simulating correctly, or the linear range of the Mixer is much smaller than you think. 

    bennys:
     

    As for the noise contribution, I agree that DC noise can be up converted by the mixer but I don't think this is the case here for two reasons:

    1. the noise depends heavaly and is very sensitive to the size of the bias device (a very small change in size may change the NF by 10dB) so it does not make any sense

    2. I ac block the bias circuit by attaching it to a separated power supply and placing a 1Farad shunt capacitor at its output with no effect on noise contribution or actual noise... 

    I think we should first take a look at how you are simulating the circuit. It may be that the simulator is not used properly for the purpose, so it is giving you results that do not make much sense.

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  • Mon, Nov 4 2013 10:38 AM

    • Tawna
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    I recommend taking a look at the Appendix A of the SpectreRF User Guide (version 12.1.1 or later).

    There is a more thorough and up to date example of simulating a transmit mixer.   Please use that instead.  Here is the link to the appendix.

     http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs;q=/spectreRF/spectreRF13.1/AppA.html

    In the software hierarchy, there is a database that you can use to run the simulations.    This will be in the MMSIM hierarchy at <MMSIM>/tools/spectre/examples/SpectreRF_workshop/RF_Doc_Database.tar.gz

    Important note:  You will need to file a Service Request so you can obtain the GPDK180 pdk kit if you want to simulate the circuit.

    best regards,

     Tawna

     

    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
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  • Tue, Nov 5 2013 12:54 AM

    • Frank Wiedmann
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    GPDK180 is also included in the Rapid Adoption Kit "Mixed Signal Simulation with Real Number Modeling" (in the share directory), which is available from http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=wp;q=ProductInformation/Custom_IC_Design/ApplicationPackages/CIC_RAK_Home.htm.

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  • Tue, Nov 5 2013 1:04 AM

    • MicheleA
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Hi Tawna,

    thanks for the tip! Indeed, my help file pointed to version 10.xx so the appendix does not contain the TX mixer example. However the online doc does and it is quite interesting. I would be curious to verify that the SNR indeed depends on the IF level by simulating with HB and HBnoise instead of PSS and Pnoise.

     Michele 

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  • Tue, Nov 5 2013 1:22 AM

    • MicheleA
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply
    Thanks for the link Frank! M
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  • Tue, Nov 5 2013 11:08 AM

    • Tawna
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Please note that the GPDK180 is being removed from the RAK.  It was included by mistake.  To get the GPDK180, contact your local Cadence AE - Also see Article 20158565 .

    Thank you for bringing this to our attention, Frank.

     best regards,

    Tawna

    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
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  • Tue, Nov 5 2013 12:48 PM

    • Tawna
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply
    Hi Michele,

    With regards to your question:  I would be curious to verify that the SNR indeed depends on the IF level by simulating with HB and HBnoise instead of PSS and Pnoise.

    The results should be the same whether you are using hb/hbnoise vs pss/pnoise.  (If the simulations are set up properly, they should give the same answer in theory.)

    best regards,

    Tawna
    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
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  • Tue, Nov 5 2013 1:46 PM

    • MicheleA
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Hi Tawna,

    I agree 100%, the general statement is that a circuit should give the same response, no matter the way it is simulated. My sentence was not so clear however, let me try to explain myself better:

    The general point to assess is whether the Noise Figure of a TX mixer depends or not on the power level of its IF input. My intuition says that is should *not* depend on it, if we can say that the IF power level is within the linear operation region of the IF port. Moreover, we are interested in the noise contribution of the Mixer - i.e. the NF and its relatives - only when this contribution is relevant, that is when both input signal and noise power are small, so that even a small contribution by the 'amplifying' (actually 'mixing') device can be of importance. So I would even say that for increasing IF input signal the output SNR will be dominated by the input SNR, with the addittive contribution of the Mixer becoming progressively negligible.

    NF = 10*log(SNRi/SNRo) = 10*log[Pi/Po*No/Ni] = 10*log[1/G*(G*Ni+Nd)/Ni] = 10*log[1+Nd/GNi]

    ..From which I think one could see that if we increase Si by keeping SNRi constant, we have to increase Ni as well, which makes Nd - the contribution of the Mixer in this example - less important. At least, for addittive noise. 

    That's for the intuitive part.

    For the simulation part, I would have run a PSS/PNoise sim with the LO as the *only* tone, and the IF port set at a DC level. Maybe, sweep the DC both common and differential mode to see whether some non-idealities pop up. This, based on my 'theory', but also on the fact that I did not know better, in terms of simulator capabilities.

    Reading your link this morning, gave me the impression that HB could handle widely spaced tones (like 2MHz and 2GHz) in a *clever* way. I never used HB as such inside Virtuoso. Only PSS (and HB inside it sometimes). If I had to do this in PSS, I would have set the fundamental to 2MHz and 1000 harmonics, to capture the 2GHz tone. Actually, many more harmonics because I want to see the harms of 2GHz as well.

    I tried it in the past, wanting to compare ENV with PSS. Yes, results agree, but PSS this way is not very efficient :)

    From the Appendix you linked to I got the impression that HB is cleverer and can handle this math problem more efficiently, so I suggested to bennys just to try and see whether indeed an higher - but still below saturation - IF power can really affect NF. After this online deployment of mine, I am even tempted to say that NF will be *better* (i.e. lower) for higher IF power :)

    Kind Regards,

    Michele 

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  • Wed, Nov 6 2013 12:34 AM

    • Frank Wiedmann
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Tawna:

    Please note that the GPDK180 is being removed from the RAK.  It was included by mistake.  To get the GPDK180, contact your local Cadence AE - Also see Article 20158565 .

    Thank you for bringing this to our attention, Frank.

     best regards,

    Tawna

    A GPDK is included in many RAKs. In my opinion, this is a good idea, because it makes installing the RAK much easier. I assume that much fewer people would use the RAKs if you first have to file a service request to obtain the GPDK and have to install it separately. These service requests will also generate additional work for Cadence support personnel.

    • Post Points: 20
  • Wed, Nov 6 2013 2:07 AM

    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Frank,

    The reason why it is supposed to be a separate download is because there's a click-through license to use it. It used to be that the pdks were available under a special web site (pdk.cadence.com) but that site has been retired, and so accessing via a support case is a temporary measure until the new location is sorted (on support.cadence.com probably). So you needn't worry about our workload - we already are trying to address this.

    Some of the RAKs contain a much-reduced version of a GPDK.

    Regards,

    Andrew.

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  • Wed, Nov 6 2013 2:23 AM

    • Frank Wiedmann
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Andrew,

    Thanks for the explanation. By the way, there is also a click-through license when you download the RAKs (which you as a Cadence employee might not see), so this might already be covered.

    Best regards,

    Frank

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  • Wed, Nov 6 2013 9:28 AM

    • Tawna
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    Re: Noise Figure of an upconverting mixer under input IF signal Reply

    Hi Frank,

    If you see other RAKs with Cadence gpdks in them, please let me know.

    As an engineer who likes to get things done quickly, I agree that it is easier if the gpdks can be included.  :-)    However, we need to comply with our internal Processes and Procedures.   I am right in the thick of this...I will update the COS Article when the gpdk180 is available on downloads.cadence.com . 

     

    best regards,

    Tawna

    Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
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Started by bennys at 29 Oct 2013 02:59 AM. Topic has 16 replies.