Noise Figure of an upconverting mixer under input IF signal

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 

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... 

bennys said:

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 said:

 

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 said:

 

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.

M 

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

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.

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 

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

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