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The Designer's Guide Community Forum › Design › RF Design › Help: 018um WLAN PA is not working on Silicon
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Help: 018um WLAN PA is not working on Silicon (Read 7852 times)
Sarig
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Help: 018um WLAN PA is not working on Silicon
Mar 09th, 2009, 3:12pm  
Hello,
My PA in 018um process for WLAN 5GHz is not working on silicon.
Facts:
1. PA is 3 stage RFIC design on 018um process for 4.9-5.9GHz class AB.
2. PA first stage is att. (simple switches) second stage has 10dB gain and output stage also 10dB gain.
3. DC current is OK on silicon. Also the gain control is working and signal seams linear (Pin Vs. Pout)
4. problem: intead of having 20dB gain it loss 30dB (where did 50dB gone?)
5. I did some FIB (cutting stages) and found that each gain stage loss ~15dB instead of gain 10dB.
6. On stability was seen (looking on single output didn't present any stabilty)
7. output stage has 4down bond and second stage has 2 down bond

I have no idea what can cause 50dB loss in gain,
I need Help here........
Any RFIC expect can supprt me?
Thanks,
Erez
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ci
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #1 - Mar 10th, 2009, 12:30pm  
Erez,

One suggestion:  since DC currents are OK, I suggest you to check the inductance/resonance of the power distribution both on-chip and off-chip.  Modeling and simulation may help identify the root cause.

Regards,
Cosmin
NoiseCoupling.com
http://www.noisecoupling.com
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Sarig
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #2 - Mar 10th, 2009, 1:10pm  
Hi Cosmin,
I have done so and found that:
1. on chip there is only 5pF decap on the PA supply.
2. VDD_TX is is only one wire bond
3. only down bond on VSS_TX (means that there are no wore bonds)
4. Board VDD_TC line is about 1cm from to board 1uF capacitor to the chip pad but even when simulating with large inductor from the supply it doesnt show any impact

Again, we are talking about 50dB difference between sim to silicon

Thanks,
Erez
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ci
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #3 - Mar 10th, 2009, 1:54pm  
Hi Erez,

What model do you use in sim for power supply?  Does it include the parasitics of the interconnects and decoupling capacitors?  Is the loop inductance included?  You may be able to simulate or even measure (if the bandwidth allows) the frequency characteristic of the power supply impedance as seen by your circuit on chip.  This may rule out the effect of power distribution impedance in your troubleshooting process.

Regards,
Cosmin
NoiseCoupling.com
http://www.noisecoupling.com
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Sarig
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #4 - Mar 10th, 2009, 2:20pm  
Hi Cosmin,
I will try to answer you:
I am at this moment ramping the RC extraction simulation (it was never been done before, (Don't ask me why I got this PA as is)
the extraction is only RC but still I can't understnd how parasitics can kill 50dB.
For the wire bond I use R,C,L package model
What is loop inductance?
Do you think that there is a chance that this PA which is differintail is effected by the power supply imedance seen by my block? and how can I masure it on silicon for my PA is a "black box"
By the Way, have you ever seen such disaster in PA design? instead of 20dB gain is has -30dB loss.
I can tell you one thing. for the last 8 years I have been an Analog designer and for the last year an RFIC designer and I have never been in a state were I am lost in debugging

Thanks,
Erez
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ci
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #5 - Mar 11th, 2009, 9:42pm  
Hi Erez,

What results did you get using RC extracted netlist?  Did they show your problem?  Loop inductance is composed of self inductance and mutual inductance, and it is the inductance seen by your amplifier when looking into the power supply.  

This is a simplified view of loop inductance: when the return supply current flows close to the forward supply current the mutual inductance tends to cancel out the self inductance reducing significantly the loop inductance.  Problems come when the return current is routed far from the forward current (and this includes pins assignments, vias, power planes, and decoupling capacitors.  These problems can be easily seen at the frequencies of your application.  

You can find more details about loop inductance in various text books including mine available at www.noisecoupling.com.

Regards,
Cosmin
NoiseCoupling.com
http://www.noisecoupling.com
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Sarig
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #6 - Mar 12th, 2009, 12:30pm  
Hi Cosmin,
Regarding R+C+C extraction:
PA gain drop to 5dB (remember pre extraction was 23dB)
Another thing seen post extraction is that freq. center moved to 4.5GHz.
How is this correlate to silicon? well, first silicon shows Gain droped to -20dB. second, the freq. center is OK (5.3GHz).

Regarding the Bond issue, My model include all bond & wire bond. (and mutual ind.)

I enclosed PA layoutimage with remarks, this might give you a clue.

There is a small chance that parasitic inductance can decrease more the gain.

Thanks,
Erez
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PA.jpg  
 
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AnalogAroma
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #7 - Mar 13th, 2009, 9:37am  
Hi  Sarig,

    OUTP and OUTN  pins should be adjacent to one another. But , in your layout , those pins are quite far apart. Why ?
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ci
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #8 - Mar 13th, 2009, 12:17pm  
Hi Erez,

I may be able to suggest you some possible causes for the gain loss if I take a look at the extracted netlist and simulation test bench netlist.

Regards,
Cosmin
NoiseCoupling.com
http://www.noisecoupling.com
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Sarig
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #9 - Mar 13th, 2009, 1:38pm  
Hi,
I have no Idea why the outputs are far from each other. I got this PA as is.

Cosmin, I will be happy to mail you the extracted netlist and the models (Spectre). please send me your mail?
Thanks,
Erez
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ci
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Re: Help: 018um WLAN PA is not working on Silicon
Reply #10 - Mar 14th, 2009, 10:38am  
Erez, use the "Ask Cosmin" form on the NoiseCoupling.com web site to contact me.

Regards,
Cosmin
NoiseCoupling.com
http://www.noisecoupling.com
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