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Design >> RF Design >> Cadence's Balun Vs ADS's TF3
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Message started by wccheng on Feb 28^th, 2007, 10:23pm

Title: Cadence's Balun Vs ADS's TF3
Post by wccheng on Feb 28^th, 2007, 10:23pm

Dear all,

I am designing differential LNA. In the simulation, I want to use a balun to make a single input and differential output for differential LNA input. In the Cadence's balun model, it could allow me to set the voltage level and the impedance of the balun's balance and unbalance port. However, ADS's TF3 just can allow me to set voltage level and cannot set the balance and unbalance port impedance. How could I set it in order to have 50ohms unbalance and 100ohms balance impedance in TF3. So, it could be similar to the actual balun.

Thanks

wccheng

Title: Re: Cadence's Balun Vs ADS's TF3
Post by ACWWong on Mar 1^st, 2007, 2:11am

have you looked under the analysis section of this site http://www.designers-guide.org/Analysis/ ?
I think if you look at the documents under testbenches might help you.

Title: Re: Cadence's Balun Vs ADS's TF3
Post by wccheng on Mar 1^st, 2007, 5:59am

Dear Sir,

I have seen this document. This is using Cadence SpectreRF. Sure, this balun model allows me to set the impedance at balanced and unbalanced port. However, the model in ADS is not allow. How could I fix it in ADS?

thanks

wccheng

Title: Re: Cadence's Balun Vs ADS's TF3
Post by ACWWong on Mar 1^st, 2007, 8:15am

Figure 5 in http://www.designers-guide.org/Analysis/diff.pdf shows the structure of a balun useful for diffential circuit testbenches. It is not limited to use with Cadence spectreRF, and can be recontructed in ADS using two TF components.
Then the impedance connected to node D will represent the differential impedance between nodes P and N. The impedance connected to node C will represent the common-mode impedance at nodes P and N. Of couse the converse is true as well (as shown in Ken's documents where it is uded for inputs and outputs of DUT).
You can use resistors in series with voltage sources at D and C to define source signals and impdences. In Cadence you can also use the port component. In ADS you can use the term component.
hope this helps.
cheers
aw

Title: Re: Cadence's Balun Vs ADS's TF3
Post by wccheng on Mar 1^st, 2007, 10:04pm

Dear Sir,

I guess you are mis-understand my meaning. I mean the impedance setting in Balun (balance and un-balance port). Not the source impedance.

Thanks

wccheng

Title: Re: Cadence's Balun Vs ADS's TF3
Post by ACWWong on Mar 2^nd, 2007, 2:22am

Hi wccheng,

You mean you are using rfLib/balun which is a behavioural (verilogA) model ?
If so you should be able to run rfLib/balun verilogA model directly in ADS. Maybe http://www.ece.osu.edu/~bibyk/ee894z/verilogaADS.pdf will help you.

In any case I would still recommend you use the balun (made using ideal transformers) as decribed in my previous post. This allows you to set impedance of balanced and unbalanced ports very easily.

cheers
aw

Title: Re: Cadence's Balun Vs ADS's TF3
Post by RFICDUDE on Mar 3^rd, 2007, 8:14pm

You could just use the ideal transformer model "TF" in ADS and set the turns ratio to get the impedance transformation you want.
(Zin/Zout)=(Nin/Nout)^2
You can use this type of relationship with the TF3 model as well.

Title: Re: Cadence's Balun Vs ADS's TF3
Post by wccheng on Mar 3^rd, 2007, 9:29pm

Dear ACWWONG,

Yes, I mean the one in rfLib/balun in Cadence. This balun allows me to set the impedance of balance and unbalance ports. However, the TF3 in ADS does not provide this setting to me. It just allows me to set the number of turn in primary and secondary coil. I have tried to add a series 50ohms resistor in the TF3 and simulate the s-paramter. It does not give me a 50ohms matching. However, it is correct in the rfLib/balun in Cadence as I set the correct impedance in rfLib/balun setting. If I wanted the TF3 with same impedance, how could I do?

Thanks

wccheng

Title: Re: Cadence's Balun Vs ADS's TF3
Post by RFICDUDE on Mar 5^th, 2007, 7:58pm

The TF3 element couples the primary to two secondaries. The relationship is
Primary <-> 1/T1 secondary turns ratio
Primary <-> 1/T2 secondary turns ratio

So you can series the the secondaries which would look like a step up of 1:2 if T1=T2=1.
If T1=T2=2 that makes each secondary look like a step down of 2 so you get a 1:1 ratio if the to secondaries are series connected.

I tried it out and you get a perfect match.