balun cadence rflib
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
you can use another idea 1:2 balun to transfer the differential port to single port, then you can simualte balun as two port element
Dear dingjingfeng,
Could you tell me the name of this model? Does it allow me to set the impedance of balanced and unbalanced port?
Thanks
wccheng
In ADS TF3 parameters are 1/number of turns on secondary, being 1 turn on primary.
You just have to calculate how these parameters are linked to single end/differential impedance ratio.
I hope it can help.
Mazz
As to the Balun, I have another question about it. That is NF calculation. For example, I have a differential RF PGA to do the simulation. I connected source, which is a port, to the PGA with a balun. In order to do the matching, I use a 50Ohm resistor at the port side. The question is what is the real NF value? Because, the balun will generate voltage gain! -- 3dB, if we assume the power is constant at the balun two side.
There is the same situation when using transformer. the secondary side turns number will CHANGING THE NF !
anyone can give explanation?
I will try to distinguish two issues:
1. NF is by definition referred to a source impedance, usually 50 Ohm. If, for example, your RF PGA has 200 Ohm input impedance, please be sure that the noise figure that you've been requested is referred to 50 Ohm. If so, use the balun 1:4 (or the transformer with proper turn ratio) and simulate.
2. NF is affected by return loss. Tipically in LNAs books is done the complete analysis. Briefly I can say that the optimum source impedance (for noise) is different from 50 Ohm, while the optimum source impedance (for power) is usually 50 Ohm.
Changing the impedance ratio (or the transformer turn ratio) you will change your CIRCUIT source impedance (after the balun, I mean), having different noise figure (but also different gain and return loss).
I hope it can help.
Mazz
Dear Sir,
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
wccheng
take care of definition of paramters in TF3.
Number of turns of primary is always one, and the two parameters are both referred to secondary (see ADS help for more details).
I hope it can help.
Mazz
Dear Mazz,
I know the parameters in TF3. How about the impedance setting at balance and unbalance port? Since the rfLib/balun in cadence allow me to set the balance and unbalance port impedance setting.
Thanks
wccheng
to a ideal 1:1 balun
the impedence is 1:2 if you want maximum power transfer
you can change ratio upon you design with similar ratio I think
Added after 1 minutes:
the impedence is set by changing the impedence if the term
wccheng
I thought you know equations on ideal transformer:
v1/v2=N1/N2=-i2/i1 (assuming currents entering the transformer)
Z1=v1/i1=((N1/N2)^2)*(-v2/i1)
but ZL=-v2/i1
so Z1/ZL=(N1/N2)^2.
Remember that N1=1 in TF3 and N2=1/T2+1/T3
I hope it can help.
Max