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Design >> RF Design >> Single-to-Differential conversion Off-chip
https://designers-guide.org/forum/YaBB.pl?num=1181988621

Message started by sandman on Jun 16^th, 2007, 3:10am

Title: Single-to-Differential conversion Off-chip
Post by sandman on Jun 16^th, 2007, 3:10am

I've read that it's possible to drive a differential LNA by directly connecting it to a (loop or dipole) antenna or to a SAW filter with single-to-differential conversion, therefore eliminating an expensive balun.

Unfortunately I've not been able to re-trace this source. Would anyone have more information on this ?

I'm trying to keep cost and power to a minimum by using a single-ended LNA (overcomes the need for a balun) and a passive differential mixer, but at the same time I do not want to rule out the differential LNA topology, for the obvious performance aspects (improvements).

Any comments/inputs would be appreciated.

Thanks in advance.

Title: Re: Single-to-Differential conversion Off-chip
Post by didac on Jun 16^th, 2007, 7:51am

Hi,
A loop and a dipole if feeded at the center are by construction balanced so you can directly connect to them with for example a twin-lead wire(one wire for V+ and the other for V-) but you will find coupling of interferences due to the unshielded nature of this kind of wire that's the cause of the use of coax wire for TV so it's not as straightforward as it seems to eliminate the coax and the off-chip balun.

Title: Re: Single-to-Differential conversion Off-chip
Post by sandman on Jun 18^th, 2007, 4:29am

Thanks for your response !

So you're saying that because it's quite complicated to overcome the intereference due to these straight wires, people usually resort to using the off-chip balun ?

Does this mean that a differential LNA cannot be connected without a balun, assuming I would not want to trouble myself with issues of intereference between the wires ?

Title: Re: Single-to-Differential conversion Off-chip
Post by didac on Jun 18^th, 2007, 12:35pm

Hi,
In classical dipole,loop antennas I will say yes that's the cause that people prefers to use baluns(altough antennas are not my specialitzation), another feasible option depending on your operation frequency will be the use of antennas printed on your PCB(I think a kind of it that allows differential feeding is called 'bowtie'), of course you then need to use something like differential coplanar lines(GND RF+ GND RF- GND) between your IC and the antenna(and probabaly you must do the design of the antenna with a magnetic simulator), I also have read some papers that directly they are integrating(or trying to integrate) the antenna inside the IC(but at frequencies above 20GHz due to the area reduction with increasing frequency).
Hope it helps,

Title: Re: Single-to-Differential conversion Off-chip
Post by sandman on Jun 20^th, 2007, 12:21am

Thanks a ton for your inputs ! They're very useful !