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Design >> RF Design >> single input - differential output RF circuit
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Message started by wccheng on Jun 11^th, 2007, 7:35am

Title: single input - differential output RF circuit
Post by wccheng on Jun 11^th, 2007, 7:35am

Dear all,

Could anyone suggest the circuit for single input and differential output in RF band? Since I see some circuit building block, which use single ended antenna and then feed into the RF chip. In the RF chip, it has one building block for single input - differential output. Then connected to differential LNA.

However, I don't know what kind of the circuit could make a single input - differential output for RF high frequency. Could anyone tell me?

Thanks

wccheng

Title: Re: single input - differential output RF circuit
Post by ACWWong on Jun 11^th, 2007, 9:17am

some ideas would be:

Passive transformer based circuits (i.e. balun) would work at high frequencies (>1GHz) on chip can be used to do the single ended to differential convserion, but are area hungry

An active single ended LNA output can be coupled to a diffential input following stage, where the unused 2nd stage input is capacitor coupled to the LNA output's common-mode level to give a psuedo differential interface.

Title: Re: single input - differential output RF circuit
Post by wccheng on Jun 11^th, 2007, 5:58pm

Dear ACWWong,

Do you have any suggested paper to me for reading?

Thanks

wccheng

Title: Re: single input - differential output RF circuit
Post by RFICDUDE on Jun 11^th, 2007, 7:52pm

Barry Gilbert published a cool circuit for single ended input differential output mixer. He coined the overall circuit the "micromixer." The input stage is a combination common base (or equivalently common gate) for the inphase path and a current mirror turn around which acts sort of a common emitter (or common source) amp for the 180 degree path. The 180 split is not completly balanced at RF, but hardly any of the other techniques are either.

The MICROMIXER: a highly linear variant of the Gilbert mixer using a bisymmetric Class-AB input stage
Gilbert, B.
Solid-State Circuits, IEEE Journal of, Vol.32, Iss.9, Sep 1997
Pages:1412-1423

Title: Re: single input - differential output RF circuit
Post by carlgrace on Jun 12^th, 2007, 12:57pm

If you want to do the single-ended to differential conversion on the chip, you really should do it in the LNA. Otherwise, you will have to add the insertion loss of the se-diff converter which would be a killer. Probably a better solution is to use a balun on the board and drive the chip differentially. If you are doing this for a class, you can design an on-chip balun with a tapped inductor. I wouldn't suggest this for a commercial part, however, unless you have a really good reason to do it. Off-chip baluns are far better (lower loss) than on-chip baluns.

Title: Re: single input - differential output RF circuit
Post by sandman on Jun 20^th, 2007, 12:26am

Additionally one could also connect the single ended antenna to one terminal of the differential mixer and connect the other terminal of the mixer by a large capacitance to ground. But of course, you will have to deal with the significant loading of the LNA by the mixer (which is mostly capacitive... and large) at it's input. This way you would only loose 3dB in the signal.