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Measurements >> RF Measurements >> Single-side band mixer to measure I/Q accuracy
https://designers-guide.org/forum/YaBB.pl?num=1203338485

Message started by neoflash on Feb 18^th, 2008, 4:41am

Title: Single-side band mixer to measure I/Q accuracy
Post by neoflash on Feb 18^th, 2008, 4:41am

Hi, folks:

I'm calculating how much spur will SSB mixer see, due to I/Q inaccuracy? For example, if there is 1 degree mismatch for I/Q, how much spur should I see?

Title: Re: Single-side band mixer to measure I/Q accuracy
Post by ACWWong on Feb 18^th, 2008, 6:24am

Spur will be about -41 dBc.
In the absense of any other mismatches, nonlinearities or crosstalk you can calculate it by 20*LOG10[tan(IQphaseerror/2)]
Often such things are represented by "image" sideband contours levels on IQ amplitude and phase mismatch Cartesian axis.

Title: Re: Single-side band mixer to measure I/Q accuracy
Post by neoflash on Feb 24^th, 2008, 2:15am

Thanks for the reply.

I agree with -41dbc. However, I think the equation should be sin(delta) instead of tan(delta). They have similar value when mismatch (delta) is small.

Title: Re: Single-side band mixer to measure I/Q accuracy
Post by ACWWong on Feb 24^th, 2008, 5:57am

No it is tan(phaseerror/2) not sin

It comes from the 10*LOG10{[1-cos(phaseerror)]/[1+cos(phaseerror)]} which is equal to 20*LOG10[tan(IQphaseerror/2)]

Look at Razavi RF microelectronics for derivation or

http://ieeexplore.ieee.org/iel5/4138799/4138800/04138864.pdf

A key thing is that in a SSB IQ modulator error vectors which cause the "image" sideband, i.e quadrature phase error and quadrature amplitude error, are orthogonal. Therefore you can null both phase and amplitude error independantly.

Title: Re: Single-side band mixer to measure I/Q accuracy
Post by neoflash on Feb 25^th, 2008, 3:33am

Oh... Yes. I think i assumed cos(mismatch)=1 during derivitation. Thanks for the head up.