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Design >> RF Design >> IQ mixer in direct converter
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Message started by nxing on Apr 15^th, 2011, 10:04pm

Title: IQ mixer in direct converter
Post by nxing on Apr 15^th, 2011, 10:04pm

Hello,

I have a question for I/Q mixer in zero IF receiver: since for zero -if, signal's image is itself, then what's the advantage of using I/Q mixer?

Thanks,

Nyranx.

Title: Re: IQ mixer in direct converter
Post by rfidea on Apr 16^th, 2011, 1:34am

Saying that the image is the same as the signal for a zero-if is not exactly true. By using an IQ-demodulator and then in the digital domain phaseshift I or Q another 90 degrees you get the received signal downconverted to zero frequency. The upper sideband of the received signal is now placed above zero frequency and the lower sideband is placed belove zero frequency.

In all modern modulation methods with high requirement on spectrum efficiency the upper and lower sidbands are not the same.

So, to be able to separate the upper and lower sidbands of the received signal with a zero-if receiver an IQ-demodulator is needed.

Title: Re: IQ mixer in direct converter
Post by RFICDUDE on Apr 16^th, 2011, 5:03am

For single carrier channels (UMTS uplink, GSM, EDGE CDMA reverse link, etc.) the image is just the signal itself, but the is not necessarily the same signal in multicarrier systems (UMTS downlink, CDMA forward link, WLAN, LTE, WiMAX, etc.). Multicarrier systems generally will have unique subcarriers or channels at the image. In these cases the ZIF demodulator is still providing two real signals, I and Q, that the modem uses to separate out the upper and lower sideband carriers and/or channels.

ZIF is just as beneficial (reduced parts count and power) for OFDM and multicarrier systems as it is for single carrier systems.

Title: Re: IQ mixer in direct converter
Post by rfidea on Apr 16^th, 2011, 6:03am

Maybe I should clarify myselt. Of course in a singel carrier system both upper and lower sideband is part of the same signal. My point is that the two sidebands are not identical. Therefor both I and Q is needed to be able to separate the two sidebands. Using only I or Q in s single carrier ZIF receiver will fold the two sidebands on top of eachother.

For example, if only "1":s are transmitted in the GMSK modulation, used by GSM, the carrier will be shifted +62.5kHz, and -62.5kHz of only "0":s are transmitted. The only way to distinguise between the two cases is to look at the phase difference between the 62.5kHz signals at I and Q.