With a clearer understanding of how to detect spectrum inversion, it is now time to explore available methods for achieving it. As mentioned earlier, the 3GPP2 standard requires that the signal spectrum of NCDMA be inverted prior to transmission and after reception. Three simple methods can be used for spectrum inversion.

In the first method, spectrum inversion is achieved by adopting a positive phase LO. This is the modulation format recommended by the 3GPP2 standard (Fig. 2):

Im = cos(ωmt)

and

Qm = sin(ωm)

and

LOITX = cos(ωLOt)

and

LOQTX = sin(ωLOt)

VTX = cos(ωmt)cos(ωLOt)  + sin(ωmt)sin(ωLOt)

VTX = 0.5cos[(ωm) - ωLO)]t + 0.5cos[(ωm + ωLO)]t + 0.5sin[(ωm - ωLO)]t + 0.5sin[(ωm + ωLO)]t

VTX = cos[(ωm) - cos(ωLO)t]

As shown here, the RF output frequency is lower than the LO frequency. Therefore, the spectrum has been inverted.

In method 2, spectrum inversion is achieved by inverting the polarity of the Q baseband signal:

Im = cos(ωmt)

and

Qm = -sin(ωmt)

LOITX = cos(ωLOt)

and

LOQTX = -sin(ωLOt)

VTX = cos(ωmt)cos(ωLOt)  + sin(ωmt)sin(ωLOt)

VTX = 0.5cos[(ωm) - ωLO)]t + 0.5cos[(ωm + ωLO)]t + 0.5cos[(ωm - ωLO)]t - 0.5cos[(ωm + ωLO)]t

VTX = cos[(ωm - ωLO)t]

In the third method, spectrum inversion is achieved by swapping the I and Q transmit baseband signals:

Im = sin(ωmt)

and

Qm = cos(ωmt)

LOITX = cos(ωLOt)

and

LOQTX = -sin(ωLOt)

VTX = cos(ωmt)cos(ωLOt)  + sin(ωmt)sin(ωLOt)

VTX = 0.5sin[(ωm) + ωLO)]t + 0.5sin[(ωm - ωLO)]t - 0.5sin[(ωm + ωLO)]t + 0.5sin[(ωm - ωLO)]t

VTX = cos[(ωm - ωLO)t]

Method 1 is the approach recommended by the 3GPP2 standard for performing spectrum inversion. However, as shown above, other methods can achieve the same result and still be compliant with the specifications. In all cases for parts derived from the MAX2553 WCDMA transceivers from Maxim Integrated™, the baseband input and output pins are labeled so that no spectrum inversion occurs in the transceiver—either in the transmitter or receiver path. For compliance with the 3GPP2 standard, one should be aware of this and apply one of the above methods for inverting the transmit spectrum. For the receive path, a corresponding inversion is also required in most cases, unless the baseband is already programmed to accept inverted spectrum signals.