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Fabricante

RF9678

W-CDMA TRANSMIT MODULATOR AND IF AGC

RF Micro Devices 

RF9678

Ref Lvl

0 dBm

1.5

IMAG

T1

CF

380 MHz Meas Signal

SR

3.84 MHz Constellation

Demod

QPSK

A

EXT

Preliminary

5

-1.5

-1.875

Date:

6.FEB.2001 00:38:12

REAL

1.875

Figure 3. DC Offset Error (Carrier Feedthrough)

As is shown above, the EVM performance of a modulator can be correlated to it's carrier and sideband suppression per-

formance.

Unadjusted Performance

A Wideband CDMA signal is a noise-like signal occupying a channel bandwidth of 3.84MHz. When viewed on a spec-

trum analyzer the channel appears as a plateau raised above the noise floor (see Figure 4). In some cases, it is normal

to see a spike over the center of the W-CDMA plateau. This will occur when the absolute power level of the unadjusted

carrier feedthrough is higher than that of the W-CDMA channel level. The cause of this phenomenon can be understood

by examining the relative powers of the carrier signal and the W-CDMA channel power.

For Example, a 1Hz channel with a power of 0dBm has an absolute power level of 0dBm when viewed on a spectrum

analyzer. When that 0 dBm channel power is spread over a 3.84MHz channel, as in W-CDMA, it results in an absolute

channel power level of -65.8 dBm (-10*log (BW)). The absolute channel level displayed on the spectrum analyzer will

increase with the resolution bandwidth setting on the instrument although the integrated channel power will remain con-

stant. A resolution bandwidth (RBW) of 30kHz will increase the displayed power level by 44.7dB (+10*log(RBW)) to -

21.0dBm. With a desired signal output power of +2.0dBm and a carrier suppression of >20dBc, the absolute carrier level

can be as high as -18.0dBm (POUT-Suppression=Carrier Level). This will result in a 3dB carrier spike above the W-

CDMA channel level (see Figure 4). (Note: The relative height of the carrier spike above the W-CDMA channel level is

directly related to the RBW of the spectrum analyzer being used. The example above assumes a 30kHz RBW.)

The following equations may be used to calculate W-CDMA channel and carrier feedthrough levels.

W-CDMA Channel Level=Channel Power (Integrated over BW)-[10*log*(BW)]+[10*log*(RBW)]

Carrier Feedthrough=POUT (Desired Sideband)-Carrier Suppression

The next section describes a procedure that may be used to dramatically reduce carrier feedthrough by tuning or opti-

mizing the modulator input signals.

5-96

Rev A4 010622

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