3. TECHNICAL BRIEF
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(3) Transmitter Part
To minimize the post-PA filtering requirements and any additional post-power amp losses, the transmit
path consists of a vector modulator and a frequency translation loop. The translation loop consists of
the following:
•
PFD and charge pump
•
In-loop modulator
•
One programmable divider
•
Integrated transmit loop filter
•
Two transmit VCOs and output buffers
A. Translation Loop
The translational loop includes a vector modulator and a frequency translation loop to minimize the post-PA
filtering requirements. the loop functions as a PLL with a mixer in the feedback path and a modulator in the
reference path. The loop provides a PFD and charge pump, integrated loop filters, two transmit VCOs,
down-conversion mixer in the feedback path, a frequency divider for frequency plan flexibility, and the
modulator. The mixers in the feedback path provide either high side or low side injection to provide flexibility
in the frequency plan. The modulator in the reference path uses a vector summing technique to reject the
unwanted image and to also sufficiently attenuate the 3rd and 5th harmonics. Therefore, no external IF
filters are required. The loop filter required for the transmit VCOs is integrated in the SKY74400.
B. Transmit VCOs
Two integrated transmit VCOs are designed to meet GSM850, EGSM900, DCS1800, and PCS1900
requirements. The transmit VCOs use the same DFC technique as the synthesizer section to lock the
translation loop. The rising edge on TXENA initializes the transmit DFC. The output buffers feed the signal
to the PAs. Two transmit buffers are provided, one for the low band VCO and the other for the high band
VCO.
Figure. 3-2 Crystal Oscillator BLOCK DIAGRAM
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