Texas Instruments TRF3703, Руководство пользователя, Страница: 8 / 22

C002
1.4.1.3 Carrier Feedthrough
1.4.1.4 GSM (EDGE EVM Measurements)
Overview
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Figure 2. Optimized Sideband Suppression
Carrier feedthrough is the amount of the LO that leaks onto the output spectrum of the modulator. Ideally
for the TRF3703, inputs (I, I, Q, and Q) must be at approximately 3.3 V for TRF3703-33 and 1.5 V for
TRF3703-15. The DAC dc settings are also useful to correct the dc mismatch between I and I and
between Q and Q to correct for the LO feedthrough. If using TI's DAC568X, then the internal controls for
the IQ offsets provide excellent carrier suppression (very low LO leakage). Alternatively, if an ESG is
being used, adjust the I and Q voltage offsets in mV steps until you obtain the minimum carrier
feedthrough. A typical carrier feedthrough value exceeds 50 dBm. See
Figure 2 .
1. Provide a GSM edge signal of the desired frequency into the differential baseband inputs (example
sample rate = 4.33 MHz).
2. Use a spectrum analyzer with edge personality to measure the transmit power to either 0 or –5 dBm.
3. PSA: Mode
→
GSM(w/ EDGE)
→
measure
→
Transmit Pwr(usually 0 or –5 dBm)
→
more
→
EDGE
EVM.
4. ESG: Mode setup
→
select waveform
→
highlight EDGE
→
select waveform
→
ARB setup
→
type
4.33333 MHz
→
I/Q
→
I/Q output control
→
Common mode I/Q offset
→
(set to either 1.65 V for
TRF3703-33 or to 0.75 V for TRF3703-15)
→
I/Q
→
I/Q output control
→
I/Q output atten (adjust to get
desired transmit power to either 0 or –5 dBm).
See Figure 3 .
8 TRF3703 Quadrature Modulator Evaluation Module SLWU042B – October 2006 – Revised August 2008
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