TRF1112_07 Datasheet, PDF (14/25 Pages) Texas Instruments – Dual VCO/PLL Synthesizer With IF Down-Conversion

TRF1112
TRF1212
SLWS175A – APRIL 2005 – REVISED DECEMBER 2005
www.ti.com
The fractional spurious level varies with the offset from the LO since these spurious signals are attenuated by the
loop filter response. The larger the offset from the LO, the lower the spur level. In general, spurs at offsets
greater than 3 or 4 MHz are below –75 dBc and are not a concern. The worst fractional spurs levels occur when
they are located at 1 MHz offset from the LO1 frequency. (Note: the fractional spur is offset from the LO1
frequency by 1 MHz when the difference between the LO1 and an integer multiple of the reference frequency is 1
or 17 MHz.)
Although both synthesizers have fractional spurs, for most applications the spurious signals from the UHF (LO2)
synthesizer can be ignored because these spurs are attenuated by frequency dividers that are placed after the
LO2 generation. In some frequency plans it is possible to offset LO1 and LO2, in a complementary manner, to
avoid worst-case fractional spurs (i.e., 1-MHz offsets) on LO1 synthesizer.
VCO Tuning Characteristics
The TRF1121 / TRF1221 internal VCOs have the following frequency vs tune voltage characteristics.
2500
700
2250
600
2000
Frequency
500
1750
400
1500
1250
1000
1
Mod Sense
−40 ºC
+25 ºC
+85 ºC
1.5
2
2.5
3
3.5
4
4.5
Tuning Voltage (V)
Figure 11. TRF1112 LO1 Frequency vs LO1TUN Voltage
300
200
100
5
14