LMX2581_14 Datasheet, PDF (20/56 Pages) Texas Instruments – Wideband Frequency Synthesizer

LMX2581
SNAS601G – AUGUST 2012 – REVISED SEPTEMBER 2014
www.ti.com
8.3.10 0–Delay Mode
When the VCO divider is used, an ambiguous phase relationship is created between the OSCin and RFout pins.
0–Delay mode can be enabled to eliminate this ambiguity.
When this mode is used, special care needs to be taken because it does interfere with the VCO calibration if not
done correctly. The correct way to use 0–Delay mode is as follows:
1. If N is not divisible by VCO_DIV, reduce the phase detector frequency to make it so.
2. Program as normal and lock the PLL.
3. Program the NO_FCAL =1.
4. Program 0_DLY = 1. This will cause the PLL to lose lock.
5. Program the PLL_N value with PLL_N* / VCO_DIV, where PLL_N* is the original value.
6. The PLL should now be locked in zero delay mode.
8.3.11 Programmable RF Output Buffers
The output states of the RFoutA and RFoutB pins are controlled by the BUFEN pin as well as the BUFEN_DIS
programming bit. If the pin is powered up, then output power can be programmed to various levels with the
OUTx_PWR words.
OUTA_PD
OUTB_PD
1
0
Table 6. Output States of the RFoutA and RFoutB Pins
BUFEN_DIS
X
0
1
BUFEN PIN
X
X
Low
High
OUTPUT STATE
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8.3.11.1 Choosing the Proper Pull-Up Component
The first decision is to whether to use a resistor or inductor for a pull up.
• The resistor pull-up involves placing a 50 Ω resistor to the power supply on each side, which makes the
output impedance easy to match and close to 50 Ω. However, it is a higher current for the same output
power, and the maximum possible output power is more limited. For this method, the OUTx_PWR setting
should be kept about 30 or less (for a 3.3-V supply) to avoid saturation. The resistive pull-up is also
sometimes more desirable when the output frequency is lower.
• The inductor pull-up involves placing an inductor to the power supply. This inductor should look like high
impedance at the frequency of interest. This method offers higher output power for the same current and
higher maximum output power. The output power is about 3 dB higher for the same OUTx_PWR setting than
the resistor pull-up. Since the output impedance will be very high and poorly matched, it is recommended to
either keep traces short or to AC couple this into a pad for better impedance matching.
If an output is partially used or unused:
• If the output is unused, then power it down in software. No external components are necessary.
• If only one side of the differential output is used, include the pull-up component and terminate the unused
side, such that the impedance as seen by this pin looks similar to the impedance as seen by the used side.
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