HMC701LP6CE Datasheet, PDF (17/46 Pages) Hittite Microwave Corporation – 8 GHz 16-Bit Fractional-N Synthesizer

v03.0709
HMC701LP6CE
8 GHz 16-Bit Fractional-N Synthesizer
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VCO Tuning, Calibration & Temperature Correction
The HMC701LP6CE supports the use of an optional external step tuned VCO. A step tuned VCO allows the advantages
of low phase noise associated with a low kvco, a consistent loop voltage on the tuning varactor from part to part, and
lower charge pump voltages for a given range of VCO operation than would be possible with a conventional VCO with
only varactor tuning. This feature also helps to reduce synthesizer charge pump non-linearity by keeping the output
voltage of the charge pump as close as possible to the mid-rail condition.
Stepped Resonator VCO Calibration & Auto Tuning
In order to use a step tuned VCO in a closed loop, the VCO must be calibrated such that the synthesizer knows which
switch position on the VCO is optimum for the desired output frequency. The HMC701LP6CE supports the automatic
calibration of the step tuned VCO. The calibration fixes the VCO tuning voltage at the optimum mid-point of the charge
pump output, then measures the free running VCO frequency on each switch setting and selects the setting which
results in the free running output frequency that is closest to the desired phase locked frequency. This procedure
results in a phase locked oscillator that locks over a very narrow voltage range on the varactor.
The calibration can be run automatically every time that a change of frequency is desired. This ensures optimum
selection of resonator settings vs. time and temperature. The accuracy desired in the calibration affects the amount
of time required to calibrate the VCO. The calibration routine searches for the best step setting that locks the VCO at
the current programmed frequency, while at a varactor voltage that is closest to mid-rail.
If it is desirable to switch frequencies very quickly it is possible to eliminate the calibration time by calibrating the VCO
in advance and storing the calibration information in the host. It will then be necessary for the host to run a routine that
interrogates the VCO over the desired range of operation and records the optimum resonator switch settings for the
desired frequency range. The calibration is then only run once on power up, and the settings stored in the host. The
host must then program the switch settings directly when changing frequencies.
Frequency programming of the synthesizer by the host, including switch selection, can be done in the background,
while the VCO is locked to the current frequency. The synthesizer normally writes the frequency control information
to the VCO, via the VCO Serial Port, upon receipt of a command that changes the PLL frequency, hence eliminating
calibration delay from the locking time.
A large change of frequency may require Main Serial Port writes to:
1. The dsm_seed register (Reg 11h Table 28),
2. VCO switch register spivco_direct_data (Reg0Bh Table 22),
3. Integer register dsm_intg (Reg0Fh Table 26) and
4. The fractional register dsm_frac (Reg10h Table 27).
This last register triggers the frequency change in fractional mode.
Smaller steps in frequency may require fewer register writes. Hence a maximum of 4 Main Serial Port transfers to the
synthesizer could be required to change frequencies. If the start phase of the fractional modulator is not of concern,
then it is not necessary to write the dsm_seed register. Similarly if the frequency step is small and the integer or the
vco switch settings do not change, then these registers may also be left constant. In all cases, in fractional mode, it is
necessary to write to the fractional register for frequency changes.
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