© 2019 Infinite Electronics, Inc. Pasternack is a registered trademark of Infinite Electronics, Inc.
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a) ramp_busy is readable via read-only register
<5>, and
b) ramp_busy may also be monitored on GPIO2 (hardware pin 2) by setting
<3:0> = 8.
4.4.5 Autosweep Mode
The autosweep mode is similar to the two-way sweep shown in (figure) except that triggers are not required.
The autosweep mode is enabled by setting ramp_repeat_en (
<3>); once enabled the autosweep
initiates the first trigger, steps the number of times given by ramp_steps_number (
) at a rate of
one step per divided reference clock cycle. It then waits the dwell time of ramp_dwell_time (
periods of the divided reference clock, and then automatically triggers the ramp in the opposite direction.
The sweep process continues, alternating sweep directions, until disabled.
4.5 Charge Pump
The up and down charge pumps of the synthesizer can each be adjusted to trade between fractional mode
spurious levels and phase noise performance. Optimal values will vary across frequency and are best
determined empirically for a particular application.
4.5.1 Charge Pump Gain
Up and down charge pump gains are set by cp_UPcurrent_sel and cp_DNcurrent_sel respectively (
). Normally the registers are set to the same value. Each of the UP and DN charge pumps consist of 5-
bit charge pumps with lsb of 125 μA. The current gain of the pump,
in Amps/radian, is equal to the gain
setting of this register divided by 2π.
For example if both cp_UPcurrent_sel and cp_DNcurrent_sel are set to ’01000’ h the output current of each
pump will be 1 mA and the gain Kp = 1 mA/2π radians, or 159 μA/rad.
Optimum phase noise performance is generally obtained using low gain at lower VCO frequencies and high
gain at higher VCO frequencies.
4.5.2 Charge Pump Gain Trim
In most applications Gain Trim is not used. However it is available for special applications.
Each of the UP and DN pumps may be trimmed separately to more precise values to improve current
source matching of the UP and DN values, or to allow finer control of pump gain.
The pump trim controls are 3-bits, binary weighted for UP and DN, in cp_UPtrim_sel and cp_DNtrim_sel
respectively (
). LSB weight is 14.7 μA, 000h = 0 trim, 001h = 14.7 μA added trim, 111 h = 102.9
μA.
4.5.3 Charge Pump Phase Offset
Either of the UP or DN charge pumps may have a DC leakage or “offset” added. The leakage forces the
phase detector to operate with a phase offset between the reference and the divided VCO inputs. It is
recommended to operate with a phase offset when using fractional mode to reduce non-linear effects from
the UP and DN pump mismatch. Phase noise in fractional mode is strongly affected by charge pump offset.
In addition, reference spurs in both integer and fractional mode are affected by the offset.
