HMC704LP4E Datasheet, PDF (25/44 Pages) Hittite Microwave Corporation – 8 GHz fractionaL-N PLL

v04.0215
HMC704LP4E
8 GHZ FRACTIONAL-N PLL
“Reg 0Bh”[4]=0 sets the phase detector polarity for use with a passive loop filter together with a VCO with a positive
tuning slope (increasing tuning voltage increases VCO frequency).
“Reg 0Bh”[4] = 1 inverts the phase detector polarity. This is most often used if an inverting op-amp is used in an active
loop filter together with a VCO with a positive tuning slope.
Charge Pump Tri-state
“Reg 0Bh”[5]=”Reg 0Bh”[6]=0 tri-states the charge pump. This effectively freezes charge on the loop filter and allows
the VCO to run open loop.
Charge Pump Gain
“Reg 09h”[6:0] and “Reg 09h”[13:7] program current gain settings for the charge pump. Pump ranges can be set from
0uA to 2.54 mA in 20uA steps. Charge pump gain affects the loop bandwidth. The product of VCO gain (K_vco) and
charge pump gain (K_cp) can be held constant for VCO’s that have a wide ranging K_vco by adjusting the charge pump
gain. This compensation helps to keep the loop bandwidth constant.
In addition to the normal CP current as described above, there is also an extra output source of current that offers im-
proved noise performance. HiK_cp provides an output current that is proportional to the loop filter voltage. This being the
case HiK_cp should only be operated with active op-amp loop filters that define the voltage as seen by the charge pump
pin. With 2.5V as observed at the charge pump pin, the HiK_cp current is 3.5 mA.
There are several configurations that could be used with the HiK_cp feature. For lowest noise, HiK_cp could be used with-
out the normal charge pump current (the charge pump current would be set to 0). In this case, the loop filter would be
designed with 3.5 mA as the effective charge pump current.
Another possible configuration is to operate with both the HiK_cp and normal charge pump current sources. In this case
the effective charge pump current would be 3.5 mA + programmed normal charge pump current which could offer a
maximum of 6 mA.
With passive loop filters the voltage seen by the charge pump pin will vary which would cause the HiK_cp current to vary
widely. As such, HiK_cp should not be used on passive loop filter implementations.
A simplified diagram of the charge pump is shown in Figure 34. The current gain of the pump in Amps/radian is
equal to the gain setting of this register divided by 2π.
Charge Pump Offset
“Reg 09h”[20:14] controls the charge pump current offsets. “Reg 09h”[21] and “Reg 09h”[22] enable the UP and DN
offset currents respectively. Normally only one is used at a time. As mentioned earlier charge pump offsets affect
fractional mode linearity and the Lock Detect window selection.
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