HMC703LP4E Datasheet, PDF (35/58 Pages) Hittite Microwave Corporation – 8 GHZ FRACTIONAL SYNTHESIZER

v02.0813
HMC703LP4E
8 GHz fractional synthesizer
PD Polarity
Reg 0Bh[4]=0 sets the PD 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 PD 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, provided that CP offset is also disabled.
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 0 µA
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
improved 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 2.5 V 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
without 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 3.5mA + 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 39. 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 . Offset polarity should be chosen such that the divided VCO lags the reference signal. This means
down for non-inverting loop filters.
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