HMC704LP4E【PDF 18/44 页】IC FRACT-N PLL 16BIT 24QFN

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Sheet目录1996

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5 - 25

HMC704LP4E

v03.1211

8 GHz fractionaL-n PLL

For price, delivery, and to place orders: Hittite Microwave Corporation,20 Alpha Road, Chelmsford, MA 01824

Phone: 978-250-3343

Fax: 978-250-3373

Order On-line at www.hittite.com

Application Support: Phone: 978-250-3343 or apps@hittite.com

PD Polarity

“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 2.54mA in 20uA steps. Charge pump gain affects the loop bandwidth. The product of VCO gain (Kvco) and

charge pump gain (Kcp) can be held constant for VCO’s that have a wide ranging Kvco 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. HiKcp provides an output current that is proportional to the loop filter voltage. This being the

case HiKcp 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 HiKcp current is 3.5mA.

There are several configurations that could be used with the HiKcp feature. For lowest noise, HiKcp 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 3.5mA as the effective charge pump current.

Another possible configuration is to operate with both the HiKcp 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 6mA.

With passive loop filters the voltage seen by the charge pump pin will vary which would cause the HiKcp current to vary

widely. As such, HiKcp 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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