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LANSDALE Semiconductor, Inc.

ML12179

The ML12179 is intended for applications where a fixed local

oscillator is required to be synthesized. The prescaler on the

ML12179 operates up to 2.8GHz which makes the part ideal for

many satellite receiver applications as well as applications in the

2nd ISM (Industrial, Scientific, and Medical) band which covers

the frequency range of 2400MHz to 2483MHz. The part is also

intended for MMDS (Multi–channel Multi–point Distribution

System) block downconverter applications. Below is a typical block

diagram of the complete PLL.

As can be seen from the block diagram, with the addition of a

VCO, a loop filter, and either an external oscillator or crystal, a

complete PLL sub–system can be realized. Since most of the PLL

function is integrated into the ML12179, the user's primary focus is

on the loop filter design and the crystal reference circuit. Figure 13

and Figure 14 illustrate typical VCO spectrum and phase noise

characteristics. Figure 17 and Figure 18 illustrate the typical input

impedance versus frequency for the prescaler input.

Crystal Oscillator Design

The ML12179 is used as a multiply–by–256 PLL circuit which

transfers the high stability characteristic of a low frequency refer-

ence source to the high frequency VCO in the PLL loop. To facili-

tate this, the device contains an input circuit which can be config-

ured as a crystal oscillator or a buffer for accepting an external sig-

nal source.

In the external reference mode, the reference source is AC–cou-

pled into the OSCin input pin. The input level signal should be

between 500–2200 mVpp. When configured with an external refer-

ence, the device can operate with input frequencies down to 2

MHz, thus allowing the circuit to control the VCO down to 512

MHz. To optimize the phase noise of the PLL when used in this

mode, the input signal amplitude should be closer to the upper

specification limit. This maximizes the slew rate of the input signal

as it switches against the internal voltage reference.

In the crystal mode, an external parallel–resonant fundamental

mode crystal is connected between the OSCin and OSCout pins.

This crystal must be between 5.0 MHz and 11 MHz. External

capacitors, C1 and C2 as shown in Figure 1, are required to set the

proper crystal load capacitance and oscillator frequency. The values

of the capacitors are dependent on the crystal chosen and the input

capacitance of the device and any stray board capacitance.

In either mode, a 50k

Ω resistor must be connected between the

OSCin and the OSCout pins for proper device operation. The value

of this resistor is not critical so a 47k

Ω or 51kΩ ±10% resistor is

acceptable.

Since the ML12179 is realized with an all–bipolar ECL style

design, the internal oscillator circuitry is different from more tradi-

tional CMOS oscillator designs which realize the crystal oscillator

with a modified inverter topology. These CMOS designs typically

excite the crystal with a rail–to–rail signal which may overdrive the

crystal resulting in damage or unstable operation. The ML12179

design does not exhibit these phenomena because the swing out of

the OSCout pin is less than 600mV. This has the added advantage

of minimizing EMI and switching noise which can be generated by

rail–to–rail CMOS outputs. The OSCout output should not be used

to drive other circuitry.

The oscillator buffer in the ML12179 is a single stage, high

speed, differential input/output amplifier; it may be considered to

be a form of the Pierce oscillator. A simplified circuit diagram is

seen in Figure 4.

OSCin drives the base of one input of an NPN transistor differ-

ential pair. The non–inverting input of the differential pair is inter-

nally biased. OSCout is the inverted input signal and is buffered by

an emitter follower with a 70

µA pull–down current and has a volt-

age swing of about 600 mVpp. Open loop output impedance is

about 425

Ω. The opposite side of the differential amplifier output

is used internally to drive another buffer stage which drives the

phase/frequency detector. With the 50 k

Ω feedback resistor in

place, OSCin and OSCout are biased to approximately 1.1V below

VCC. The amplifier has a voltage gain of about 15 dB and a band-

width in excess of 150 MHz. Adherence to good RF design and

layout techniques, including power supply pin decoupling, is

strongly recommended.

A typical crystal oscillator application is shown in Figure 1. The

crystal and the feedback resistor are connected directly between

OSCin and OSCout, while the loading capacitors, C1and C2, are

connected between OSCin and ground, and OSCout and ground

respectively. It is important to understand that as far as the crystal

is concerned, the two loading capacitors are in series (albeit

through ground). So when the crystal specification defines a spe-

cific loading capacitance, this refers to the total external (to the

crystal) capacitance seen across its two pins.

This capacitance consists of the capacitance contributed by the

amplifier (IC and packaging), layout capacitance, and the series

combination of the two loading capacitors. This is illustrated in the

equation below:

Legacy Applications Information

Figure 3. Typical Block Diagram of Complete PLL

External Ref

10.0 MHz

ML12179 PLL

φ/Freq

Det

Charge

Pump

2560.00 MHz

Loop

Filter

VCO

P

256

Figure 4. Simplified Crystal Oscillator/Buffer Circuit

OSCin

Bias

Source

VCC

OSCout

To Phase/

Frequency

Detector