ML2712CH Просмотр технического описания (PDF) - Micro Linear Corporation

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ML2712CH

2.4GHz RF Transceiver

Micro Linear Corporation 

PRELIMINARY

ML2712

MODES OF OPERATION (CONTINUED)

Any modulation on SMOD is duplicated at the final

frequency, provided is it inside the control loop

bandwidth. The spectrum of SMOD and the final output

frequency are shown in Figure 4.

SMOD

TRANSMIT

SIGNAL

PSD

1IF

F1LO

1IF

FREQUENCY

Figure 4. Simplified Spectrum of Directly

Modulated VCO Transmitter

The PLL loop bandwidth is at least four times that of the

modulation rate. Meeting the IEEE802.11 FHSS

specification for 1Msymbol/sec requires a PLL control

loop bandwidth greater than 4MHz. To achieve the

Wideband PLL dynamics and the RF channel spacing

requirements, the Transmit VCO is down converted with

the 1LO signal (at Frequency F1LO). The RF channel

spacing can be achieved by stepping the 1LO VCO. This

allows the same VCOs and PLLs to be used in both

Transmit and Receive Modes. For typical WLAN operation

the receive frequency and transmit frequency are the same

(between frequency hops). Therefore the 1LO (and 2LO)

frequencies do not require re-tuning when switching from

transmit to receive. As a result the transmit to receive turn-

round time is very rapid. It is determined by the power on,

settling, and lock up times of the Wideband PLL. For

802.11 FH systems the requirement is less than 2msec.

TRANSMITTER PLL

The transmitter wideband PLL is shown in Figure 5.

An external VCO provides a nominal 2.45GHz signal

which is coupled to the Transmit RF mixer using a

directional coupler or similar external circuit. The

transmiter RF mixer is used to down-convert the transmiter

VCO signal using the frequency doubled 1LO VCO to

TO TX POWER

AMPLIFIER

VCO AND LOOP FILTER

COUPLER

TX RF

MIXER

TANK

RX 1LO

VCO AND

PLL

SAW IF

DIGITALLY

GENERATED

MODULATED

TONE

ANTI ALIAS

FILTER

TANK

2LO VCO

AND PLL

ML2712

ML2713

Figure 5. Wideband Phase Locked

produce a 260MHz (nominal) signal. This signal is fed via

a low pass filter to the Wideband Phase Comparator where

is it compared to the Transmit IF reference signal, supplied

by the ML2713. The Wideband Phase Comparator output

controls the transmit VCO via an external loop filter. The

bandwidth of the Wideband Phase Comparator is high

enough to enable a relatively low tolerance Transmit VCO

to be used.

The Transmit RF input is a single ended design with a

nominal 50 ohms input impedance. The Transmit RF down-

converter and Wideband PLL are only enabled in

TRANSMIT Mode.

The Wideband PLL is designed to ensure the Transmit

VCO is within the pull-in range of the Wideband PLL

(so it will lock when transmitting), to achieve

required lock up time, and to set the loop

bandwidth. The Wideband PLL loop bandwidth

requirement is 4 times the symbol rate at a minimum.

The ML2712 is capable of loop bandwidth greater

than 5MHz. Since the bandwidth of the Wideband PLL

can be >4MHz, the lock up time for 802.11

applications is typically less than 2msec. The maximum

frequency pull-in range for the transmit VCO is

500MHz to ensure that the down-converted signal

passes through the low pass filter.

Design of the ML2712 enables the large pull-in frequency

in the Wideband PLL. When TS (Pin 47)is asserted the

Wideband charge pump output is first clamped to mid-

rail. A PFD detector is then used to ensure frequency

lock. Finally the output switches to an XOR detector for

accurate phase tracking. After TS is asserted WBCP is

clamped to a nominal voltage of mid rail of VQWB (the

charge pump supply voltage) for 0.25msec for 16MHz and

32MHz at the reference input, or 0.2msec for 20MHz or

40MHz. This pulls the VCO to mid range.

When the clamp is disabled a phase frequency detector

(PFD) pulls the VCO to frequency lock for 1msec with 16

MHz and 32MHz reference input, or 0.8msec with 20MHz

or 40MHz reference input. This rapidly pulls the VCO to

frequency lock.

If the Wideband bit (Control Word B b11) is set low, the

PFD is then disabled and an XOR phase detector is used

until the TS signal is de-asserted. A factor of four

difference between the PFD and XOR charge pump

currents keeps KD, the phase detector gain, constant. The

PFD charge pump current is nominally 2mA and the XOR

charge pump current is nominally 0.25mA.

A lock detect output from the Wideband PLL

indicates Frequency Lock. An indication of the

transmitter not locked is required by some regulatory

authorities.

January, 2000 PRELIMINARY DATASHEET

9

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