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ML2721

Low IF Digital Cordless Transceiver

Micro Linear Corporation 

PRELIMINARY

MODES OF OPERATION (CONTINUED)

ILO

DATA FILTER

QLO

F

V

∫

ML2721

FREF or

FREF/2

CAL I

TONE

GEN

Q

FCOMP

TRACKING

COUNTER

Figure 2. Dual Modulus Signal Divider

The signal flow in RECEIVE mode is from the RF input,

through an image reject mixer, IF filter, hard limiter,

Frequency to Voltage converter, data filter and data slicer.

The ML2721 uses a single conversion super heterodyne

receiver with a nominal IF of 1.024 MHz. An AGC

subsystem extends the dynamic range of the receiver. The

IF frequency and filter bandwidths are given in Table 3.

Reference Division Ratio

6

12

Parameter

RD0 = 0 RD0 = 1

IF filter center frequency

REF/6 REF/12

IF filter bandwidth

REF/4.4 REF/8.8

Receive data filter

REF/8 REF/16

F to V converter center frequency REF/6 REF/12

Table 3. Filter Parameters to Division Ratio

Major features of the receiver are:

n High dynamic range mixers with 10dB noise figure, –

15dBm IP3, 35dB image rejection (typical)

n 6-pole bandpass IF filter, with accurate Gaussian to

12dB response

n Center frequency 1.024MHz, Bandwidth 1.408 MHz

(nominal)

n Limiting IF amplifiers with >80dB gain, and excellent

AM rejection

n AGC to extend the dynamic range of the integrated

filters & RSSI

n Logarithmic RSSI output from Limiter; AGC state is

information added to give 80dB range

n FM demodulation by a linear F-to-V converter

n 5-pole low-pass data filter with an accurate Gaussian

response and 3dB cutoff at 768kHz (nominal)

n 2 level data slicer with DC offset removal

The output of the receiver is quantized in amplitude (to 1

bit) but there is no internal timing recovery. Timing

recovery is performed in the external baseband circuits.

FILTER ALIGNMENT

In RXCAL mode the receiver is not functional. Instead, the

ML2721 filter alignment function tunes all the internal

filters using the reference frequency from the REF pin. See

Figure 2.

The IF filter, data filters, and the F-to-V converter all have

their frequency responses governed by RC time constants.

Every capacitor that affects the frequency response

includes a binary weighted capacitor array controlled by

a 7 bit tuning bus. In RXCAL mode a reference tone at the

IF center frequency is routed to the F-to-V converter. A

digital tracking loop then adjusts the tuning word until

there is mid-rail output from the F-to-V converter. This

tracking loop may take up to 300µs to tune the filter when

XCEN is asserted, but the tracking loop will continue to

make small adjustments whenever the ML2721 is in

RXCAL mode. Because all the filters in the chip are tuned

in this manner, centering the F-to-V converter sets up the

correct center frequencies and bandwidths for all the

filters.

The ML2721 must be placed in RXCAL mode for 300µs

whenever XCEN is asserted to give time for the filters to

align completely.

Data Slicer

The data slicer is a comparator that is AC coupled to the

receive data filter output. The output is logic high or logic

low. This circuit is designed to rapidly acquire valid data

at the beginning of a received packet of data. The

nominal time constant for the AC coupling is 9µs. This

limits the maximum recommended run length to four ones

or zeroes at 1.3Mbps. Longer run lengths require the use

of a more complex external data slicer circuit.

Data Output Drive

The ML2721 DOUT pin is designed to drive a PCB trace

and a single logic input with controlled slew rates. Buffer

the output when driving any logic load greater than 5pF

(i.e., more than an ´10 oscilloscope probe or a single

CMOS logic input pin).

January, 2000 PRELIMINARY DATASHEET

11

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