MICRF002 datasheet(8/17 Pages) MICREL

8 / 17 page

Micrel, Inc.

MICRF002/RF022

July 2008

M9999-070808

MICRF002, allowing the device to operate in applications

where

significant

transmitter-receiver

frequency

misalignment may exist. The transmit frequency may vary

up to ±0.5% over initial tolerance, aging, and temperature.

In sweep-mode a band approximately 1.5% around the

nominal transmit frequency is captured. The transmitter

may drift up to ±0.5% without the need to retune the

receiver and without impacting system performance.

The swept-LO technique does not affect the IF bandwidth,

therefore noise performance is not degraded relative to

fixed-mode. The IF bandwidth is 430kHz whether the

device is operating in fixed- or sweep-mode. Due to

limitations imposed by the LO sweeping process, the upper

limit on data rate in sweep mode is approximately 5.0kbps.

Similar performance is not currently available with

crystalbased superheterodyne receivers which can operate

only with SAW- or crystal-based transmitters. In sweep-

mode, a range reduction will occur in installations where

there is a strong interferer in the swept RF band. This is

because the process indiscriminately includes all signals

within the sweep range. An MICRF002 may be used in

place of a superregenerative receiver in most applications.

Step 2: Selecting the Reference Oscillator

All timing and tuning operations on the MICRF002 are

derived from the internal Colpitts reference oscillator.

Timing and tuning is controlled through the REFOSC pin in

one of three ways:

1. Connect a ceramic resonator

2. Connect a crystal

3. Drive this pin with an external timing signal

The specific reference frequency required is related to the

system transmit frequency and to the operating mode of

the receiver as set by the SWEN pin.

Crystal or Ceramic Resonator Selection

Do not use resonators with integral capacitors since

capacitors are included in the IC, also care should be taken

to ensure low ESR capacitors are selected. Application Hint

34 and Application Hint 35 provide additional information

and recommended sources for crystals and resonators.

If operating in fixed-mode, a crystal is recommended. In

sweep-mode either a crystal or ceramic resonator may be

used. When a crystal of ceramic resonator is used the

minimum voltage is 300mVPP. If using an externally applied

signal it should be AC-coupled and limited to the operating

range of 0.1VPP to 1.5VPP.

Selecting Reference Oscillator Frequency fT (Fixed-

Mode)

As with any superheterodyne receiver, the mixing

between the internal LO (local oscillator) frequency fLO and

the incoming transmit frequency fTX ideally must equal the

IF center frequency. Equation 1 may be used to compute

the appropriate fLO for a given fTX:

(1)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

315

0.86

Frequencies fTX and fLO are in MHz. Note that two values of

fLO exist for any given fTX, distinguished as “high-side

mixing” and “low-side mixing”. High-side mixing results in

an image frequency above the frequency of interest and

low-side mixing results in a frequency below.

After choosing one of the two acceptable values of fLO, use

Equation 2 to compute the reference oscillator frequency fT:

(2)

64.5

T =

Frequency fT is in MHz. Connect a crystal of frequency fT to

REFOSC on the MICRF002. Four-decimal-place accuracy

on the frequency is generally adequate. The following table

identifies fT for some common transmit frequencies when

the MICRF002 is operated in fixed mode.

Transmit Frequency fTX

Reference Oscillator

Frequency fT

315MHz

4.8970MHz

390MHz

6.0630MHz

418MHz

6.4983MHz

433.92MHz

6.7458MHz

Table 2. Fixed-Mode Recommended Reference Oscillator

Values for Typical Transmit Frequencies

(high-side mixing)

Selecting REFOSC Frequency fT (Sweep-Mode)

Selection of the reference oscillator frequency fT in sweep-

mode is much simpler than in fixed mode due to the LO

sweeping process. Also, accuracy requirements of the

frequency reference component are significantly relaxed.

In sweep-mode, fT is given by Equation 3:

(3)

64.25

T =

In sweep-mode a reference oscillator with frequency

accurate to two-decimal-places is generally adequate. A

crystal may be used and may be necessary in some cases

if the transmit frequency is particularly imprecise.

Transmit Frequency fTX

Reference Oscillator

Frequency fT

315MHz

4.88MHz

390MHz

6.05MHz

418MHz

6.48MHz

433.92MHz

6.73MHz

Table 3. Sweep-Mode Recommended Reference Oscillator

Values for Typical Transmit Frequencies

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