Micrel

MICRF009

January 18, 2005

7

M9999-011805

(408) 955-1690

MICRF009, 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 680kHz 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 1250Hz.

Similar performance is not currently available with crystal-

based superheterodyne receivers, which can operate only

with SAW- or crystal-based transmitters.

In sweep-mode, a range reduction will occur in installation

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 MICRF009 may be

used in place of a super-regenerative receiver in most

applications.

Step 2: Selecting The Reference Oscillator

All timing and tuning operations on the MICRF009 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 crystals are selected. Contact Micrel RF

Applications for suggested suppliers and part numbers.

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

sweep-mode either a crystal or ceramic resonator may be

used. When a crystal or ceramic resonator is used, the

applied signal, it should be AC-coupled and limited to the

(Fixed-Mode)

As with any superheterodynes receiver, the difference

center frequency. Equation 1 may be used to compute the

315

⎛

⎝

⎜

⎞

⎠

⎟

(1)

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. There is

generally no preference of one over the other.

Equation 2 to compute the reference oscillator frequency

64.5

(2)

to REFOSC on the MICRF009. Four-decimal-place

accuracy on the frequency is generally adequate. The

frequencies when the MICRF009 is operated in fixed-

mode.

Reference Oscillator

315MHz

9.7941MHz

390MHz

12.1260MHz

418MHz

12.9966MHz

433.92MHz

13.4916MHz

Table 1. Fixed-Mode Recommended Reference Oscillator

Values For Typical Transmit Frequencies (high-side mixing)

(Sweep Mode)

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.

64.25

(3)

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.

Reference Oscillator