MICRF002BM Ver la hoja de datos (PDF) - Micrel

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componentes Descripción

Fabricante

MICRF002BM

300-440MHz QwikRadio™ASK Receiver

Micrel 

MICRF002/RF022

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 de-

rived 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 capaci-

tors 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 incom-

ing transmit frequency fTX ideally must equal the IF center

frequency. Equation 1 may be used to compute the appropri-

ate fLO for a given fTX:

(1)

fLO

=

fTX

±





0.86

fTX

315





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)

fT

=

fLO

64.5

Micrel

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

315MHz

390MHz

418MHz

433.92MHz

Reference Oscillator

Frequency

fT

4.8970MHz

6.0630MHz

6.4983MHz

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 fre-

quency reference component are significantly relaxed.

In sweep mode, fT is given by Equation 3:

(3)

fT

=

fLO

64.25

In SWEEP mode a reference oscillator with frequency accu-

rate 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

315MHz

390MHz

418MHz

433.92MHz

Reference Oscillator

Frequency

fT

4.88MHz

6.05MHz

6.48MHz

6.73MHz

Table 3. Recommended Reference Oscillator Values

For Typical Transmit Frequencies (sweep-mode)

MICRF002/RF022

8

March 2003

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