QwikRadio

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88

December 1998b

MICRF011

MICRF011

Micrel

[By adding resistance from the CAGC pin to VDDBB or

VSSBB in parallel with the CAGC capacitor, the ratio of

decay-to-attack time-constant may be varied, although the

value of such adjustments must be studied on a per-

application basis.

Generally the design value of 10:1 is

adequate for the vast majority of applications.]

See

“Application Note 22”.

To maximize system range, it is important to keep the AGC

control voltage ripple low, preferably under 10mVpp once

the control voltage has attained its quiescent value. For this

reason capacitor values

≥ 0.47uF are recommended.

Reference Oscillator (REFOSC) and External Timing

Element

All timing and tuning operations on the MICRF011 are

derived from the REFOSC function.

This function is a

single-pin Colpitts-type oscillator. The user may handle this

pin in one of three possible ways:

(1) connect a ceramic resonator, or

(2) connect a crystal, or

(3) drive this pin with an external timing signal.

The third approach is attractive for further lowering system

cost if an accurate reference signal exists elsewhere in the

system (e.g., a reference clock from a crystal or ceramic

resonator-based microprocessor).

An externally applied

signal should be AC-coupled, and resistively-divided down

(or otherwise limited) to approximately 0.5Vpp. The specific

reference frequency required is related to the system

transmit frequency, and the operating mode of the device as

set by the SWEN control pin.

See “Application Note 22,

MICRF001 Theory of Operation” for a discussion of

frequency selection and accuracy requirements.

MICRF011 Frequency and Capacitor Selection

Selection of the REFOSC frequency ft, Slicing Level (CTH)

capacitor, and AGC capacitor are briefly summarized in this

section.

Please see Application Note 22 for complete

details.

1. Selecting REFOSC Frequency ft (FIXED Mode)

As with any superheterodyne receiver, the difference

between the (internal) Local Oscillator (LO) frequency flo

and the incoming Transmit frequency ftx must ideally equal

the IF Center frequency.

Equation (1) may be used to

compute the appropriate flo for a given ftx:

flo = ftx

± 1.064 * (ftx / 390)

(1)

where 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”, and there is generally no preference

of one over the other.

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

equation (2) to compute the REFOSC frequency ft:

ft = flo / 64.5.

(2)

Here ft is in MHz. Connect a crystal of frequency ft to the

REFOSC pin of the MICRF011. 4 decimal-place accuracy

on the frequency is generally adequate. The following table

identifies ft for some common Transmit frequencies when

the MICRF011 is operated in FIXED mode.

Transmit Freq. ftx (MHz)

REFOSC Freq. ft (MHz)

315

418

433.92

4.8970

6.4983

6.7458

2.

Selecting REFOSC Frequency ft (SWP Mode)

Selection of REFOSC frequency ft in SWP mode is much

simpler than in FIXED mode, due to the LO sweeping

process. Further, accuracy requirements of the frequency

reference component are significantly relaxed.

In SWP mode, ft is given by equation (3):

ft = ftx / 64.25.

(3)

Connect a ceramic resonator of frequency ft to the REFOSC

pin of the MICRF011. 2-decimal place accuracy is generally

adequate. (A crystal may also be used if desired, but may

be necessary to reduce the Rx frequency ambiguity if the Tx

frequency ambiguity is excessive. See Application Note 22

for further details.)

3. Selecting Capacitor CTH

First step in the process is selection of a Data Slicing Level

timeconstant.

This selection is strongly dependent on

system issues, like system decode response time and data

code structure (e.g., existence of data preamble, etc.). This

issue is too broad to discuss here, and the interested reader

should consult the Application Note 22.

Source impedance of the CTH pin is given by equation (4),

where ft is in MHz:

Rsc = 118k

Ω * (4.90 / ft).

(4)

Assuming that a Slicing Level Timeconstant TC has been

established,

capacitor

CTH

may

be

computed

using

equation (5):

CTH = TC / Rsc.

(5)