MICRF004BM View Datasheet(PDF) - Micrel

Part Name

Description

Manufacturer

MICRF004BM

QwikRadio™ Low-Power VHF Receiver

Micrel 

MICRF004/RF044

Transmit

Frequency

fTX

149.675MHz

184.225MHz

Reference Oscillator

Frequency

fT

4.6318MHz

5.7010MHz

Table 2. Common Transmitter Frequencies

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

32.25

Connect a ceramic resonator of frequency fT to the REFOSC

pin on the MICRF004. Two-decimal-place accuracy is gener-

ally adequate. A crystal may be used. A crystal may be

mandatory in some cases to reduce receive frequency ambi-

guity if the transmit frequency ambiguity is excessive.

Use Equation 3a to compute sweep-mode frequency band

coverage (fBC):

(3a) fBC = 0.5fT + 2fIF + fBW

Example:

fTX = 170MHz

fT = 5.27MHz

fIF

=

170

150

0.86MHz

fBW

=

170

150

0.43MHz

then:

fBC = 5.07MHz

centered symmetrically about 170MHz.

Selecting Capacitor CTH

The first step in the process is selection of a data-slicing-level

time constant. This selection is strongly dependent on sys-

tem issues including system decode response time and data

code structure (that is, existence of data preamble, etc.). This

issue is covered in more detail in Application Note 22.

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

where fT is in MHz:

(4)

RSC

=

124kΩ

4.65

fT

Assuming that a slicing level time constant τ has been

established, capacitor CTH may be computed using equation

(5)

CTH

=

τ

RSC

Micrel

A standard ±20% X7R ceramic capacitor is generally suffi-

cient.

Selecting CAGC Capacitor in Continuous Mode

Selection of CAGC is dictated by minimizing the ripple on the

AGC control voltage by using a sufficiently large capacitor.

Factory experience suggests that CAGC should be in the

vicinity of 0.47µF to 4.7µF. Large capacitor values should be

carefully considered as this determines the time required for

the AGC control voltage to settle from a completely dis-

charged condition. AGC settling time from a completely

discharged (zero-volt) state is given approximately by Equa-

tion 6:

(6) ∆t = 1.333CAGC − 0.44

where:

CAGC is in µF, and ∆t is in seconds.

Selecting CAGC Capacitor in Duty-Cycle Mode

Use of 0.47µF or greater is strongly recommended for best

range performance. Use low-leakage type capacitors (dipped

tantalum, ceramic, or polyester)for duty-cycled operation to

minimize AGC control voltage droop.

Generally, droop of the AGC control voltage during shutdown

should be replenished as quickly as possible after the IC is

“turned-on”. As described in the functional description, for

about 10ms after the IC is turned on, the AGC push-pull

currents are increased to 45 times their normal values.

Consideration should be given to selecting a value for CAGC

and a shutdown time period such that the droop can be

replenished within this 10ms period.

Polarity of the droop is unknown, meaning the AGC voltage

could droop up or down. Worst-case from a recovery stand-

point is downward droop, since the AGC pullup current is

1/10th magnitude of the pulldown current. The downward

droop is replenished according to the Equation 7:

(7)

I = ∆V

CAGC ∆t

where:

I = AGC pullup current for the initial 10ms (67.5µA)

CAGC = AGC capacitor value

∆t = droop recovery time

∆V = droop voltage

For example, if user desires ∆t = 10ms and chooses a 4.7µF

CAGC, then the allowable droop is about 144mV. Using the

same equation with 200nA worst case pin leakage and

assuming 1µA of capacitor leakage in the same direction, the

maximum allowable ∆t (shutdown time) is about 0.56s for

droop recovery in 10ms.

MICRF004

12

February 9, 2000

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