MICRF007 Просмотр технического описания (PDF) - Micrel

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MICRF007

QwikRadio® Low-Power UHF Receiver

Micrel 

MICRF007

appropriate fLO for a given fTX:

fLO = fTX± ���1.18 43f3TX.92���

(1)

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. 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 reference oscillator frequency

fT:

fT

=

fLO

64.5

(2)

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

REFOSC on the MICRF007. Four-decimal-place accuracy

on the frequency is generally adequate. The following table

identifies fT for some common transmit frequencies.

Transmit

Frequency fTX

315MHz

Reference

Oscillator Frequency fT

4.8970MHz

390MHz

6.0630MHz

418MHz

6.4983MHz

433.92MHz

6.7458MHz

Table 2. Recommended Reference Oscillator Values

for Typical Transmit Frequencies (high-side mixing)

Step 2: Selecting CTH Capacitor

Extraction of the DC value of the demodulated signal for

purposes of logic-level data slicing is accomplished using the

external threshold capacitor CTH and the on-chip switched

capacitor “resistor” RSC, shown in the block diagram.

Slicing level time constant values vary somewhat with de-

coder type, data pattern, and data rate, but typically values

range from 5ms to 50ms.This issue is covered in more detail

in “Application Note 22.” Optimization of the value of CTH is

required to maximize range.

τ of 5x the bit-rate is recommended. The effective resistance

of RSC is listed in the electrical characteristics table as 110kΩ

at 433.92MHz, This value scales inversely with frequency.

Source impedance of the CTH pin at other frequencies is

given by equation (3), where fT is in MHz:

6.7458

RSC = 110kΩ fT

(3)

Since slicing level time constant τ has been established

as 5 times bit rate, capacitor CTH may be computed using

equation (4),

CTH=



RSC

(4)

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

cient. Refer to “Application Hint 42” for CTH and CAGC selec-

tion examples.

Micrel

Step 3: Selecting CAGC Capacitor

The signal path has automatic gain control (AGC) to increase

input dynamic range. The attack time constant of the AGC is

set externally by the value of the CAGC capacitor connected

to the CAGC pin of the device. To maximize system range,

it is important to keep the AGC control voltage ripple low,

preferably under 10mVPP once the control voltage has at-

tained its quiescent value. For this reason, capacitor values

of at least 0.47µF are recommended.

The AGC control voltage is carefully managed on-chip to al-

low duty-cycle operation of the MICRF007. When the device

is placed into shutdown mode (SHUT pin is pulled high), the

AGC capacitor floats to retain the voltage. When operation

is resumed, only the voltage droop due to capacitor leakage

must be replenished. A relatively low-leakage capacitor is

recommended when the devices are used in duty-cycled

operation.

To further enhance duty-cycled operation, the AGC push

and pull currents are boosted for approximately 10ms im-

mediately after the device is taken out of shutdown. This

compensates for AGC capacitor voltage droop and reduces

the time to restore the correct AGC voltage. The current is

boosted by a factor of 45.

Selecting CAGC Capacitor in Continuous Mode

A CAGC capacitor in the range of 0.47µF to 4.7µF is typically

recommended. Caution! If the capacitor is too large, the

AGC may react too slowly to incoming signals. AGC set-

tling time from a completely discharged (zero-volt) state

is given approximately by this equation:

∆t = 1.333 × CAGC –0.44

(5)

where:

CAGC is in µF, and ∆t is in seconds.

Selecting CAGC Capacitor in Duty-Cycle Mode

Voltage droop across the CAGC capacitor during shutdown

should be replenished as quickly as possible after the IC is

enabled. As mentioned above, the MICRF007 boosts the

push-pull current by a factor of 45 immediately after start-up.

This fixed time period is based on the reference oscillator

frequency fT. The time is 10.9ms for fT = 6.00MHz, and varies

inversely with fT. The value of CAGC capacitor and the duration

of the shutdown time period should be selected 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. The worst-case from a recovery

standpoint is downward droop, since the AGC pull-up current

is 1/10th magnitude of the pull-down current. The downward

droop is replenished according to the Equation 6:

CAGC=

It

V

(6)

where:

I = AGC pull-up current for the initial 10ms (67.5µA)

CAGC = AGC capacitor value

∆t = droop recovery time

February 17, 2005

7

M9999-021705

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