QwikRadio

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99

December 1998b

MICRF011

MICRF011

Micrel

3. Selecting CAGC Capacitor

Selection of CAGC is dictated by minimizing the ripple on

the AGC control voltage, by using a sufficiently large

capacitor. It is Micrel’s experience that CAGC should be in

the vicinity of 0.47

µF 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 discharged condition. AGC settling time from a

completely discharged (0-volt) state is given approximately

by equation (6):

∆T = (1.333 * CAGC) – 0.44

(6)

where CAGC is in microfarads, and

∆T is in seconds.

I/O Pin Interface Circuitry

Interface circuitry for the various I/O pins of the MICRF011 is

shown in Figures 1 through 6.

Specific information

regarding each of these circuits is discussed in the following

sub-paragraphs.

Not shown are ESD protection diodes

which are applied to all input and output pins.

1. ANT Pin

The ANT pin is internally AC-coupled via a 3pF capacitor, to

an RF N-channel MOSFET, as shown in Figure 1.

Impedance on this pin to VSS is quite high at low

frequencies, and decreases as frequency increases. In the

UHF frequency range, the device input can be modeled as

6.3k

Ω in parallel with 2pF (pin capacitance) shunt to

VSSRF.

2. CTH Pin

Figure 2 illustrates the CTH pin interface circuit. CTH pin is

driven from a P-channel MOSFET source-follower biased

with approximately 10µA of bias current.

Transmission

gates TG1 and TG2 isolate the 6.9pF capacitor.

Internal

control signals PHI1/PHI2 are related in a manner such that

the impedance across the transmission gates looks like a

“resistance” of approximately 118k

Ω. The DC potential on

the CTH pin is approximately 1.6V.

3. CAGC Pin

Figure 3 illustrates the CAGC pin interface circuit. The AGC

control voltage is developed as an integrated current into a

capacitor CAGC.

The attack current is nominally 15

µA,

while the decay current is a 1/10th scaling of this,

approximately 1.5

µA. Signal gain of the RF/IF strip inside

the IC diminishes as the voltage on CAGC decreases. By

simply adding a capacitor to CAGC pin, the attack/decay

time constant ratio is fixed at 1:10. Further discussion on

setting the attack time constant is found in “Application Note

22,

MICRF001

Theory

of

Operation”,

section

6.5.

Modification of the attack/decay ratio is possible by adding

resistance from CAGC pin either to VDDBB or VSSBB, as

desired.

4. DO Pin

The output stage for the Data Comparator (DO pin) is shown

in Figure 4. The output is a 10µA push-10µA pull, switched

current stage. Such an output stage is capable of driving

CMOS-type

loads.

An

external

buffer-driver

is

recommended for driving high capacitance loads.

5. REFOSC Pin

The REFOSC input circuit is shown in Figure 5.

Input

impedance is quite high (200k

Ω).

This is a Colpitts

oscillator, with internal 30pF capacitors.

This input is

intended

to

work

with

standard

ceramic

resonators,

connected from this pin to VSSBB, although a crystal may

be used instead, where greater frequency accuracy is

required.

The resonators should not contain integral

capacitors, since these capacitors are contained inside the

IC.

Externally applied signals should be AC-coupled,

amplitude limited to approximately 0.5Vpp. The nominal DC

bias voltage on this pin is 1.4V.

6. Control Inputs (SEL0, SEL1, SWEN)

Control input circuitry is shown in Figure 6. The standard

input is a logic inverter constructed with minimum geometry

MOSFETs (Q2, Q3). P-channel MOSFET Q1 is a large

channel length device which functions essentially as a

“weak” pullup to VDDBB. Typical pullup current is 5

µA,

leading to an impedance to the VDDBB supply of typically

1M

Ω.

Figure 1 ANT Pin