MICRF501 datasheet(12/18 Pages) MICREL

12 / 18 page

MICRF501

Micrel

MICRF501

March 2003

signals. The receive mode loop filter comprises C115, C116,

R109, R101 and C101.

Modulation Outside PLL (Open Loop)

In this mode the charge pump output is tri-stated. The loop is

open and will therefore not track the modulation. This means

that the loop filter can have a relatively high bandwidth, which

give short switching times. However, the loop voltage will

decrease with time due to current leakage. The transmit time

will therefore be limited and is dependent on the bandwidth of

the loop filter. High bandwidth gives low capacitor values and

the loop voltage will decrease faster, which gives a shorter

transmit time.

The loop is closed until the PLL is locked on the desired

frequency and the power amplifier is turned on. The loop

immediately opens when the modulation starts. The loop will

not track the modulation, but the modulation still needs to be

DC free due to the AC coupling in the modulation network.

Transmit

Power Amplifier (PA)

The power amplifier is biased in class AB. The last stage has

an open collector, and an external load inductor (L5) is

therefore necessary. The DC current in the amplifier is

adjusted with an external bias resistor (R14). A good starting

point when designing the PA is a 1.5k

Ω bias resistor which

gives a bias current of approximately 50

µA. This will give a

bias current in the last stage of about 15mA. R14 is optimized

to 1k

Ω, as shown in the application circuit.

The impedance matching circuit will depend on the type of

antenna used, but should be designed for maximum output

power. For maximum output power the load seen by the PA

must be resistive and should be about 100

Ω. The output

power is programmable in eight steps, with approximately

3dB between each step. This is controlled by bits Pa2 - Pa0.

To prevent spurious components from being transmitted the

PA should be switched on/off slowly, by allowing the bias

current to ramp up/down at a rate determined by the external

capacitor C25 connected to Pin 24. The ramp up/down

current is typically 1.1

µA, which makes the on/off rate for a

2.8V power supply 2.6

µs/pF. Turning the PA on/off affects the

PLL. Therefore the on/off rate must be adjusted to the PLL

bandwidth.

PA Buffer

A buffer amplifier is connected between the VCO and the PA

to ensure that the input signal of the PA has sufficient

amplitude to achieve the desired output power. This buffer

can be bypassed by setting the bit Gc to 0.

Receive

Front End (LNA and Mixers)

A low noise amplifier in the RF receiver is used to boost the

incoming signal prior to the frequency conversion process.

This is important in order to prevent mixer noise from domi-

nating the overall front end noise performance. The LNA is a

two stage amplifier and has a nominal gain of 25dB at

434MHz. The LNA has a dc feedback loop, which provides

bias for the LNA. The external capacitor C26 decouples and

stabilizes the overall dc feedback loop, which has a large low

frequency loop gain. Figure 8 shows the input impedance of

the LNA.

Figure 8. Input Impedance

Input matching is very important to get high receive sensitiv-

ity. The LNA can be bypassed by setting bit LNA to ‘1’. This

is useful for very strong signal levels. The RSSI signal can be

used to drive a microcontroller to create a subroutine when a

strong income signal is present to bypass the LNA. This will

increase the dynamic range by approximately 25dB.

The mixers have a gain of about 15dB at 434MHz. The

differential outputs of the mixers are available at Pins 34, 35

and at Pins 38, 39. The output impedance of each mixer is

about 30k

Ω.

Sallen-Key Filter and Preamplifier

Each channel includes a preamplifier and a prefilter, which is

a three-pole elliptic Sallen-Key lowpass filter with 20dB

stopband attenuation. It protects the following gyrator filter

from strong adjacent channel signals. The preamplifier has a

gain of 35dB and output voltage swing is about 200mV

PP.

The third order Sallen-Key lowpass filter is programmable to

four different cut-off frequencies according to the table below:

Fc1

Fc0

Cut-Off Frequency

Recommended

(kHz)

Channel Spacing

±2.5

25kHz

±7.5

100kHz

±15

200kHz

200

±50

700kHz

For the 10kHz cut-off frequency the first pole must be gener-

ated externally by connecting a 330pF capacitor between the

outputs of each mixer.

As the cut-off frequency of the gyrator filter can be set by

varying an external resistor, the optimum channel spacing

will depend on the cut-off frequencies of the Sallen-Key filter.

The table above shows the recommended channel spacing

depending on the different bit settings.

MICRF501 Datasheet(PDF) 12 Page - Micrel Semiconductor

MICRF501 Datasheet(HTML) 12 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL