micrf501 Micrel Semiconductor, micrf501 Datasheet - Page 12

• Manufacturer Part Number: micrf501
• Description: 300mhz To 600mhz Radiowire? Rf Transceiver
• Manufacturer: Micrel Semiconductor

MICRF501
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 1.5kΩ bias resistor which
gives a bias current of approximately 50µA. This will give a
bias current in the last stage of about 15mA. R14 is optimized
to 1kΩ, 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 100Ω. 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 1.1µA, which makes the on/off rate for a
2.8V power supply 2.6µs/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
MICRF501
12
stabilizes the overall dc feedback loop, which has a large low
frequency loop gain. Figure 8 shows the input impedance of
the LNA.
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 30kΩ.
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
The third order Sallen-Key lowpass filter is programmable to
four different cut-off frequencies according to the table below:
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.
Fc1
0
0
1
1
Fc0
0
1
0
1
Figure 8. Input Impedance
Cut-Off Frequency
200 ±50
10 ±2.5
30 ±7.5
60 ±15
(kHz)
Channel Spacing
Recommended
100kHz
200kHz
700kHz
25kHz
March 2003
Micrel
PP
.