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MICRF500
Transmit
Power Amplifier (PA)
The power amplifier is biased in class AB. The last stage has
an open collector, and an external load inductor (L2) 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 ? bias resistor which
gives a bias current of approximately 50 μ A. This will give a
bias current in the last stage of about 15mA.
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 ? . 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 μ A, which makes the on/off rate for a
3.0V power supply 2.6 μ s/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
Micrel
Figure 8. Input Impedance
Sallen-Key Filter and Preamplifier
Each channel includes a preamplifier and a prefilter, which is
a three-pole elliptic Sallen-Key low pass filter with 20dB
stopband attenuation. It protects the following gyrator filter
from strong adjacent channel signals. The preamplifier has a
gain of 20dB when bit Gc = 0 and 30dB when bit Gc = 1. The
output voltage swing is about 200mV PP for the 30dB gain
setting and 1V PP for the 20dB gain setting.
The third order Sallen-Key low pass filter is programmable to
four different cut-off frequencies according to the table below:
Front End (LNA and Mixers)
A low noise amplifier in RF receivers 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 23dB at
900MHz. The LNA has a dc feedback loop, which provides
Fc1
0
0
1
1
Fc0
0
1
0
1
Cut-Off Frequency
(kHz)
10 ± 2.5
30 ± 7.5
60 ± 15
200 ± 50
Recommended
Channel Spacing
25kHz
100kHz
200kHz
700kHz
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. Input matching is very important to get high receive
sensitivity.
The LNA can be bypassed by setting bit ByLNA to ‘ 1 ’ . This is
useful for very strong signal levels.
The RSSI signal can be used to drive a microcontroller in a
way when a strong RF income signal is present the LNA can
be bypassed. This will increase the dynamic range by ap-
proximately 25dB.
The mixers have a gain of about 12dB at 900MHz. 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 15k ? .
For the 10kHz cut-off frequency the first pole must be gener-
ated externally by connecting a 820pF capacitor between the
outputs of each mixer. For the 30kHz cut-off frequency a
68pF capacitor is needed between the outputs.
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.
Gyrator Filter
The main channel filter is a gyrator capacitor implementation
of a seven-pole elliptic low pass filter. The elliptic filter
minimizes the total capacitance required for a given selectiv-
ity and dynamic range. An external resistor can adjust the cut-
off frequency of the gyrator filter. The following table shows
how the cut-off frequency varies with bias resistor:
MICRF500
12
March 2003
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