micrf506 Micrel Semiconductor, micrf506 Datasheet - Page 17

• Manufacturer Part Number: micrf506
• Description: 410mhz And 450mhz Ism Band Transceiver
• Manufacturer: Micrel Semiconductor

Receiver
The receiver is a zero intermediate frequency (IF)
type in order to make channel filtering possible with
low-power integrated low-pass filters. The receiver
consists of a low noise amplifier (LNA) that drives a
quadrature mixer pair. The mixer outputs feed two
identical signal channels in phase quadrature. Each
channel include a pre-amplifier, a third order Sallen-
Key RC lowpass filter from strong adjacent channel
signals and finally a limiter. The main channel filter is
a switched-capacitor implementation of a six-pole
elliptic lowpass filte. The elliptic filter minimizes the
total capacitance required for a given selectivity and
dynamic range. The cut-off frequency of the Sallen-
Key RC filter can be programmed to four different
frequencies: 100kHz, 150kHz, 230kHz and 340kHz.
The demodulator demodulates the I and Q channel
outputs and produces a digital data output. If detects
the relative phase of the I and Q channel signal. If
the I channel signal lags the Q channel, the FSK
tone frequency lies above the LO frequency (data
‘1’). If the I channel leads the Q channel, the FSK
tone lies below the LO frequency (data ‘0’). The
output of the receiver is available on the DataIXO
pin. A RSSI circuit (receive signal strength indicator)
indicates the received signal level.
Front End
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 dominating the overall
front-end noise performance. The LNA is a two-
stage amplifier and has a nominal gain of
approximately 23dB at 434MHz. The front end has a
gain of about 35dB to 38dB. The gain varies by 1-
1.5dB over a 2.0V to 2.5V variation in power supply.
The LNA can be bypassed by setting bit LNA_by to
‘1’. This can be useful for very strong input signal
levels. The front-end gain with the LNA bypassed is
about 12dB. The mixers have a going of about 10dB
at 434MHz. The differential outputs of the mixers
can be made available at pins IchOut and QchOut.
The output impedance of each mixer is about 8k_.
The input impedance is close to 50_ as shown in
Figure 12, giving an input reflection of about -20dB.
The receiver does not require any matching network.
0000000
A6..A0
Micrel
September 2004
LNA_by
D7
PA2
D6
PA1
D5
PA0
D4
Sync_en
D3
Mode1
D2
Mode0
D1
Load_en
D0
17
0000001
A6..A0
Sallen-Key Filters
Each channel includes a pre-amplifier and a prefilter,
which is a three-pole Sallen-Key lowpass filter. It
protects the following switched-capacitor filter from
strong adjacent channel signals, and it also works as
an anti-aliasing filter. The preamplifier has a gain of
22.23dB. The maximum output voltage swing is
about 1.4Vpp for a 2.25V power supply. In addition,
the IF amplifier also performs offset cancellation.
Gain varies by less than 0.5dB over a 2.0 – 2.5V
variation in power supply. The third order Sallen-Key
lowpass filter is programmable to four different cut-
off frequencies according to the table below:
Modulation1
PF_FC1
D7
0
0
1
1
Figure 12. LNA Input Impedance
Modulation0
D6
PF_FC0
0
1
0
1
D5
‘0’
D4
‘0’
RSSI_en
D3
Cut-off Freq. (kHz)
LD_en
[phone]
M9999-092904
D2
100
150
230
340
MICRF506
PF_FC1
D1
PF_FC0
D0