January
1999, Issue 102
Wires,
Wires Everywhere
The
RF Solution
MICROFUN
The
MICRF001 (I don’t know if Micrel intended it, but I
find myself saying "microphone") is a complete
radio receiver on a single chip. As stated in the datasheet,
you don’t have to be an RF expert to design it in (or,
fortunately for me, to write about it).
Again
quoting the datasheet, the chip is a true antenna-in,
data-out monolithic device. As Figure 1 shows, building
a complete receiver around the MICRF001 requires as
little as adding a crystal (possibly getting away with
an even lower cost ceramic resonator) and two capacitors.
|
(Click
here to enlarge)
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Figure
1—What does it take to get on the air with the MICRF001?
Little more than a clock reference (crystal, resonator,
or external input) and a couple of capacitors. |
The
specs reflect targeted applications like keyless entry,
security systems, garage-door openers, and so on. Operating
in the 300- to 440-MHz (UHF) frequency band, the chip
can typically receive data at up to 4.8 kbps over 100
m. The exact data rate depends on the RF frequency selected
and one of four selections made by jumpering the SEL0
and SEL1 pins.
For
instance, if the frequency is 418 MHz, data-rate options
are 4.8, 2.4, 1.2, and 0.6 kbps. As for range, actual
results depend heavily on antenna design (more on this
later) and, of course, the presence or absence of interference
and obstacles. Performance also depends on transmission
characteristics such as the presence or absence of a
preamble, minimum pulse width, and such (more on this
later, too).
Figure
2 shows all the components of a classic radio receiver
design on-chip. Starting at the ANT (antenna) input,
the raw RF (ftx) is downconverted to a lower intermediate
frequency (fif) using a mixer in conjunction with a
programmable synthesizer also known as the LO (local
oscillator, flo).
|
(Click
here to enlarge)
|
Figure
2—The MICRF001 integrates all the components needed
to grab RF from an antenna connected to the ANT
pin and deliver digital data out the DO pin.
|
The
downconverted data is amplified, subjected to automatic
gain control (AGC), and passed to the demodulator section
where it’s filtered and sliced into good old 1s and
0s (i.e., baseband) for delivery out the data out (DO)
pin.
I
just described a classic superheterodyne (SH) receiver.
The MICRF001 can work in an even simpler super-regenerative
(SR, or homodyne) mode that dispenses with the need
for the LO because conversion is direct to baseband
from RF without an intermediate step.
The
MICRF001 works with on-off key (OOK) modulation in which
the transmitter simply turns the RF carrier on and off,
rather than modulating its amplitude (AM), frequency
(FM), or phase (PM).
SH
and SR schemes each exhibit relative advantages and
disadvantages. An SR transmitter doesn’t call for especially
high transmit-frequency accuracy (e.g., the transmitter
can use a cheap LC oscillator). So, the SR transmitter
is usually only appropriate for applications where receiver
frequency can be manually tuned.
In
contrast, SH setups require accurate timing as well
as crystals or especially accurate SAW resonators. The
benefit is that you can dispense with the need for manual
tuning.
Micrel
has managed to combine the best of both the SH and SR
worlds. For example, the device can be configured in
sweep or fixed modes by jumpering the sweep-enable (SWEN)
pin appropriately. Sweep mode varies the LO symmetrically
to broaden the RF bandwidth permitting operation with
drift-prone LC-based transmitters.
Of
course, being less selective about what’s received implies
more susceptibility to interference. But if there’s
no need to achieve accurate timing, as when a crystal-based
reference already exists in the system, conventional
fixed mode is OK.
