Micrel, Inc.
MICRF002/RF022
MICRF002, allowing the device to operate in applications
the appropriate fLO for a given fTX:
where
significant
transmitter-receiver
frequency
fTX
⎛
⎞
misalignment may exist. The transmit frequency may vary
up to ±0.5% over initial tolerance, aging, and temperature.
In sweep-mode a band approximately 1.5% around the
nominal transmit frequency is captured. The transmitter
may drift up to ±0.5% without the need to retune the
receiver and without impacting system performance.
⎜
⎟
⎟
(1)
fLO = fTX ± 0.86
⎜
315
⎝
⎠
Frequencies fTX and fLO are in MHz. Note that two values of
fLO exist for any given fTX, distinguished as “high-side
mixing” and “low-side mixing”. High-side mixing results in
an image frequency above the frequency of interest and
low-side mixing results in a frequency below.
The swept-LO technique does not affect the IF bandwidth,
therefore noise performance is not degraded relative to
fixed-mode. The IF bandwidth is 430kHz whether the
device is operating in fixed- or sweep-mode. Due to
limitations imposed by the LO sweeping process, the upper
limit on data rate in sweep mode is approximately 5.0kbps.
After choosing one of the two acceptable values of fLO, use
Equation 2 to compute the reference oscillator frequency fT:
fLO
(2)
fT =
64.5
Frequency fT is in MHz. Connect a crystal of frequency fT to
REFOSC on the MICRF002. Four-decimal-place accuracy
on the frequency is generally adequate. The following table
identifies fT for some common transmit frequencies when
the MICRF002 is operated in fixed mode.
Similar performance is not currently available with
crystalbased superheterodyne receivers which can operate
only with SAW- or crystal-based transmitters. In sweep-
mode, a range reduction will occur in installations where
there is a strong interferer in the swept RF band. This is
because the process indiscriminately includes all signals
within the sweep range. An MICRF002 may be used in
place of a superregenerative receiver in most applications.
Transmit Frequency fTX
Reference Oscillator
Frequency fT
315MHz
390MHz
4.8970MHz
6.0630MHz
6.4983MHz
6.7458MHz
Step 2: Selecting the Reference Oscillator
All timing and tuning operations on the MICRF002 are
derived from the internal Colpitts reference oscillator.
Timing and tuning is controlled through the REFOSC pin in
one of three ways:
418MHz
433.92MHz
Table 2. Fixed-Mode Recommended Reference Oscillator
Values for Typical Transmit Frequencies
(high-side mixing)
1. Connect a ceramic resonator
2. Connect a crystal
3. Drive this pin with an external timing signal
Selecting REFOSC Frequency fT (Sweep-Mode)
The specific reference frequency required is related to the
system transmit frequency and to the operating mode of
the receiver as set by the SWEN pin.
Selection of the reference oscillator frequency fT in sweep-
mode is much simpler than in fixed mode due to the LO
sweeping process. Also, accuracy requirements of the
frequency reference component are significantly relaxed.
Crystal or Ceramic Resonator Selection
In sweep-mode, fT is given by Equation 3:
fLO
Do not use resonators with integral capacitors since
capacitors are included in the IC, also care should be taken
to ensure low ESR capacitors are selected. Application Hint
34 and Application Hint 35 provide additional information
and recommended sources for crystals and resonators.
(3)
fT =
64.25
In sweep-mode a reference oscillator with frequency
accurate to two-decimal-places is generally adequate. A
crystal may be used and may be necessary in some cases
if the transmit frequency is particularly imprecise.
If operating in fixed-mode, a crystal is recommended. In
sweep-mode either a crystal or ceramic resonator may be
used. When a crystal of ceramic resonator is used the
minimum voltage is 300mVPP. If using an externally applied
signal it should be AC-coupled and limited to the operating
range of 0.1VPP to 1.5VPP.
Transmit Frequency fTX
Reference Oscillator
Frequency fT
315MHz
390MHz
4.88MHz
6.05MHz
6.48MHz
6.73MHz
Selecting Reference Oscillator Frequency fT (Fixed-
Mode)
418MHz
As with any superheterodyne receiver, the mixing
between the internal LO (local oscillator) frequency fLO and
the incoming transmit frequency fTX ideally must equal the
IF center frequency. Equation 1 may be used to compute
433.92MHz
Table 3. Sweep-Mode Recommended Reference Oscillator
Values for Typical Transmit Frequencies
M9999-070808
July 2008
8
MICREL [ MICREL SEMICONDUCTOR ]
