MICRF010BM View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MICRF010BM Datasheet PDF : 14 Pages
| |||
Micrel
MICRF010
is desired, use crystals with lower ESR, which normally are
bigger in size, like the HC49 package. “Application Hints
35” provides additional information and recommended
sources for crystals. If using an externally applied signal, it
should be AC-coupled and limited to the operating range of
0.2VPP to 1.5VPP.
Selecting Reference Oscillator Frequency fT
As with any super-heterodyne receiver, the difference
between the internal LO (local oscillator) frequency fLO and
the incoming transmit frequency fTX, should equal the IF
center frequency. Equation 1 may be used to compute the
appropriate fLO for a given fTX:
fLO
=
fTX
±
⎛
⎝⎜0.86
fTX
315
⎞
⎠⎟
(1)
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. There is
generally no preference of one over the other.
After choosing one of the two acceptable values of fLO, use
Equation 2 to compute the reference oscillator frequency
fT:
fT
=
2×
fLO
64.5
(2)
Frequency fT is in MHz. Connect a crystal of frequency fT
to REFOSC on the MICRF010. Four-decimal-place
accuracy on the frequency is generally adequate. The
following table identifies fT for some common transmit
frequencies.
Transmit Frequency (fTX)
Reference Oscillator
Frequency (fT)
315.0 MHz
9.7941 MHz
390.0 MHz
12.1260 MHz
418.0 MHz
12.9966 MHz
433.92 MHz
13.4916 MHz
Table 1. Recommended Reference Oscillator Values For
Typical Transmit Frequencies (high-side mixing)
Step 2: Selecting CTH Capacitor
Extraction of the DC value of the demodulated signal for
purposes of logic-level data slicing is accomplished using
the external threshold capacitor CTH and the on-chip
switched capacitor “resistor” RSC, as shown in the block
diagram.
Slicing level time constant values vary somewhat with
decoder type, data pattern, and data rate, but typically
values range from 5ms to 50ms. This issue is covered in
more detail in “Application Note 22.” Optimization of the
value of CTH is required to maximize range.
Selecting Capacitor CTH
The first step in the process is selection of a data-slicing-
level time constant. This selection is strongly dependent
upon system issues including system decode response
time and data code structure (that is, existence of data
preamble, etc.) This issue is also covered in more detail in
“Application Note 22.”
The effective resistance of RSC is listed in the electrical
characteristics table as 150kΩ at 315MHz, this value
scales inversely with frequency. RSC value at other
frequencies is given by equation (4), where fT is in MHz:
R SC
= 150Ω 9.7941
fT
(4)
CTH can be calculated using equation (5) with the
knowledge of Rsc and τ.
CTH
=
τ
RSC
(5)
Recommended τ is 5x the bit-rate.
A standard ±20% X7R ceramic capacitor for CTH is
generally sufficient. Refer to “Application Hint 42” for CTH
and CAGC selection examples.
Step 3: Selecting CAGC Capacitor
The signal path has AGC (automatic gain control) to
increase input dynamic range. The attack time constant of
the AGC is set externally by the value of the CAGC
capacitor connected to the CAGC pin of the device. To
maximize system range, it is important to keep the AGC
control voltage ripple low, preferably under 10mVPP once
the control voltage attains its quiescent value. For this
reason, capacitor values of at least 0.47µF are
recommended.
The AGC control voltage is carefully managed on-chip to
allow duty-cycle operation of the MICRF010. When the
device is placed into shutdown mode (SHUT pin is pulled
high), the AGC capacitor floats to retain the voltage. When
operation is resumed, only the voltage drop, due to
capacitor leakage, must be replenished. A relatively low-
leakage capacitor such as a ceramic type is recommended
June 2005
7
M9999-063005
(408) 955-1690
Share Link: 