micrf004 Micrel Semiconductor, micrf004 Datasheet - Page 12

• Manufacturer Part Number: micrf004
• Description: Micrf004/micrf044 Qwikradio? Low-power Vhf Receiver
• Manufacturer: Micrel Semiconductor

MICRF004/RF044
Selecting REFOSC Frequency f
(Sweep Mode)
Selection of the reference oscillator frequency f
mode is much simpler than in fixed mode due to the LO
sweeping process. Also, accuracy requirements of the fre-
quency reference component are significantly relaxed.
In sweep mode, f
(3)
Connect a ceramic resonator of frequency f
pin on the MICRF004. Two-decimal-place accuracy is gener-
ally adequate. A crystal may be used. A crystal may be
mandatory in some cases to reduce receive frequency ambi-
guity if the transmit frequency ambiguity is excessive.
Use Equation 3a to compute sweep-mode frequency band
coverage (f
(3a) f
Example:
then:
Selecting Capacitor C
The first step in the process is selection of a data-slicing-level
time constant. This selection is strongly dependent on sys-
tem issues including system decode response time and data
code structure (that is, existence of data preamble, etc.). This
issue is covered in more detail in Application Note 22.
Source impedance of the CTH pin is given by equation (4),
where f
(4)
Assuming that a slicing level time constant
established, capacitor C
(5)
MICRF004
centered symmetrically about 170MHz.
R
C
f
f
f
f
f
f
Table 2. Common Transmitter Frequencies
IF
T
BC
TX
T
BW
BC
T
SC
TH
is in MHz:
5
32.25
170
150
BC
170
0.5f
.27
5.07MHz
f
170
150
LO
124k
1
1
R
F
):
4
8
T
SC
e r
MHz
9
4
MHz
0.86MHz
T
a r
6 .
2 .
0.43MHz
q
T
n
f
u
7
2
T
s
2f
X
e
5
5
is given by Equation 3:
4.65
m
n
M
M
IF
f
c
T
t i
H
H
y
z
z
TH
TH
f
BW
may be computed using equation
R
e
e f
T
e r
F
4
5
6 .
7 .
e r
n
c
3
0
q
e
1
1
u
f
8
0
T
O
e
M
M
n
T
s
H
H
c
c
to the REFOSC
y
z
z
l l i
t a
T
r o
has been
in sweep
12
A standard 20% X7R ceramic capacitor is generally suffi-
cient.
Selecting C
Selection of C
AGC control voltage by using a sufficiently large capacitor.
Factory experience suggests that C
vicinity of 0.47 F to 4.7 F. Large capacitor values should be
carefully considered as this determines the time required for
the AGC control voltage to settle from a completely dis-
charged condition. AGC settling time from a completely
discharged (zero-volt) state is given approximately by Equa-
tion 6:
(6)
where:
Selecting CAGC Capacitor in Duty-Cycle Mode
Use of 0.47 F or greater is strongly recommended for best
range performance. Use low-leakage type capacitors (dipped
tantalum, ceramic, or polyester)for duty-cycled operation to
minimize AGC control voltage droop.
Generally, droop of the AGC control voltage during shutdown
should be replenished as quickly as possible after the IC is
“turned-on”. As described in the functional description, for
about 10ms after the IC is turned on, the AGC push-pull
currents are increased to 45 times their normal values.
Consideration should be given to selecting a value for C
and a shutdown time period such that the droop can be
replenished within this 10ms period.
Polarity of the droop is unknown, meaning the AGC voltage
could droop up or down. Worst-case from a recovery stand-
point is downward droop, since the AGC pullup current is
1/10th magnitude of the pulldown current. The downward
droop is replenished according to the Equation 7:
(7)
where:
For example, if user desires t = 10ms and chooses a 4.7 F
C
same equation with 200nA worst case pin leakage and
assuming 1 A of capacitor leakage in the same direction, the
maximum allowable t (shutdown time) is about 0.56s for
droop recovery in 10ms.
AGC
, then the allowable droop is about 144mV. Using the
C
I = AGC pullup current for the initial 10ms (67.5 A)
C
C
t = droop recovery time
V = droop voltage
AGC
AGC
t 1.333C
AGC
I
is in F, and t is in seconds.
= AGC capacitor value
AGC
AGC
V
t
Capacitor in Continuous Mode
AGC
is dictated by minimizing the ripple on the
0.44
AGC
February 9, 2000
should be in the
Micrel
AGC