.
OG_LMD-400-R-AB_v11e Circuit Design, Inc.
11
OPERATION GUIDE
How to calculate the setting values for the PLL register
The PLL IC manual shows that the PLL frequency setting value is obtained with the following equation.
f
vco
= [(M x N)+A] x f
osc
/ R -- Equation 1
f
vco
: Output frequency of external VCO
M: Preset divide ratio of the prescaler (64 or 128)
N: Preset divide ratio of binary 11-bit programmable counter (3 to 2,047)
A: Preset divide ratio of binary 7-bit swallow counter (0
≤
A
≤
127 A<N))
f
osc
: Output frequency of the reference frequency oscillator
R: Preset divide ratio of binary 14-bit programmable reference counter (3 to 16,383)
With LMD-400-R, there is an offset frequency (f
offset
) 21.7 MHz for the transmission RF channel frequency f
ch
.
Therefore the expected value of the frequency generated at VCO (f
expect
) is as below.
f
vco
= f
expect
= f
ch
– f
offset ----
Equation 2
The PLL internal circuit compares the phase to the oscillation frequency f
vco.
This phase comparison
frequency (f
comp
) must be decided. f
comp
is made by dividing the frequency input to the PLL from the reference
frequency oscillator by reference counter R. LMD-400-R uses 21.25 MHz for the reference clock f
osc.
f
comp
is
one of 6.25 kHz, 12.5 kHz or 25 kHz.
The above equation 1 results in the following with n = M x N + A, where “n” is the number for division.
f
vco
=n*f
comp
---- Equation 3 n = f
vco
/f
comp
---- Equation 4 note: f
comp
= f
osc
/R
Also, this PLL IC operates with the following R, N, A and M relational expressions.
R=f
osc
/f
comp
---- Equation 5 N = INT (n / M) ---- Equation 6 A = n - (M x N) ---- Equation 7
INT: integer portion of a division.
As an example, the setting value of RF channel frequency f
ch
458.000 MHz can be calculated as below.
The constant values depend on the electronic circuits of LMD-400-R.
Conditions:
Channel center frequency:
f
ch
= 458.000 MHz
Constant: Offset frequency:
f
offset
=21.7 MHz
Constant: Reference frequency:
f
osc
=21.25 MHz
Set 12.5 kHz for Phase comparison frequency and 64 for Prescaler value M
The frequency of VCO will be
f
vco
= f
expect
= f
ch
- f
offset
= 458.000 –21.7 = 436.300 MHz
Dividing value “n” is derived from Equation 4
n = f
vco
/ f
comp
= 436.300 MHz/12.5 kHz = 34904
Value “R” of the reference counter is derived from Equation 5.
R = f
osc
/f
comp
= 21.25 MHz/12.5 kHz = 1700
Value “N” of the programmable counter is derived from Equation 6.
N = INT (n/M) = INT(34904/64) = 545
Value “A“ of the swallow counter is derived from Equation 7.
A = n – (M x N) =34904 – 64 x 545 = 24
The frequency of LMD-400-R is locked at a center frequency f
ch
by inputting the PLL setting values N, A and
R obtained with the above equations as serial data. The above calculations are the same for the other
frequencies.
Excel sheets that contain automatic calculations for the above equations can be found on our web site
(www.circuitdesign.jp).
The result of the calculations is arranged as a table in the CPU ROM. The table is read by the channel
change routine each time the channel is changed, and the data is sent to the PLL.
