User Manual

PROGRAMMING
3-4
June 1997
Part No. 001-3412-002
3.2.5 A0-WORD CALCULATION (24-BITS)
The A0-Word is sent last (see Figure 3-4). The
A0-Word contains the data for the loop dividers and is
programmed on a channel-by-channel basis. The
Fractional-N (NF) word is a 3-bit word that programs
the synthesizer to the fractional steps determined by
the fractional modulus selection flag FMOD
(1 = modulus 8) and the loop comparison frequency
(50 kHz). The frequency resolution is:
50 kHz ÷ 8 = 6.25 kHz.
NF= (3-bits)* Fractional increment for modulus 8
(3-bits)
NM1=(12-bits)*Number of main divide cycles when
prescaler modulus equals 64 (12-bits)
NM2=(8-bits)* Number of main divide cycles when
prescaler modulus equals 65
(4-bits, PR=01)
NM3=(4-bits)* Number of main divide cycles when
prescaler modulus equals 72
(4 bits, PR=10)
NOTE: NM3 is not used because
dual modulus is selected by PR=01.
* Indicates frequency/channel dependant variable.
The Fractional-N increment (NF) is a 3-bit word
that is channel dependant. NF is used to program the
synthesizer to channels (frequency steps) that are
below the comparison frequency (FCM), i.e. frequen-
cies that are not divisible by FCM. When the D-word is
programmed with FMOD = 8, and NR = 350 as
described in Section 3.2.2, the fractional increment is:
50 kHz ÷ 8 = 6.25 kHz.
FCM = Ftcxo ÷ NR = 17.5 MHz ÷ 350 = 50 kHz
The synthesizer resolution (min. step) is 6.25 kHz.
Example Calculation:
Program the transceiver to receive at 457.01875 MHz:
RX = 457.01875 MHz
LO = RX + 52.95 MHz = 509.96875 MHz
(LO is high-side injection)
N = LO ÷ FCM = 509.96875 ÷ 0.05 = 10199 (integer)
NM2 = 64 x FRAC [N ÷ 64]
= 64 x FRAC [10199 ÷ 64]
= 64 x 0.359375
= 23
NM1 = INTEGER [N ÷ 64] - 2 - NM2
= 159 - 2 - 23
= 134
NF = 18.75 kHz ÷ 6.25 kHz = 3
NOTE: Channel steps of 10 kHz can also be achieved by
setting the Fractional-N modulus flag (FMOD) in the D-
word to "0" which sets FMOD to 5. NR remains set to
350. This produces 10 kHz steps as NF is incremented.
Figure 3-4 A0-WORD
4
321 141312111098765 2019181715 16 24232221
ADDRESS
BITS
0/1 0/1 0/1 0/1 0/1 0/1 0/1 0/10
NF
0/10/10/10/1 0/1 0/1 0/1 0/1 0/1 0/1 0/1 0/1
NM1
(NUMBER OF MAIN DIVIDER CYCLES
WHEN PRESCALER MODULUS = 64)
(NUMBER OF MAIN DIVIDER CYCLES
NM2
WHEN PRESCALER MODULUS = 65)
0/10/10/1