User Manual

PROGRAMMING
3-2
June 1997
Part No. 001-3492-001
3.2.4 C-WORD CALCULATION (24 BITS)
The C-Word enables the auxiliary prescaler, and
sets the auxiliary divide ratio for the secondary (Sec-
ond LO) loop (refer to Figures 3-4 and 3-7).
PA = 0 Sets aux prescaler mode to ÷ 4
NA = 437 (0x1B5) Auxiliary divide ratio
87.4 MHz ÷ (4 x 437) = 50 kHz
C-Word = 0x91 0xB5 0x00
3.2.5 B-WORD CALCULATION (24 BITS)
The B-Word programs the Fractional-N charge
pump current setting factor. The Binary acceleration
factors (CL/CK) and prescaler type (modulus 3).
The value of CN should be interpolated for fre-
quencies between the band edges. With these recom-
mended values of CN, the transceiver should have the
fractional spurs minimized far below the levels needed
to make 70 dB adjacent channel Rx or Tx specifica-
tions.
The Charge Pump Current setting (CN) could be
changed on a channel-by-channel basis for ultimate
rejection of the Fraction N spurious responses close
into the carrier frequency. the 3492 synthesizer has an
adjust (R823) for the fractional compensation cur-
rent. The factory preset value will allow CN to be set
to the following ranges:
(Refer to Figures 3-3 and 3-7)
Frequency in a Band CN
Lowest Tx 110
Highest Tx 115
Lowest Rx 100
Highest Rx 105
CN = * (Channel/Frequency dependent variable)
(110-115 Tx) (100-105 Rx)
CK = 0000 Binary acceleration factor for integral
charge pump
CL = 00 Binary acceleration factor for
proportional charge pump
PR = 10 Selects modulus 3 prescaler
B-Word = 0x80 (8 bit CN) 0x02
3.2.6 A0-WORD CALCULATION (24 BITS)
The A0-Word is sent last (see Figure 3-5). The
A0-Word contains the data for the loop dividers and is
programmed on a channel-by-channel basis. The
Functional-N (NF) word is a 3 bit word that programs
the synthesizer to the fractional steps determined by
the fractional modulus selection flag (1 = modulus 8)
and the loop comparison frequency (50 kHz). The fre-
quency resolution (i.e. step size) is then 50 kHz ÷ 8 =
6.25 kHz.
NF=* Fractional increment for modulus 8 (3 bits)
NM1=* Number of main divider cycles when
prescaler modulus equals 64 (12 bits)
NM2=* Number of main divider cycles when
prescaler modulus equals 65 (4 bits, PR=10)
NM3=* Number of main divider cycles when
prescaler modulus equals 72 (4 bits, PR=10)
* Indicates frequency/channel dependant variable.
EXAMPLE: To program an 18.75 kHz channel:
NF = 18.75 kHz ÷ 6.25 kHz
NF = 3
NM1, NM2 and NM3 are calculated as follows:
N = (NM1 + 2) x 64 + NM2 x 65 + (NM3 +1) x 72
Where:
N = Total division ratio
NM1 = Number of main divider cycles when
prescaler modulus equals 64
NM2 = Number of main divider cycles when
prescaler modulus equals 65
NM3 = Number of main divider cycles when
prescaler modulus equals 72
Example:
Calculate NM1, NM2 and NM3 to Rx 944.150 MHz
Rx LO = 944.15 - 87.85 = 856.3 MHz (Synth Freq)
(87.85 MHz IF with Low Side Injection)
N = Rx LO ÷ FCM = 856.3 ÷ 0.05 = 17126
(FCM = Loop Reference Frequency)