Service Manual

PROGRAMMING
3-2
Part No. 001-3474-002
3.2.2 B-WORD
The B-Word is 24-bits long (see Figure 3-2). It contains the Address, Charge Pump setting factor (CN),
Binary Acceleration factors (CK, CL), and Prescaler Type (PR).
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 3474 synthesizer has an adjust (R855)
for the fractional compensation current. The factory preset value will allow CN to be set to the following ranges:
Frequency in a Band CN
Lowest TX 86
Highest TX 90
Lowest RX 96
Highest RX 100
The value of CN should be interpolated for frequencies between the band edges. With these recommended
values of CN, the transceiver should have the fractional spurs minimized far below the levels needed to make ETSI
70 dB adjacent channel RX or TX
specifications.
Example:
Model 3474-530 is a 450-466 MHz transceiver.
458 MHz TX CN = 88 01011000 Binary
458 MHz RX CN = 98 01100010 Binary
3.2.3 A-WORD
The A-Word must be sent last (see Figure 3-3). The A-Word contains new data for the loop dividers and is
programmed for every channel. The A-Word can be a 24-bit or 32-bit word depending on the state of the flag
LONG in the D-Word. The 24-bit word (A0) is sent if LONG=0 and the 32-bit word (A1) is sent if LONG=1. The
extra 8-bits in A1 are the CN charge pump settings. Upon power up the D-, B- and A-Words must be sent, but after
that only the A1 word needs to be sent.
The Fractional-N increment (NF) is a 3-bit word that is channel dependent. NF is used to program the sub-
channels below the 50 kHz Loop Reference frequency. FCM = 50 kHz and if FMOD = 8, then the Fractional-N
increment is: 50 kHz ÷ 8 = 6.25 kHz
To program an 18.75 kHz channel:
NF = 18.75 kHz ÷ 6.25 kHz
NF = 3