Appendix A Synthesizing Desired Frequencies
Appendix A
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Prescalar Div is a multiselection dual modulus to limit the frequency of the Prescalar to below 250
MHz. The setting are 8/9, 6/17, 32/33, and 64/65. The TRF3762 has an integer-N PLL synthesizer,
and because of its flexibility (14-bit R, 6-bit A, 13-bit B counter, and dual modulus prescalar), is
ideal for synthesizing virtually any desired frequency. Assume that one needs to synthesize a
900-MHz local oscillator, with spacing capability (minimum frequency increment) of 200 kHz, as in a
typical GSM application. The choice of the external reference oscillator to be used is beyond the
scope of this user's guide, but assuming that a 10-MHz reference is selected, one can calculate the
settings that yield the desired output frequency and channel spacing. Although more than one
solution can often apply to a specific set of conditions, the following is one means of achieving the
desired result.
First, select the appropriate R counter value. Because a channel spacing of 200 kHz is desired, the
PFD can also be set to 200 kHz. Calculate the R value through R = REFIN/PFD = 10 MHz/ 200
kHz = 50. Assume that a prescalar value of 8/9 is selected. This is a valid choice because the
prescalar output will be well within the 200-MHz limit (900 MHz / 8 = 112.5 MHz). Select the
appropriate A and B counter values. It is known that RFOUT = Fpfd
×
N = (F
REFIN
/ R)
×
(A + P
×
B). Therefore, solve the following equation:
900 MHz = 200 kHz x (A + 8 x B)
Clearly, many solutions exist for this single equation with two unknowns; however, because 3
≤
B
≤
8191, and also B
≥
A, there are some basic constraints on the solution. So, if A = 4, solving the
equation yields B = 562. Thus, one complete solution would be to choose: R = 50, A = 4, B = 562,
and P = 8/9, resulting in the desired N = 4500.
Synthesizing Desired Frequencies
10
SLWU056 – June 2008
