SKY73112 Datasheet, PDF (5/13 Pages) Skyworks Solutions Inc. – 750-850 MHz High Performance VCO/Synthesizer With Integrated Switch

PRELIMINARY DATA SHEET • SKY73112 VCO/SYNTHESIZER
Synthesizer Output Switch
An on-chip switch is integrated into the SKY73112 RF output after
the balun and is controlled by the SW_EN signal (pin 4) as
indicated below:
SW_EN Input
High
Low
Synthesizer Output
On
Off
The switch provides >50 dB isolation at the synthesizer RF
output. This allows the SKY73112 to be used for GSM
applications.
Synthesizer Programming
To program the synthesizer to the correct frequency, values for
the N-counter (both M and A portions), fractional divisor (FN), and
fractional modulus extender (ME) are needed. These values are
used to determine the total divider ratio, D_Total, according to
Equation 1:
D_Total = N_actual + FN_actual + ME_actual + 3.5
(1)
Where: N_actual = the actual value of the N-counter
FN_actual = the actual fractional divisor
ME_actual = the actual fractional modulus extender
Because of the way the ∆Σ modulator is implemented in the
SKY73103, the number 3.5 must be added to the division number
to obtain the final division ratio.
The calculated value for D_Total can then be used to determine the
correct synthesizer frequency, RF:
RF =
F_REF
R1
× D_Total
(2)
Where: F_REF = the reference frequency
R1 = the reference divider radio
The 6-bit M-counter and the 4-bit A-counter portions of the N-
counter are calculated according to the following relationships:
N_actual is the actual N-counter value and is the integer portion of
(D_Total – 3.5):
N_actual = M_actual × P + A_actual
(3)
If: M = M_actual (binary number, fit to six bits)
A = A_actual (binary number, fit to four bits)
Then: N = M × 2⁴ + A
Where: N is the number to be programmed into the N-counter.
The synthesizer has a selectable prescaler of 8/9 or 16/17. If the
16/17 prescaler is used:
P = 2⁴ = 16
In this case, N is the same as N_actual, M is equal to the six MSBs of
N_actual, and A is equal to the four LSBs of N_actual.
If the 8/9 prescaler is used:
P=8
Here, N is not equal to N_actual. The A-counter portion only uses the
three LSBs (the 4^th bit of the A-counter is a “don’t care” bit).
The fractional divisor code (FN) sets the fractional-N modulo up to
256 modulo according to the following equation:
FN _actual
=
D₇
⎜⎛
⎝
1
2
⎟⎞
⎠
+
D₆
⎜⎛
⎝
1
2²
⎟⎞
⎠
+
D₅
⎜⎛
⎝
1
2³
⎟⎞
⎠
+
•
•
•
+
D₀
⎜⎛
⎝
1
2⁸
⎟⎞
⎠
(4)
The value of FN is equal to the binary representation of 256 (or 2⁸)
× FN_actual, or:
FN = D₇ × 2⁷ + D₆ × 2⁶ + D₅ × 2⁵+ . . . D₀
The fractional modulo can be extended up to 2²³ using the modulo
extender (ME) if required:
ME_actual = D₁₄(1/2⁹) + D₁₃(1/2¹⁰) + D₁₂(1/2¹¹) + . . . + D₀(1/2²³)
The value of ME is equal to the binary representation of the
integer part of 2²³ × ME_actual, or:
ME = D₁₄ × 2¹⁴ + D₁₃ × 2¹³ + D₁₂ × 2¹² + . . .D₀
Example 1:
A desired synthesizer frequency of 2640.45 MHz is required using
a crystal frequency of 16 MHz and a 16/17 prescaler. Since the
maximum internal reference frequency is 25 MHz, the crystal
frequency does not need to be divided. However, a reference
divider ratio of 2 is used for this example.
Restating Equation 2 as a function of D_Total:
D_Total = (2640.45 × 2)/16 = 330.05625
Where: RF = 2640.45
R1 = 2
F_REF = 16
Determine N_actual by subtracting 3.5 from D_Total and removing the
fractional portion:
D_Total – 3.5 = 326.55625
Using Equation 3:
N_actual = 326 = M_actual × P + A_actual
Where: M_actual = 20
P = 16
A_actual = 6
M = M_actual = 20 = 010100b (the six MSBs)
A = A_actual = 6 = 0110b (the four LSBs)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
200736A • Skyworks Proprietary and Confidential Information • Products and Product Information are Subject to Change Without Notice • July 9, 2007
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