Cypress CY25566 manual Modulation Rate, S3, S2 CDiv Output Frequency, Document # 38-07429 Rev. *B

CY25566

Modulation Rate

Spread Spectrum clock generators utilize frequency modulation (FM) to distribute energy over a specific band of frequencies. The maximum frequency of the clock (Fmax) and minimum frequency of the clock (Fmin) determine this band of frequencies. The time required to transition from Fmin to Fmax and back to Fmin is the period of the Modulation Rate, Tmod. Modulation Rates of SSCG clocks are generally referred to in terms of frequency or Fmod = 1/Tmod.

The input clock frequency, Fin, and the internal divider count, Cdiv, determine the Modulation Rate. The CY25566 utilizes two different modulation rate dividers, depending on the range selected on S2 and S3 digital control inputs. Refer to the example below.

S3, S2	CDiv	Output Frequency
0,0	1166	1X
0,1	1166	2X
1,0	2332	1X
1,1	N/A	N/A

Example:

Device = CY25566 Fin = 65 MHz

Range = S3 = 0, S2 = 1, S0 = 0

Then: modulation rate = Fmod = 65 MHz/1166 = 55.7 kHz

The CY25566 has three frequency groups to select from. Each combination of frequency and bandwidth can be selected by programming the input control lines, S0–S3, to the proper logic state.

Group 1 is the 1X low-frequency range and operates from 25 to 100 MHz.

Group 2 is the 1X high-frequency range and operates from 50 to 200 MHz.

Group 3 is the 2X low frequency range and operates from 25 to 50 MHz and 50 to 100 MHz output.

Modulation Profile	Spectrum Analyzer
Figure 3. SSCG Clock, CY25566, 65 MHz
Document #: 38-07429 Rev. *B	Page 4 of 9

[+] Feedback