The clock generator devices like Si5340/41/91 as well as the jitter attenuator devices like Si5345/95 are both designed to provide low jitter output clocks. But a major difference is that the jitter attenuators, as the name suggests, will attenuate the noise/jitter at the input while for the clock generators, the noise at the input passes through to the output and contributes to the output jitter value.

Therefore, in case of the clock generator devices, the total RMS jitter at the output will tremendously depend on the jitter of the input source and also on the input frequency. In order to get the best jitter performance, it is recommended to use an input frequency between 48MHz to 54MHz at the XAXB pins of these devices and also to avoid using low frequencies like 20MHz or 25MHz at the input. Lower input frequencies cause an increase in noise at the clock output which is seen as increased RMS jitter.

The above applies to both the XAXB pins and the clock inputs pins for the clock generators.

Also, the signal generator or the oscillator used at the input should have minimum possible jitter. The Rohde & Schwarz SMA 100 signal generator is one of the very low jitter signal generators available

Now in case of the jitter attenuating clocks, any allowed input frequency and any signal generator can be used to produce the input signal and it will not have any effect on the output clock jitter.

Frequency accuracy is an important parameter when selecting a reference clock for any application. In general frequency accuracy is the difference in the measured value of the crystal/XO/TCXO/OCXO frequencies from the ideal expected value.

Following factors contribute to the accuracy measurement:

1. Initial accuracy: It is the deviation of the clock frequency from the ideal value when measured at room temperature. Any crystal/XO/TCXO/OCXO datasheets specify this value as Initial Tolerance.

2. Accuracy over temperature: This is the deviation of the clock frequency from the reference frequency (Ref Freq).

Frequency of a crystal/XO/TCXO/OCXO is measured after placing the device in a temperature-controlled chamber and then varying the temperature from -40°C to 85°C. Then the frequency accuracy at a particular temperature is calculated in ppm as follows:

Facc = Frequency accuracy at particular temperature

Ft = Frequency at particular temperature

Fref = Frequency at 25°C

This Ref freq can be calculated using any of the following two techniques (this is usually specified in the datasheet for the device):

1. The frequency measured at 25°C (room temperature) is taken as reference
2. Reference frequency is the average of frequency measured at the extreme temperatures. For example:

DCO mode allows the user to dynamically change an output clock frequency in any existing device or OPN if required. The output frequency can be modifying by changing the N divider value which is assigned to the particular clock output. However manually writing to the N dividers is not a recommended way to use the DCO mode. There are certain factors which need to be considered.

The N divider which is associated with the DCO enabled output should be in fractional mode. If the N divider is integer in the original project file, then it needs to be changed to operate in the fractional mode. The register PIBYP[4:0] enables to change the N divider mode. Each bit in this register is assigned to one N divider. If N0 needs to be in fractional mode, write 0 to PIBYP[0]. Note that a soft reset should be performed after changing the above mentioned register.

The next step involves calculating the frequency step word and then also the corresponding values for either the N divider numerator or the N divider denominator. All these calculations are explained in detail in the following application note:

https://www.silabs.com/documents/public/application-notes/AN959.pdf

In order to avoid all these complications, the best and easiest way to implement DCO mode is using Clock Builder Pro DCO mode tool. It will perform all the calculations and determine the required register values.

Many applications require output frequencies that are not integers and are represented using very specific decimal values. Clock generators and Jitter attenuators (Si534x/9x) devices are capable of generating all types of frequency outputs. When a required input-output configuration file is generated, Clock Builder Pro calculates the values of the M, N and P dividers according to the input-output frequencies.

The M and N dividers are in the form of a multiplication ratio. Calculating these values is very tricky when the output frequencies are not integers. In order to get the correct multiplication ratio, it is important to express the frequency values in an exact manner. This means that the decimal frequencies should not be rounded. They should be expressed in terms of fractions. Many times, the output frequency is in terms of repeating decimals. If these are rounded, then there is possibility of getting incorrect divider values.

For example, if a 33.3333… MHz output is required, it should be expressed as 100/3 MHz and not rounded to 33.333 MHz. The following table will show the difference in the M and N divider register values between 33.333 MHz and 100/3 MHz output frequency.

The left side shows register values when the output frequency is 33.333 MHz and the right side shows when it’s 100/3 MHz. As seen the M divider and N divider values are different and in order to get a perfect multiplication ratio, decimal frequencies should be expressed as fractions.