Silicon Frequency Controller (SFC) – the darling of the crystal replacement market

introduction
The main component of the crystal is silica, commonly known as quartz. Quartz has extraordinary mechanical and piezoelectric properties that have made it a fundamental clock device since the mid-1840s. Today, whenever a clock is needed, the first thing that engineers think of is the crystal, but with the deepening of the application, some inherent defects of the crystal are also exposed. Today, new technologies are constantly emerging and bringing about great changes.

Crystal characteristics and parameters
 
package
The encapsulation of the crystal is shown in Figure 1 and consists of three parts: a metal upper cover, a quartz plate with electrodes and a ceramic base. In general, it is also necessary to fill the sealed shell with nitrogen.

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 1 Crystal package diagram

Almost all ceramic seals are now supplied by three Japanese companies, but production was severely affected by the earthquake and tsunami in Japan. It will be difficult to meet market demand for a long time in the future.

Quartz material
Quartz is the dominant part of the crystal with its inherent piezoelectric properties. However, it must be cut and polished before it can be used. Because of its very thin thickness, although protective measures have been taken, its shock resistance has always been a concern.

precision
The so-called accuracy is the degree to which the actual clock frequency deviates from the standard clock frequency. The formula is expressed as:
Error (PPM) = (Factual-Ftarget) / Ftarget * 10E6
Error: precision
Factual: actual frequency
Ftarget: standard frequency
PPM: one part per million

In the application of crystals, there are several aspects to consider:
1) Frequency tolerance: It is the value by which the actual frequency deviates from the standard frequency at the usual ambient temperature (25°C+/-5°C).
2) Frequency temperature characteristics: it is the value that the actual frequency deviates from the standard frequency in the entire temperature variation range. There are now generally three temperature ranges: 0°C–70°C, -20°C–70°C and -40°C–85°C.
3) Aging: The deviation of the frequency caused by the change of the internal characteristics of the crystal over time is called the aging of the crystal. Generally speaking, the accuracy of the crystal is affected by aging to 5PPM in the first year, and about 1-3PPM per year thereafter. If the design life cycle of a product is 10 years, the frequency accuracy change due to aging can be up to 32PPM.
4) Changes in frequency due to changes in load capacitance accuracy: This factor is often easily overlooked. There are two operating modes in the application of crystal, serial oscillation mode and parallel oscillation mode. Due to the flexible design of parallel mode and high output precision, it has now become the mainstream of the market. Figure 2 is the equivalent circuit diagram of the parallel oscillation mode:

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 2 Equivalent circuit diagram of parallel oscillation mode

R1: dynamic impedance
C1: Dynamic Capacitance
L1: Dynamic inductance
C0: static capacitance
CL: load capacitance

The frequency of the parallel oscillation mode can be based on the following formula:

FL=[1/2π√(L1*C1))]*√[1+C1/(C0+CL)]

in[1/2π√(L1*C1))]is the frequency of the crystal serial oscillator mode

According to Taylor expansion:
FL=[1/2π√(L1*C1)]*[1+C1/2(C0+CL)] (1)

As can be seen from the formula, the frequency is related to C0, C1 and CL.

In the fundamental frequency resonance, C1 is 10-30fF, and the general value is 20fF. The value of C0 is related to the size of the crystal, and the general value is 5pF. However, the calculation of CL is related to the external capacitance of the crystal and the PCB design and materials.The following figure is the reference circuit diagram

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 3 Schematic diagram of external load capacitance of crystal

From the above circuit, it can be concluded that:
1/(C11+CS1)+1/(C12+CS2)=1/(CL) (2)

Among them, C11 and C12 are external capacitors, that is, two capacitors that are grounded on both sides of the crystal in the circuit design. CS1 and CS2 are parasitic capacitances, which are related to the PCB traces, pads and pins of the chip. Generally 5-10pF (can be set to 8pF in this calculation). For C11 and C12, there is no definite value (15pF-30pF), which is related to the actual design, for example, take 18pF.

If CL changes, the frequency of the parallel oscillation mode also changes, see Figure 4

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 4 The relationship between load capacitance change and frequency

From formula (1), the frequency change can be obtained as:
(FCL1-FCL2)/FCL1=C1/2* [1/(C0+CL1)-1/(C0+CL2)] * 10E6 (3)

From formula (2) and formula (3), it can be known that the accuracy of C11 and C12 will affect the accuracy of the frequency. The specific data are shown in Table 1. The values ​​of the parameters are as above: C1=20fF, C0=5pF, CS1=CS2=8pF, C11=C12=18pF.

Capacitance Accuracy

CL1

CL2

affects frequency accuracy (PPM)

0.50%

12.955

13.045

3

1%

12.910

13.090

6

5%

12.550

13.450

28

10%

12.100

13.900

56

20%

11.200

14.800

112

Table 1 Relationship between capacitance accuracy and frequency accuracy

In many applications, the capacitance accuracy is 5%. It can be seen from the above table that its influence on the frequency accuracy can reach 28PPM. This is easily overlooked in the design.

5) Other factors: such as the influence of reflow soldering, the influence of humidity, the influence of atmospheric pressure, etc. These factors have little effect and will not be described in detail here.

The total frequency accuracy of crystal oscillation is the sum of the above five aspects.

Silicon Frequency Controller (SFC)

Principle of SFC
Due to the limitations of the quartz material and its oscillation principle, in recent years, people have been exploring new technologies to replace it. Such as MEMS technology, but its center oscillation frequency is not very high (such as 16MHz), so if a high frequency output is required, it must go through a PLL, which increases the cost, phase noise and power consumption.

IDT has done in-depth research in this field and has introduced an all-silicon frequency controller using a patented CMOS Harmonic Oscillator (CHO). Its core is a high-frequency oscillation module, and different output frequencies can be obtained according to different frequency division coefficients. In this way, neither quartz is needed as an oscillation source nor PLL for frequency multiplication.

The SFC operating state requires power and the crystal does not. However, since the ASIC must provide a crystal start-up circuit, the crystal also increases the power consumption of the ASIC accordingly.

Parameters of Silicon Frequency Controller (SFC)

precision
The frequency tolerance of the silicon frequency controller is 50PPM. -20-70°C frequency temperature characteristic is 50PPM. Silicon frequency controllers do not use quartz, so there are no aging issues. Accuracy only needs to consider two aspects. Refer to the examples in Table 2 below. For the value of crystal precision, please refer to the calculation above.

crystal

SFC

Frequency tolerance

50ppm

50ppm

temperature characteristics

50ppm

50ppm

Ageing

32ppm/10yr

Load capacitance variation +/-5%

28ppm

other

total

160ppm

100ppm

Table 2 Comparison of the two types of products

It can be seen from this example that although the frequency tolerance and temperature characteristics are all 50PPM, the accuracy of the calculated crystal can reach 160PPM. The accuracy of the silicon frequency controller is 100PPM.

the simplest design
Silicon frequency controllers do not require any auxiliary devices to work. The crystal must be connected to two external capacitors to work properly, which not only saves costs, but also saves valuable space, which is in line with the trend of product miniaturization.

Ultra-low supply current
Under operating conditions, the supply current is 1.9mA. The quiescent current is only 1uA. Other crystal and MEMS-based products are 4-10 times larger. This is even more a boon for handheld devices. Because for the same battery capacity, low current means longer usage time, which is increasingly valued by product manufacturers.

fast startup time
The standard startup time is 400uS. The startup time of the crystal sometimes reaches 10mS. Faster startup times allow the product to quickly enter normal operation from power-on or standby. This also enhances the competitiveness of the product and occupies a favorable position in the market.

Wide frequency output range
No PLL is required, the silicon frequency controller can output 4-50MHz. For common frequencies, such as 10M, 14.31818M, 19.44M, 20M, 25M, 33M, 33.3333M, 40M, 48M, 50M, etc., you can directly order them. For uncommon or special frequencies, you can get the output frequency division coefficient through the factory settings .

Higher working reliability
Nowadays, fashionable products are always liked to be carried around by people. Small accidents such as falling on the ground or being hit by hard objects are inevitable. This often causes the crystal to stop vibrating and the product fails. Since the silicon frequency controller does not use a crystal, it is not affected by vibration and extrusion. This is very important for the long-term stability of the product. And that’s one of the reasons why device makers are increasingly loving it. In addition, the silicon frequency controller does not have high impedance input pins, which is more conducive to passing EMI tests.

better jitter metrics
The following example is the result of the connection performance test on SATA GEN2. It can be seen from this that the jitter of the silicon frequency controller is smaller than that of the crystal.

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 5. Measured graph of crystal jitter index

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 6. The measured graph of the jitter index of the silicon frequency controller SFC

Plastic case package
The packaging of the silicon frequency controller can use a lower cost plastic case. The crystals must be sealed with ceramics. So the cost of the silicon frequency controller can be lower. Moreover, the resources of plastic case packaging are very rich, and are not limited by the manufacturer’s supply cycle and quantity.

The size of the silicon frequency controller is usually 2.5*2*0.55mm. And there will be a smaller size launch. It is well known that crystals in small packages increase the cost considerably. But this is very easy to implement for silicon frequency controllers and may be less expensive.

faster lead times
As mentioned earlier, the core of the silicon frequency controller is a stable high-speed oscillation module. The factory can produce in advance, and then configure the output frequency according to the customer’s needs, which can greatly shorten the product supply cycle.

SFC crystal source
The silicon frequency controller is a CMOS technology, which is the same process as the ASIC, so the ASIC can easily integrate the crystal source of the silicon frequency generator. If it is quartz, because it is a different process, it will bring a lot of process and reliability problems.

Silicon Frequency Controller Product Application
The appearance of the silicon frequency controller is shown in Figure 7

Silicon Frequency Controller (SFC) – the darling of the crystal replacement market
Figure 7 Appearance of silicon frequency controller

The pin distribution of the silicon frequency controller is 2.5*2*0.55mm as an example, as shown in Figure 8

Figure 8, Pin layout of silicon frequency controller (2.5*2*0.55mm)

The Links:   LM170E03-TLG3 RM500CZ-24