Why is choosing the right crystal oscillator output signal important?
The output signal types of your crystal oscillators can provide different benefits and drawbacks depending on what you are trying to accomplish with your product. With that said, it's important to understand signal types to avoid as much attenuating and distorting of your clock signal as possible. You'll also want to be sure you're receiving more benefits than drawbacks from the signal to meet your specific design needs.
TL;DR: Here's a summary...
Differential vs. Single Ended
- Sinewave generally offers the best phase noise followed by CMOS then differential
- Differential signals have better rise and fall time
- Differential signals are more resistant to common mode noise
- Less EMI concerns with differential
Ease of Use:
- LVDS requires only a single resistor at the receiver where as LVPECL requires termination at both transmitter and receiver ends
Fastest Speed:
- LVDS is faster than CMOS. HCSL and LVPECL are faster but can require more power
Lowest Power Consumption:
- LVPECL is faster but consumes more power, so we recommend using CMOS or LVDS for low power consumption.
Best Jitter Performance:
- You can get the best jitter performance from LVPECL, LVDS, and then CMOS in that order.
Learn the Basics of Oscillator Output Signals:
Before understanding the basics of output signal types, it is probably a good idea for you to have a decent understanding of the different crystals in your frequency control oscillators. (Click Here to learn the basics of different crystals inside your oscillators).
This post will only touch on the basics of output signal types. This should help you get an overall understanding to assist you in choosing the correct clock output for your application.
Great! Let's move on...
The common signal types are broken down into two categories. Single Ended and Differential Logic Output Families. Here is a quick list of them in their correct category:
Single Ended:
- Sinewave and Clipped Sinewave
- CMOS (Complementary Metal Oxide Semiconductor), HCMOS (High Speed CMOS), and LVCMOS (Low Voltage CMOS)
- TTL (Transistor to Transistor Logic)
Differential Logic:
- ECL (Emitter Coupled Logic)
- PECL (Positive Emitter Coupled Logic) and LVPECL (Low Voltage PECL)
- CML (Current-Mode Logic)
- LVDS (Low Voltage Differential Signaling)
- HCSL (High Speed Current Steering Logic)
Which Oscillator Output Signal Type is Best for Your Needs?
It all depends on what your specific needs are for your application. Let's look more in depth at the above signal types to better understand their benefits and trade offs. This will give you a better idea of which type is best for your needs.
Let's start with the single ended options first:
Sinewave and Clipped Sinewave
This is your stan
dard or 'natural' signal output of a crystal oscillator circuit. It consists of just one fundamental frequency (without any harmonics present). This will provide the maximum spectoral fluxuation that you can expect from your oscillator. A sinewave output is great for demanding low phase noise applications.
CMOS, HCMOS and LVCMOS
CMOS outputs are suitable for shorter trace lengths and lower frequency clock sources (below 200MHz). This will allow for a direct connection between the clock output and receiver input. In most cases, a low value series resistor can be used to effectively reduce reflections and maintain a reliable signal. There are also High Speed and Low Voltage CMOS that may fit better with your specific needs.
Transistor to Transistor Logic (TTL)
For the most part, TTL is outdated by CMOS. This is because CMOS provides lower costs and better noise immunity than the TTL. Both CMOS and TTL are great for low power consumption, higher output swing, and relatively low cost needs. However, higher frequencies tend to require differential signals (see differential signal types).
Now for the differential signal options:
Emitter Coupled Logic (ECL)
ECL was mainly introduced as a good alternative to TTL. ECL circuits can change state very rapidly which makes it more capable of very high speed data transmission needs.
As for disadvantages, ECL needs fairly high currents to operate. ECL also uses a negative power supply during use. This can create challenges when trying to connect to positive base power supply devices.
PECL and LVPECL
PECL outputs are frequently used in high-speed clock distribution circuits. This is because PECL provides high noise immunity, the ability to drive high data rates over long line lengths, and good jitter performance due to the large voltage swings. However, PECL requires high power consumption to operate, this is it's main disadvantage.
LVPECL provides a good foundation for Gigabit Ethernet and Fibre Channel usage. LVPECL is similar to LVDS electrically, but provides a larger differential voltage swing and slightly less power efficiency. Some challenges my arise with the output from LVPECL because termination is needed to emit a voltage. Also be aware that differential receivers from different manufacturers can have different input tolerances. So make sure to do your homework on which receiver will work best for you!
Current-Mode Logic (CML)
CML has similar performance to that of a LVPECL. The main difference here is that CML do not require an external bias. This makes CML a good alternative to LVPECL needs when low power is a concern.
Low Voltage Differential Signaling (LVDS)
LVDS is similar to LVPECL output, however the power consumption for LVDS is lower and tends to have smaller voltage swings. LVDS is typically used for high speed data transfer needs like clock distribution or backplane transceivers. For higher data rates HCSL, CML or LVPECL are usually required instead but will require more power consumption than LVDS. Other benefits include reduced susceptibility to noise and are easy to implement in CMOS ICs.
A disadvantage to LVDS is its reduced jitter performance compared to PECL, but new technology is being looked at to achieve the same level of jitter performance as LVPECL.
High Speed Current Steering Logic (HCSL)
HCSL has a newer output standard that is similar to LVPECL. One advantage of HCSL is its high impedance output with quick switching times. A 10 to 30 ohm series resistor is recommended to reduce possible overshoot and ringing. Other advantages include the quickest switching speeds, average power consumption (between that of LVDS and LVPECL), and average to good phase noise performance.
Final Thoughts on Oscillator Output Signals:
Hopefully you now have a very basic understanding of the different output signal types and have an idea of what will work best for different applications. It's important to note that since this is such a basic overview, make sure to do more research on signal types of interest before making a final decision. There are some more fine details that go into choosing the very best signal type.
Have something to add or a question regarding signal types? Leave us a comment below!
