When we select and purchase a quartz crystal oscillator, or when we look at the crystal oscillator specifications provided by the manufacturer, there will be an indicator of output mode (Output Type) or output waveform. Commonly seen output modes include CMOS, TTL, Sine Wave, etc. What do these output modes mean? When we are shopping, how should we make the right choice? Let's lead you to understand the various output modes of the crystal oscillator.
1. Definition of each output mode of crystal oscillator
The commonly used output modes of crystal oscillators mainly include: TTL, CMOS, ECL, PECL, LVDS, Sine Wave. These types of waveforms are commonly used in the industry. Among them, TTL, CMOS, ECL, PECL, and LVDS are all square waves, and Sine Wave is a sine wave. Generally, the square wave output power is large, the driving ability is strong, but the harmonic component is rich; the sine wave output power is not as good as the square wave, but its harmonic component is much smaller. Let us introduce the definitions of these output modes to you one by one:
(1) TTL: Transistor-Transistor Logic (transistor-transistor logic circuit), with fast transmission delay time and high power consumption, is a current control device.
(2) CMOS: Complementary Metal Oxide Semiconductor (Complementary Metal Oxide Semiconductor CMOS logic circuit), with slow transmission delay time and low power consumption, which is a voltage control device. Compared with TTL, CMOS has a larger noise tolerance, and the input impedance is much larger than TTL input impedance. Corresponding to 3.3V LVTTL, LVCMOS appears, which can directly drive each other with 3.3V LVTTL. HCMOS uses a fully static design, high-speed complementary metal oxide semiconductor process, and CMOS uses complementary metal oxide semiconductor. CMOS will eventually be replaced by HCMOS.
(3) ECL: Emitter-Couple Logic (Emitter-Couple Logic), the characteristic of this circuit is that the basic gate circuit works in an unsaturated state. The ECL circuit has a fairly high speed, with an average delay time of several nanoseconds or even sub-nanoseconds. The logic swing of the ECL circuit is small (only about 0.8V, while the logic swing of TTL is about 2.0V). When the circuit transitions from one state to another, the charging and discharging time of the parasitic capacitance will be reduced. This is an important reason for the high switching speed of the ECL circuit. However, the logic swing of ECL output is small, which is disadvantageous in anti-interference ability. In addition, the ECL circuit has a very high input impedance and a low output impedance.
(4) PECL: PosiTIve Emitter-Couple Logic (positive emitter coupling logic circuit). The ECL circuit has fast speed, strong drive capability, low noise, and can easily reach applications of several hundred MHz, but it has high power consumption and requires a negative power supply. In order to simplify the power supply, PECL (ECL structure, using positive voltage supply instead) and LVPECL output modes have appeared. LVPECL is Low Voltage PosiTIve Emitter-Couple Logic. LVPECL is developed from ECL and PECL. The typical output of LVPECL is a pair of differential signals, and their emitters are grounded through an AC source. Note when using ECL, PECL, and LVPECL: Different levels cannot be driven directly, and AC coupling, resistance network or special chip can be used for conversion in the middle. The above three types are all emitter-fed output structures, which must be pulled to a DC bias voltage by a resistor. (For example, LVPECL used for clock: 130 ohm pull-up and 82 ohm pull-down for DC matching; 82 ohm pull-up for AC matching and 130 ohm pull-down at the same time, but the DC level is 1.95V after the two methods work. about.)
(5) LVDS: Low-Voltage DifferenTIal Signaling (low-voltage differential signal), which is a differential pair input and output. There is a constant current source 3.5～4mA inside, and the direction and level are changed on the differential line to indicate "1" and "0" ". It is converted to a differential level of ±350mV through an external 100-ohm matching resistor (connected to the differential line close to the receiving end). LVDS use attention: it can reach above 600MHz, PCB requirements are high, and the differential line requires strict equal length, and the difference is preferably no more than 10mil (0.25mm); the distance between the 100 ohm resistor and the receiving end should not exceed 500mil, and it is best to control it within 300mil. The application mode of LVDS can have three forms: ①One-way point-to-point and two-way point-to-point, two-way half-duplex communication can be realized through a pair of twisted pairs; ②Multi-branch form, that is, one driver connects multiple receivers (when there are the same When data is to be transmitted to multiple loads, this application form can be used); ③Multi-point structure, at this time, the multi-point bus supports multiple drivers, or BLVDS drivers can be used, which can provide two-way half-duplex communication, but at any time Only one driver can work at a time. Therefore, the priority of transmission and the arbitration protocol of the bus need to be selected according to different applications, and different software protocols and hardware schemes.
(6) Clipped Sine Wave: Clipped Sine Wave. Compared with the square wave, the harmonic components are much less, but the driving ability is weaker. When the load is 10K//10PF, Vp-p is 0.8Vmin. It is usually the output waveform used by surface mount temperature compensated crystal oscillators in packages such as SMD 7050, SMD5032, SMD3225, etc.
(7) Sin Wave: Usually the load impedance of the crystal sine wave output is 50 ohms. The harmonic component of the waveform is very small, and the general harmonic suppression is better than -30dBc. Sine wave output crystal oscillators are usually used in applications such as radio frequency signal processing and frequency sources.
2. The difference between the output waveforms of the crystal oscillators
After understanding these definitions, according to actual usage, TTL is basically not selected due to high power consumption. We will not conduct too much analysis on TTL here. Below, let's take a look at the difference between several square wave output waveforms through the actual test results of the output waveform:
(2) LVPECL (@2.5V):
3. Test circuit for several output waveforms of crystal oscillator
Through the waveform output of the above output modes, we can also see the difference. Let's talk about the test circuit of each circuit:
(1) CMOS output test circuit:
(2) LVPECL output (@2.5V) test circuit:
(3) LVPECL output @3.3V test circuit:
(4) LVDS output (@2.5V) test circuit:
(5) LVDS output (@3.3V) test circuit:
Through the above analysis, I believe that everyone has a certain understanding of the difference between the output modes.