First, the definition
High-frequency signals: 1) Frequency domain analysis, the high frequency component signal representing the whole signal energy over 1/3;
2) time domain analysis, the rise time Tr and the transmission delay time Td
Overshoot and delayed signal transmission line related to:
"Signal Integrity": Td> 20% Tr is no need to consider the terminal wire terminating the ringing noise is generated, the circuit function will affect, if not controlled will cause signal integrity risk;
Td <When 20% Tr, ringing noise is negligible, the transmission lines do not need to match the terminal.
Current Status:
In order to avoid signal integrity problems, not terminated transmission line length requirements, Len <Tr, is converted into mil multiplying by 1000;
The current high-speed circuit, Tr is already less than 0.25ns, it requires less than a length Len 0.25in, in general, PCB traces from easily exceed 0.25in, so to design the circuit termination.
Second, the conventional method of high-speed circuit test
Third, pay attention oscilloscope measurement of high-speed signals
1) Bandwidth Selection
Bandwidth is defined: sine wave signal at the -3dB attenuating frequency;
Square: the square wave can be decomposed into odd multiple of the sine wave frequency. 1MHz square wave example is 1MHz, 3MHz, 5MHz, 7MHz and other sine wave superposition. Want to get a complete square wave information, it requires a minimum of 5 times harmonic components. Therefore, selection for selecting at least a square wave signal when the measured bandwidth of the probe and oscilloscope or more harmonics of the frequency bandwidth of 5 times.
2) Probe Selection
Common passive probe 1: probe 1 and 10: 1 two kinds of probes; two different probes in addition to the attenuation ratio outside, but also a big difference to the high-speed signal. Want to explain the problem, we need to understand the loading effect of the probe.
When not connected to the oscilloscope, a circuit diagram of the system under test:
When the oscilloscope to measure the access circuit, the input resistance and parasitic capacitance of the oscilloscope, the equivalent circuit becomes as follows:
At this time, the voltage measurement points are:
Rin is the input impedance, Cin is parasitic capacitance, W represents the frequency.
At this point the voltage test has changed.
This leads to the front and rear access probe signal itself has changed.
As can be seen by the equation, the greater the Rin, the less effect on the signal.
1 / WC is the reciprocal of a parasitic capacitance and the signal frequency multiplication, when W, the greater the impact, reduce the influence of 1 / WC only decrease the capacitance Cin.
X1 probe model is as follows:
Since the probe line length generally exceeds 1m, the cable leading to a large parasitic capacitance, approximately 100pF. In measuring high frequency signals will have a great loading effect.
Below is a model X10 probe
X10 10pF capacitor in series with the capacitance probe model and rear, so Cin must be smaller than 10pF. X1 is much smaller than the input capacitance of the probe.
And Rin has been increased to 10MΩ,
So X10 probe has higher input impedance, smaller parasitic capacitance. Greatly reducing the loading effect of the probe.
Therefore, when measuring high-speed signals, X10 or higher need to select the input impedance of the probe.
Select 3) Grounding of
Commonly used to clamp relatively long, will form a grounded inductance Lgnd. With the growth of the clip line, the inductance will increase. And this loop inductance and the oscilloscope input capacitance Cin resonance. This leads to an oscilloscope amplitude-frequency characteristic becomes uneven. Lead to inaccurate measurements. The equivalent circuit is as follows:
The spectral characteristic curve
Above 60MHz in frequency and amplitude it has produced more than 3dB overshoot, around 100MHz, maximum overshoot, if the clamp, 60MHz signal will be measured over a relatively large distortion.
Correct method is to use the ground spring. Ground spring having a very small inductance can greatly enhance the bandwidth of the probe.
Select 4) measurement locations
Measuring a position close to the terminal, since the reflected avoid superimposing no termination signal is generated to the test result is not accurate.