Led2-Ted2


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Summary: Previously, led (leading edge detector) and a ted (trailing edge detector) device models were prepared using inverters and behavioral logic gate device models. The width of the outputs was not well defined. The led signal output occurred at the rise of the input signal during the input positive going signal, and the ted output occurred after the fall of the input signal.

It is often of little consequence, but the need occurred to detect the trailing edge of the input signal during the input signal positive excursion. Also, control of the output signal width was desired. The device model was created using a delay line to meet these needs.

led2-ted2 device model:

The model for the led2-ted2 device is shown in Figure 1 following:


Figure 1
Led2-ted2 device model

The device model shown in Figure 1 assumes that the input signal in question is ground referenced, as are the signal outputs. If a need arises for differential input signal, it is simple enough to bring both ends of resistor R1 to input terminals, isolating it from ground directly, and likewise the Vled and Vted outputs if required.

The circuit is, of course, digital in nature, although it will 'square up' analog signals about a specified threshold value. Generator B1 creates a positive voltage output of 1V amplitude when the input signal is greater than a threshold parameter passed value of vt.

Transmission line T1 delays the signal. Generator B2 creates a specified voutH level when the input to the delay line is greater than the delayed output, which is the case at the leading edge of the input signal. Otherwise, the output voltage is level is voutL. Generator B3 detects the trailing edge as the interval when the delay line input is less than the delay line output with similar output levels.

Now the transmission line output is an analog signal. Consequently, there is some ambiguity in the output level due to solution noise. Without the addition of the 1u constant, there can be times when the analog voltage vdel is not precisely one nor zero. The added constant takes care of that situation with no detrimental effects.

The user defined parameters for the test circuit are as shown in Figure 2 following:


Figure 2
Led2-ted2 device model parameters

The parameters for the transmission line are shown by Figure 3 following:


Figure 3
Transmission line model parameters

Now when a device is made using this model, it must be remembered that B2SPICE creates 'flat' parameterized subcircuit models. That is, all of the parameterized devices used in a circuit, together with user defined parameters, will be presented as parameterized circuit input choices at the same level. The choices of Figure 2 will be merged with those of Figure 3 in this instance.

The values to be entered as shown in Figure 3 should NOT be altered save for the 'td' parameter. A test circuit for this model is trivial, but included here for completeness in Figure 4 following:


Figure 4
Led2/ted2 model test circuit

The V1 input signal is a 1 MHz sine wave of 1v amplitude. The output value is 5V, the output value 0V, the threshold voltage vt is 0.5v. The delay time td is set as 10nS. A graph of the output is shown in Figure 5 following:


Figure 5
Led2/ted2 model test circuit graph 1

Figure 5 shows that the led2 and ted2 signals are 10nS in duration, and lie within the v6 =1 envelope.

Conclusions:

A version2 LED/TED circuit has been created that produces controllable width and amplitude signals, suitable for use in many applications

References:

associated circuit files (v5)