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Activity: CMOS Logic Circuits, Transmission Gate XOR

Objective:

The objective of this lab activity is to reinforce the basic principles of CMOS logic from the previous lab activity titled “Build CMOS Logic Functions Using CD4007 Array”[1] and gain additional experience with complex CMOS gates. Specifically, learn how to combine CMOS transmission gates and CMOS inverters to build transmission gate exclusive OR (XOR) and XNOR logic functions.

Background:

To construct the logic functions in this lab activity you will be using the CD4007 CMOS array and discrete NMOS and PMOS transistors (ZVN2110A NMOS and ZVP2110A PMOS) from the ADALP2000 Analog Parts Kit. The CD4007 consists of 3 pairs of complimentary MOSFETs, as shown in figure 1. Each pair shares a common gate (pins 6,3,10). The substrates of all PMOSFETs are common (positive supply pin 14), as well as those of the NMOSFETs (ground pin 7). For the left pair, the NMOS Source terminal is tied to the NMOS substrate (pin 7), and the PMOS Source terminal is tied to PMOS substrate (pin 14). The other two pairs are more general purpose. For the right pair, the Drain terminal of the NMOS is tied to the Drain terminal of the PMOS on pin 12.

Figure 1: CD4007 functional diagram.

The CD4007 is a very versatile IC with many uses as we saw in the previous lab activity[1]. For example, a single CD4007 can be used to make three inverters, an inverter plus two transmission gates, or other complex logic functions such as NAND and NOR gates. Inverters and transmission gates are particularly useful for building transmission gate exclusive OR (XOR) and XNOR logic functions. The schematic symbols for XOR and XNOR logic gates are shown in figure 2.

Figure 2 XOR and XNOR schematic symbols

Static Discharge

The CD4007 like many CMOS integrated circuits, it is easily damaged by static discharge. The CD4007 includes diodes to protect it from static discharge, but it can still be damaged if it is not handled carefully. Normally one would use anti-static mats and wrist straps when working with static sensitive electronics. However, you may not have those when working at home outside a formal lab environment. A low budget way to avoid static discharge is to ground yourself before touching an IC. Discharging any built up static charge before handling a CD4007 will help ensure that you do not have a broken chip half way through the lab.

Materials:

ADALM2000 Active Learning Module
Solder-less Breadboard
1 CD4007 ( CMOS array)
2 ZVN2110A NMOS transistors
2 ZVP2110A PMOS transistors

Directions:

We will now combine the double pole single throw (SPDT) transmission gate switch and two CMOS inverters to build an XOR gate (and XNOR) as shown in Figure 3. The two transmission gates work in tandem to realize a selector operation. Depending on the state of the A input, either Input B or the inverted version of input B appears at the C (XOR) output. Another inverter, M9,10, inverts C to produce the Cbar (XNOR) output.

Build the XOR/XNOR circuit shown in figure 3 on your solder-less breadboard. Use the CD4007 CMOS array for devices M1-6 and one ZVN2110A NMOS and ZVP2110A PMOS for each of the two inverter stages M7,8 and M9,10. Use the fixed +5 V power supply from ADALM2000 to power your circuit.

There are two logic inputs A, and B to the circuit. The non-inverting XOR output is at node C and an inverting version of the output is at node Cbar to form the XNOR function.

Figure 3, Exclusive OR and XNOR gate

Hardware Setup:

Configure both AWG outputs as DC sources for the first steps of the lab. The scope channels are to be used to monitor the inputs and outputs of the circuit as needed. The fixed +5 V power supply is to be used to power your circuit. The fixed -5V supply should be disabled during this Lab.

Figure 4, Exclusive OR and XNOR gate breadboard circuit

Procedure:

Connect pin 6, which serves as the A input to AWG1. Connect pins 1,9, which serve as the B input to the output of AWG2. Connect pins 2,5,12, which serves as the C output to Scope channel 1. Connect the drain terminals of M9,10 which serves as the Cbar output to Scope channel 2. Be sure to turn on the fixed +5 V power supply.

First apply logic Low to A by opening the AWG control screen and setting AWG1 to 0 V DC. Apply logic low to the B input by setting AWG2 to 0 V DC.

Figure 5, Cout and Cbar output

Observe the output C of the gate on scope Channel 1. A steady DC voltage should appear on the scope screen. Enter your observations in the following table. Include the completed logic table in your lab report.

Input AInput BOutput COutput Cbar
00
10
01
11

Table 1 XOR / XNOR truth table

Continue filling in the table by setting AWG1 and AWG2 to logic high (+5 V) and low (0 V) values as needed.

Now configure both AWG channels as square waves with 5 V amplitudes and 2.5 V offsets ( 0 to 5 V swings). Set AWG1 to a frequency of 1 KHz and AWG2 to a frequency of 2 KHz or twice AWG1. Be sure to set the AWGs to run synchronously.

Observe both the C output and the Cbar output on the scope screen with respect to the signals at the A and B inputs. Capture the various waveforms and save a screen shot for inclusion in your lab report.

Next set AWG2 to the same 1 KHz frequency as AWG1 but set the phase of AWG2 to 90°. Observe both the C output and the Cbar output on the scope screen with respect to the signals at the A and B inputs. Capture the various waveforms and save a screen shot for inclusion in your lab report.

Questions:

How could you configure ( i.e. add to ) an XOR gate to produce an output frequency that is twice ( double ) the frequency of a single input source?

XOR Gate as phase detector:

A phase detector or phase comparator is a logic circuit that generates an analog output voltage signal which represents the difference in phase between two logic signal inputs. It is a central element of a phase-locked loop (PLL). Detecting the phase relationship between signals is an important functional block in many systems, such as motor control, radar, telecommunication, demodulators, and servo mechanisms.

A phase detector for square wave signals can be made from an exclusive-OR (XOR) logic gate. When the two signals being compared are completely in-phase, i.e. phase difference is 0 degrees, the XOR gate's output will have a constant level of zero. When the two signals differ in phase by 10° for example, the XOR gate's output will be high for 10/180th or 1/18th of a cycle, the fraction of a cycle during which the two signals differ in value. When the signals differ by 180°, that is, one signal is high when the other is low, and vice versa the XOR gate's output remains high throughout each cycle.

When an XOR gate phase detector is used in a phase locked loop ( PLL ) system it generally locks near a 90° phase difference in the middle of the phase detection range. At 90° the XOR has a 50% duty cycle square-wave output at twice the frequency of the input. The square-wave changes duty-cycle in proportion to the phase difference of the two input signals. Passing the output of the XOR gate through a low-pass filter results in an analog voltage that is proportional to the phase difference between the two signals. It requires inputs that are symmetrical square waves. If the duty cycle of one input is slightly different than the duty cycle of the other, the low pass filtered output will have an offset from the ideal middle of the range at 90° of phase difference.

Directions:

Add the RC low pass filter shown in figure 4 to your XOR breadboard circuit. Connect scope channel 1 to the output of the RC filter.

Figure 6: XOR Gate phase detector

Hardware Setup:

Configure both AWG channels as square waves with 5 V amplitudes and 2.5 V offsets ( 0 to 5 V swings). Set both AWG1 and AWG2 to a frequency of 1 KHz. Also be sure to start with the phase of both AWG1 and AWG2 set to 0°. Be sure to set the AWGs to run synchronously.

Figure 7: XOR Gate phase detector breadboard circuit

Procedure:

Connect scope channel 1 to the output of the RC filter at C1 to observe the filtered (DC) output of the phase detector. Connect scope channel 2 to output C of the XOR gate to observe the pulse width of the output of the logic gate. Record the DC output voltage in the table below. Set the phase of AWG 2 to the values listed in the table and record the DC voltage you observe at the output of the phase detector.

Figure 8: XOR Gate phase detector sample output

AWG1 PhaseAWG2 PhaseOutput Voltage
00
045
090
0135
0180
0225
0270
0360

Table 2 Phase detector output vs phase

Questions:

Move the RC low pass filter to the XNOR output at Cbar and repeat your phase sweep measurements. How is the output response of the phase detector different?

Alternate component choices:

The pair of inverters made using the four individual NMOS and PMOS transistors (ZVN2110A and ZVP2110A) could also be constructed from a second CD4007 IC or could be CMOS inverters from a Hex Inverter IC such as a 74HC04 or CD4049. The CD4066 quad SPST switch could also serve as an alternative to the switches built from the CD4007.

For Further Reading:

[1] Build CMOS Logic Functions Using CD4007 Array
Exclusive OR logic gate: [http://en.wikipedia.org/wiki/XOR_gate]
Exclusive NOR logic gate: [http://en.wikipedia.org/wiki/XNOR_gate]
Phase detector: [http://en.wikipedia.org/wiki/Phase_detector]

Return to Lab Activity Table of Contents.

university/courses/electronics/electronics-lab-30.txt · Last modified: 15 Dec 2017 03:37 by DBautista