## Extra Class question of the day: digital circuits, flip-flops

Digital circuits are used for a variety of functions in modern amateur radio equipment. Unlike analog circuits, the output voltage of an ideal digital circuit can only be one of two values. One of these voltages—normally a positive voltage—represents a digital 1. The other value—normally near 0 V—represents a digital 0.

This type of logic is generally called positive logic. Positive Logic is the name for logic which represents a logic “1” as a high voltage. (E7A11) The logic may be reversed, though. That is to say that a high voltage may represent a logic 0. Negative logic is the name for logic which represents a logic “0” as a high voltage. (E7A12)

The microcomputers that control today’s transceivers, for example, are very complex digital circuits. These complex digital circuits are made by combining many smaller building blocks called logic gates. These gates perform basic digital logic functions.

Table E7-1. This two-input NAND truth table show the output (Q) for each combination of inputs (A,B).

One of the most basic digital circuits in the NAND gate. The logical operation that a NAND gate performs is that it produces a logic “0” at its output only when all inputs are logic “1.” (E7A07)

This logical operation can be described by a truth table. A truth table is a list of inputs and corresponding outputs for a digital device. (E7A10) Table E7-1 shows a truth table that describes the operation of a two-input NAND gate. A and B are the two inputs; Q is the output.

Table E7-2.

Other types of gates perform different logical functions. The logical operation that an OR gate performs is that it produces a logic “1” at its output if any or all inputs are logic “1.” (E7A08) Table E7-2 shows a truth table that describes the logical operation of an OR gate.

The logical operation that is performed by a two-input exclusive NOR gate is that it produces a logic “0” at its output if any single input is a logic “1.” (E7A09) Table E7-3 shows a truth table that describes the logical operation of an OR gate.

Table E7-3.

Flip-flops are circuits that are made from combinations of logic gates. The output of a flip-flop is not entirely dependent on its inputs; it is also dependent on the current value of its output.

As an example, let’s look at the SR or RS flip-flop. An SR or RS flip-flop is a set/reset flip-flop whose output is low when R is high and S is low, high when S is high and R is low, and unchanged when both inputs are low. (E7A13) So, once set to a particular value, the output will not change when both inputs are set to low.

Some flip-flops are clocked. That is to say that they only change states when a clock signal input changes states. A D flip-flop is an example of this type of flip-flop. A D flip-flop is a flip-flop whose output takes on the state of the D input when the clock signal transitions from low to high. (E7A15) A JK flip-flop is a flip-flop similar to an RS except that it toggles when both J and K are high. (E7A14)

Another type of flip-flop is the T flip-flop. The T flip-flop is so called because for each transition from low to high on the flip-flop’s T input, the output “toggles” from 0 to 1 if the output was already at 0, and from 1 to 0 if the output was already at 1. Two output level changes are obtained for every two trigger pulses applied to the input of a T flip-flop circuit. (E7A02) See figure E7-1 below.

The output of a T flip-flop changes state every time a clock pulse appears on the T input. This effectively divides the input frequency by a factor of two.

A flip-flop can divide the frequency of a pulse train by 2. (E7A03) Consequently, 2 flip-flops are required to divide a signal frequency by 4. (E7A04)

A flip-flop is a bistable circuit. (E7A01) What that means is that its output is stable in either state. An astable multivibrator is a circuit that continuously alternates between two states without an external clock. (E7A05) In other words, it is an oscillator.

A monostable circuit is one that is stable in one state but not the other. One characteristic of a monostable multivibrator is that it switches momentarily to the opposite binary state and then returns, after a set time, to its original state.(E7A06) A trigger pulse causes the monostable vibrator to switch to the unstable state, and it stays in that state for a set period, no matter how long the trigger pulse.

Resources
Wikipedia: Multivibrator (https://en.wikipedia.org/wiki/Multivibrator)

The Twittersphere is kind of like the ionosphere. It helps you make contact with other hams and brings you news from far and wide. Here are a few interesting links that I found on Twitter in the last day or so:

Global Pirate HF Weekend 14-15.1.2012.  This station lists pirate SW radio stations that it expects to be on the air this weekend. They include one using the callsign WEMP. Look for it between 15.005 – 15.095 MHz. They’ll be broadcasting with 100 W to Europe: 12.00 – 16.00 UTC – (check 15.010 or 15.040 or 15.090 MHz).

The Evil Mad Scientist Laboratories Zener Diode Tutorial. Confused about zener diodes and how they work? Read this.

Monitor your Ham Radio transmitter with an oscilloscope. In this video, Alan, W2AEW builds a little adapter that lets you connect your transmitter output to a scope input so that you can see how clean its output is.

## Keep electronics safe from static discharge

Electrostatic discharge, or ESD, is perhaps the biggest danger to integrated circuits, and electronics companies spend millions of dollars each year on products to reduce static buildup and discharge and to test how susceptible their ICs are to ESD. Simliarly, when building a kit, or working on electronics that are ESD-sensitive, it’s a good idea to use a conductive  mat on your work surface and a wrist strap connected to earth ground. I use both in my shack.

Texas Instruments has a number of videos on their website that will give you a better idea why this is a good idea. They’re mostly designed to sell you their ESD protection devices, but unless you’re designing electronic systems, you can mostly ignore the sales pitch.

## How the Transistor Got Its Name

I’ve been a ham radio operator a long time, and I even have an electrical engineering degree, but I never really knew how the transistor got its name. Well, according to this post on the Adafruit blog, they actually took a vote. The entire memo has been posted to flickr.

While searching around for a picture of the first transistor, I found the video, “Does the first transistor ever built still work?” It explains how it works, and you can actually see the transistor, which is now part of the collection at the University of Illinois Spurlock Museum.

## The Shack to Get Back Into Parts Again?

From the 6/2/11 ARRL Letter:

In the Shack: RadioShack Looking for Suggestions for DIY Parts

Back in the day, RadioShack was the first place many of us looked for parts. QST construction projects routinely referenced RS part numbers, and the Shack even sold 10 and 2 meter transceivers. What goes around comes around, and RadioShack is now looking for input from us, as members of the larger DIY community, on parts we’d like to see them carry. There’s a video explaining what they’re looking for, and a place to add comments.

Now, before you all start complaining that there going to charge double for whatever it is that they will carry–and they probably will charge double for it–consider that you’ll at least be able to get the parts you need when you need them.

## Using Op Amps for Small-Signal Audio Designs

EE Times has started running a series of articles on using op amps as audio amplifiers. Taken from the book, Small Signal Audio Design, Part 1 discusses the history of op amps and then looks at various op amp properties from a perspective of audio design. Looks pretty interesting to me.

## So, When Can I Get a 100 W HT?

From the 4/1/11 issue of ACM Tech News:

The Incredible Shrinking Circuit
University of Cambridge (03/28/11)

University of Cambridge researchers have developed a technique for producing smaller microchips that can also support extremely high electrical current densities. Led by Cambridge professor John Robertson, the researchers used carbon nanotubes to replace the vertical copper connectors in integrated circuits to build smaller circuits and to reduce the size of electronics even further. They took advantage of the special arrangement of carbon atoms. Normally the atoms are arranged hexagonally and layered in sheets, but in nanotubes the sheets are rolled up to form tiny tubes with diameters equivalent to just a few carbon atoms. To make the approach feasible, the nanotubes would need to be grown in very dense bundles directly onto the substrate. Robertson and colleagues used multiple deposition and the annealing steps to grow nanotube bundles, and the method led to successive increases in nanoparticle density. The researchers say the density of the bundles is five times greater than current technology.

## Links for the New General Class Study Guide

For the next version of No-Nonsense, General Class License Study Guide I am going to include links to Web pages that readers can refer to if they really want more information on a topic. I’ve just added some links to the following sections:

• G5A – Reactance; inductance; capacitance; impedance; impedance matching
• G5B – The Decibel; current and voltage dividers; electrical power calculations; sine wave root-mean-square (RMS) values; PEP calculations
• G5C – Resistors, capacitors, and inductors in series and parallel; transformers

If you have other suggestions, I’d love to hear about them. Please send me the URLs and why you happen to like that particular website. Thanks!

I’m on the distribution list for many different electronics trade magazines. Quite often, there are articles of interest to amateur radio operators. Here are four of them—two from electronic design and two from EE Times—that I hope you’ll find interesting.

Radiated efficiency: A true measure of antenna performance
Many engineers tend to think of antennas in terms of gain, but the author argues that we’d be better off if instead we evaluated antennas in terms of efficiency, that is how well it turns the power supplied to the feedpoint into radiated energy.

And You Thought The 555 Timer Was Dead?
Recently, both Advanced Linear Devices and Semtech have redesigned the 555 timer chip, improving it in many ways and extending its usefulness—most likely—for years to come.

Melville Eastham: Workplace Innovator Crafts Early Electronic Products
Eastham was the founder of General Radio. The article points out that Eastham founded the company in 1915 to “serve the rapidly growing ham radio market.” By the late 1920s, that “boom” had subsided, and the company turned its attention to precision measurement instruments. It was very successful doing this for many, many years.

10 Technologies to Watch in 2011
This article predicts that “wireless connects for health care” will be one of the technologies to watch in 2011. Makers of medical electronics equipment, apparently, are planning to integrate their gear using Bluetooth.

## Analog-to-Digital Converter Targets Wireless Applications

This from the press release for the AD9467, an analog-to-digital converter, capable of 16-bit conversions at data rates up to 250 MHz:

Norwood, MA (09/27/2010) – The industry’s fastest 16-bit ADC (analog-to-digital converter)–at 250 MSPS (mega samples per second)–was unveiled today by data converter market share leader* Analog Devices, Inc. (ADI). The AD9467 16-bit, 250 MSPS ADC operates on 35% less power at 25% higher sampling rate than any other 16-bit data converter, providing a new level of signal processing performance for test and measurement instrumentation, defense electronics, and communications applications where high resolution over a wide bandwidth is needed.

The AD9467 delivers resolution and a fast sample rate while simultaneously achieving a high SFDR (spurious-free dynamic range) of up to 100 dBFs and SNR (signal-to-noise ratio) performance of 76.4 dBFS. The device’s SFDR of 90 dBFS up to 300 MHz analog input and 60-femtosecond rms (root mean square) jitter helps lower the signal chain bill of materials component count by allowing engineers to increase system performance at higher intermediate frequencies, thereby reducing the number of signal down-conversion stages. Download data sheet or order samples and evaluation boards.

The AD9467 can be used with ADI’s AD9523/24 low-jitter clock generators and ADL5562 3.3 GHz ultra-low distortion RF/IF differential amplifier to provide a data conversion signal chain solution.