Lesson 15

Digital Integrated Circuits

Integrated circuits are typically called IC’s. In radios you will find two different types of IC’s, analog and digital. Analog IC’s are used for RF things like op-amps and oscillators. This lesson will focus on those in more detail shortly.

Digital IC’s are things like microprocessors. These are computers on a chip. They take data in and use onboard logic to process it. You encounter microprocessor chips every day. They are CPU’s which drive your computer, mobile phone, and modern radio.

How does that onboard logic work? Digital IC’s use logic changes, called gates, to produce a single logic output. The most simple one is an AND gate. When input A AND B are on, the logic output is ON. When either or both are OFF, the logic output is OFF. 

Let’s look at the schematic symbols in figure E6-3 and talk about several of the gates.

Number 1 is the symbol for the AND gate that was just mentioned.

Number 2 is the NAND or NOT AND gate symbol. This output is ON until both input A and B are on, then it is NOT on, or NOT AND. You can remember this because it looks just like the AND gate of number 1, but also has a little circle on the end.

An OR gates output is on when either the A or B inputs are on.  It’s symbol number 3. So, if we follow the same thinking as the AND and NAND gates, number 4 is the schematic symbol for a NOR gate. That’s a NOT OR gate. This gate is only ON when A NOR B are on. The curved back represents OR, and the circle represents the NOT.

Symbol number 5 shows the NOT operation, also called a NOT gate or inversion operation. When input A is ON, output is OFF and vice-versa. The output is just the opposite of the input.

An advanced version of gates is tri-state logic. Tri-state has 3 possible outputs. They are logic devices with 0, 1 and high-impedance output states.

Much of these logic circuits go unseen to the naked eye. Most are built into chips called Programmable Logic Devices or PLDs. Most every computer has one of these chips called a CMOS. They are a type of MOSFET circuit. In a computer they store the most basic information it needs to start up, like how to read a hard drive.  

One reason that CMOS stands out among other digital logic families is that it has the lowest power consumption. CMOS circuits also have high immunity to noise on the input signal or power supply. This is because the input switching threshold is about one-half the power supply voltage. In your computer a small battery keeps your CMOS on standby for years.  

Let’s merge two semiconductors and see what we get. We’ll combine a CMOS and a bi-polar junction. That becomes a BiCMOS device. An advantage of BiCMOS logic is that it has the high input impedance of CMOS and the low output impedance of bipolar transistors.

Another type of IC is a field programmable gate array. An amateur radio application for this FPGA is a digital signal processor. Instead of being set to a single function, it can get new configurations loaded into it.  That design is done with a hardware description language (HDL).

Analog Integrated Circuits

Analog used to be the king of circuits until digital came along. Now both analog and digital are used regularly depending on circuit design needs. Analog IC’s make up many resonant circuits. 

A Monolithic Microwave Integrated Circuit sounds intimidating. Let’s call it a MMIC. As the name implies, MMIC’s are popular in microwave band circuits. They do operate in UHF and VHF as well. Their frequency range is from 300 MHz to 300 GHz. 

Diagram of MMIC

What makes MMIC’s make a popular choice for this spectrum? Their characteristics of controlled gain, low noise figure, and constant input and output impedance over the specified frequency range. 

There are two types of gallium semiconductors in use here. Which one depends on the application. Gallium arsenide has a higher electron mobility. The other option is gallium nitride.  It’s likely to provide the highest frequency of operation when used in MMICs.   

Let’s connect RF and power to our MMICs. The most common input and output impedance of circuits that use MMICs is 50 ohms. Because of this, connections are commonly made using Microstrip transmission line. The most common types of MMIC’s get power through a resistor and/or RF choke connected to the amplifier output lead. All of that gives good performance. In a low-noise UHF preamplifier, a 0.5 dB noise figure is a typical value.

Both the digital and analog IC’s discussed can come in various types of packaging.   

A DIP devices package is probably what you picture in your mind when it comes to a conventional integrated circuit package. It has spider-like legs that go through a circuit board. 

DIP package devices

Those legs are a formal characteristic of DIP packaging in IC’s. It’s a total of two rows of connecting pins placed on opposite sides of the package (Dual In-line Package) or DIP!

DIP through-hole package ICs are not typically used at UHF and higher frequencies. This is because excessive lead length can cause interference. 

Replacing through-hole technology in many applications are surface-mount components. Like the name implies, they are soldered to the face of a circuit board. The advantages surface-mount technology offers in RF compared to using through-hole components are:

  • Smaller circuit area
  • Shorter circuit-board traces and
  • Components have less parasitic inductance and capacitance

All these are correct when asked about surface-mount on the exam. 

In a radio, surface mount component package types would be most suitable for use at frequencies above the HF range.

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