Assigment 3: Modulation Methosd, FM, AM and Airband, and Digital

Overview

Last week you used an integrated program, gqrx or SDR#, to control your SDR and listen to wideband commercial FM, and narrowband FM used in police and fire radio. This week we'll listen to the some of the signals that aircraft use. These use amplitude modulation, or AM. Then we will take a first look at some ways digital data is transmitted over RF channels.

FM and AM Modulation

There are many ways to send a voice signal over the air. Last week we listened to FM signals, where the frequency of the carrier wave is varied with the amplitude of the signal. This week we are looking at AM modulation, where the amplitude is varied with the amplitude of the signal. These are the two most common ways of directly transmitting voice. These are known as analog modulation, because the signal itself is transmitted, not it's digital representation.

The basic idea is illustrated below:

The carrier is the pure RF signal when it isn't modulated. It is a constant frequency sinusoid. For last week's signals the frequency was in the 100's of MHz. The sdr's only go down to about 30 MHz, and up to 1700 MHZ.

The message is the second line. In this case it is shown as another sinusoid, which will be in the audio range. The frequencies here will only be a few kHz (the range of frequencies needed to convey voice).

For AM, all we do is add something to the message so that it is always positive, and then multiply it by the carrier. That's what we transmit. Then to receive, all we need to do is follow the envelope of the waveform. A diode and a capacitor will do this. This is all a crystal radio is. Note that the signal goes up and down in amplitude as it goes along.

For FM, all we do is slightly change the frequency of the carrier. It always has the same amplitude. The frequency increases when the signal is high (loud), and decreases when it is low (quiet). The transmitter is a little more complicated, but still pretty simple. The receiver just needs to differentiate the signal, which can be done many ways. When the signal is loud, it varies more quickly, and the derivative is larger. When the signal is low, the signal varies more slowly, and the derivative is smaller.

Airband AM Signals

The first part of the lab is to capture and decode AM signals in the air band, which is right above the commercial FM band we were decoding last week. There is a band from 108-118 MHz that mostly has navigation beacons that identify themselves by Morse code. Then from 118-137 MHz there are several bands used for communication between aircraft and the ground. Communications in these bands use AM modulation. This is because when two users try to talk on the same channel the stronger channel will come through with AM, and this is usually the tower. You will also be able to hear something of the weaker station. With FM you will just hear a buzzing if two users interfere, unless one station is very much stronger, and then you only hear stronger user. AM gives you a better idea of who else is out there.

There are many airports around here. The major ones are shown below.

These all can be heard from here. There are also lots of small airports, and Moffet Field.

The local airport is Palo Alto, which is station KPAO. It transmits on these frequencies

We will also hear traffic from San Francisco (SFO), San Jose (SJC), as well as NORCAL Approach, which coordinates the airspace. The frequencies for SFO are

and San Jose are

and Norcal Approach are

Choose a frequency where you might expect to get a signal. The ATIS frequencies are good initial candidates, because these continuously transmit information about the airport, and how to contact them. The other frequencies, such at the air traffic control frequencies, are more interesting, but are not always in use. Often a transmission lasts just a few seconds, and can be hard to capture.

One busy channel that has a strong signal around here is 310.8 MHz, SFO departures and arrivals from the south. Another active frequency is band around 135.0 MHz, which has two of the approach frequencies from the south. You'll hear all the planes overhead on these.

If you are not in the bay area, you can look your local airport up in

http://radioreference.com

and see what frequencies they use.

Use gqrx or SDR# to see if you can find any activity. If we were to extract just the audio signal from one of these channels, you would see something like this:

When the operator keys up the radio (pushes the Push To Transmit, or PTT button) the radio starts transmitting its carrier frequency. This looks like a constant, because we are tuned to that frequency. When they start to talk, you see the audio waveform added to the carrier.

This is a signal from 310.8 MHz.

Two audio clips are here:

Clip one

Clip two

This is a signal from 135.65

This was collected in Packard EE, where there is a tremendous amount of background noise (the stripes in the waterfall plot). I'd complain, but it could well be my lab!

The wikipedia page on the airband channels is interesting. You can find it here:

https://en.wikipedia.org/wiki/Airband

Particularly look at the last section about “Unauthorized Use”, and the references cited. It is an interesting story!

Tuning Your Antenna By Ear

To get clear signals, it is helpful to get the antenna length right. For last week it didn't matter too much. FM signals are essentially perfect provided they are above the threshhold where you can determine the frequency of the signal. This happens pretty abruptly, and is known as “full quieting”. This is why FM is so popular. With enough power, it is perfect.

For AM, that is not true. As you get more signal, the sound gets better. Last week we talked about calculating the right length for an antenna. For a half wave dipole (both antenna elements pointed in opposite directions) this should be half a wavelength. For the quarter-wave dipoles you have this year, it should be a quarter of the wavelength.

For AM, you can also tune the antenna length by hand. Set your sdr to the frequency you want, and then adjust the length to maximize the noise. It turns out that will also maximize your signal. Make sure you aren't touching the antenna when you compare levels. You are a lossy dialectric, and that detunes the antenna.

Digital Modulation Methods

Next week we are going to start looking at digital communications. This can be voice, but is most often used with data. Many devices use short digital packets to communicate. These are the devices we will be looking at in the next few weeks.

The basic task for digital communication is to transmit a stream of data, often bits (just ones and zeros), from one place to another. Fundamentally, these are all based on AM or FM transmission. However, there are a tremendous number of methods that have been developed to use AM or FM to carry these bits.

A simple example is Morse Code, which looks like this:

We start with a carrier frequency that we are using to transmit. We can think of this as sending a binary sequence of bits that encode english characters. The zero bit is sent by short pulse, and a one bit is a long pulse. We then use this sequence of long and short pulses to gate the carrier on and off. Here the information is in the width of the pulses, which is called Pulse Width Modulation (PWM).

There are lots of different types of modulation schemes. The Signal ID Wiki is a great place to find examples. They give you waterfall plots, as well as audio clips so you can hear what they sound like if you were to find them with your SDR.

Signal Identification Wiki

For example, if you go to the HF section, you will find lots of different examples.