Chapter 2 - CW, SSB, AM signals and Oscillators

 

Already we have seen how a radio signal moves through the atmosphere and those critical factors which influence its' propagation. Whatever be the transmitting frequency and the transmission power, no signal from a particular antenna reaches all places on the earth’s surface - strong or weak. There, you may need three artificial satellites working at a time from three different points in the space. 

The Morse Code

Generating a radio signal could be the most easy of all engineering works in Electronics but utilising it effectively as a communication media had ever been a tough job. One among the easiest of early error free long distance communication techniques belongs to Samuel Morse. With some minor changes, Morse Code is effectively used in radio communication and earlier it was an inseparable part of Amateur Life and also Amateur Station Operators Course (ASOC) Examination. 

For all grades beyond 'Restricted', a Ham aspirant had to do it at the specified speeds. In telegraph, which uses Morse code, two distinct sounds, ‘dit' and ‘dah’ are used and combining both in different patterns, letters, digits and punctuation marks are created. For example, if a ‘dit' is followed by a ‘dah’, it means ‘A’. If three ‘dit's follow a ‘dah’ it signifies ‘B’. They are not pronounced 'dit dah’ and ‘dah dit dit dit’ but didah and 'dahdididit'.

In Radio communication using Morse Code, instead of two different sounds, long and short units of the same radio signal are used; that's, a simple Radio Signal is cut in two different lengths. It is the Beat Frequency Oscillator connected to the Receiver that helps us to listen it in a sweet tone of our choice. If the time for one ‘dit’ is considered as the fundamental unit, the space between units in a letter won’t be as long as one ‘dit’. Space between two letters are three 'dits' long while space between words are 7 'dits' long. In Radio communication, Morse code is recorded on paper using 'dot's and ‘dash'es. 

Though it may seem to be difficult in the beginning, Morse code can easily be learnt. There are Morse code practice programs for both grade 1 and 2 which can be used comfortably - thirty minutes everyday for 30 days is more than enough to learn it in full. Grade 2 exam requires 5 words per minute speed in both sending and receiving while it is 13 words per minute speed that is required for Grade 1 license. What we need for practice is a push switch connected to an audio oscillator of any make. Morse code is useful to all sorts of radio listeners too. 

International Morse Code
A	. _	didah		6	-….	dahdidididi
B	_...	dahdididit		7	_ _...	dahdahdididi
C	_._.	dahdidahdi		8	_ _ _..	dahdahdahdidi
D	_..	Dahdidit		9	_ _ _ _.	dahdahdahdahdi
E	.	dit		0	_ _ _ _ _	dahdahdahdahdah
F	.._.	dididahdit		From	_...	dahdididi
G	_ _ .	dahdahdit		End of Message	._._._	didahdidahdidah
H	….	didididit
I	..	didi		End of Work	…_._	dididitdahdidah
J	._ _ _	didadadah		Wait	._...	didahdididit
K	_._	dahdidah		Invitation to transmit	_._	dahdidah
L	._..	didahdidit
M	_ _	dahdah		Error	........	didididididididit
N	_.	dahdit		Understood	…_.	didididahdit
O	_ _ _	dahdada		Received Okay	._.	didahdit
P	._ _.	Didahdahdit
Q	_ _ ._	dahdahdidah		Full Stop	._._._	didahdidahdidah
R	._.	didahdit		Semi Colon	_._._.	dahdidahdidahdi
S	…	dididit		Colon	_ _ _...	dahdahdahdididit
T	_	dah		Comma	_ _.._ _	dahdahdididahdah
U	.._	dididah		Quotes	._.._.	didahdididahdit
V	…_	didididah		Question  Mark	.._ _..	dididahdahdidit
W	._ _	didahdah
X	_.._	dahdididah		Apostrophe	._ _ _ _.	didahdahdahdahdit
Y	_._ _	dahdidahdaha		Hyphen	_...._	dahdididididah
Z	_ _..	dahdahdidit		Fraction Bar	_.._.	dahdididahdi
1	._ _ _ _	didahdahdahadah		Paranthesis	_._ _._	dahdidahdahdidah
2	.._ _ _	dididahdahdah		Under Score	.._ _ ._	dididahdahdidah
3	…_ _	didididahdah		Double Dash	_..._	dahdidididah
4	…._	dididididah		Seperation	._.._	didahdididah
5	…..	dididididi		Attention	_._._	dahdidahdidah

Modulation

In most transmission situations, a radio signal is used only as a carrier of intelligence which we propose to convey. Adding intelligence (another signal) to the carrier radio frequency is called Mixing. Actually mixing is changing the characteristics of the carrier according to the desired intelligence signal. That particular process of mixing the carrier with intelligence is called ‘Modulation'. Whatever be the intelligence we propose to transmit, picture or sound, the carrier radio frequency gets modulated accordingly. Most of the commercial radio stations change the amplitude of the carrier radio signal in tune with the audio mixed with it. This is called Amplitude Modulation (AM). Stations changing the frequency of the carrier wave in tune with the audio also are growing in number. The second type of modulation is called Frequency Modulation (FM). Ref. C-2/2

Assuming that an average beginner usually proposes to have a Receiver capable of receiving AM, FM, CW (Continuous Wave) and SSB (Single Side Band) signals, we will discuss on modifying an ordinary Commercial BC Receiver accordingly.

Side Band Transmissions

When a Radio signal is amplitude modulated, the Band Width of the carrier frequency definitely increases. For example, if a 500 Khz Radio Frequency (RF) signal is modulated with a 2.5 Khz Audio Frequency (AF) signal what we get in the out put is a Radio signal that is wide between 497.5 Khz (500 Khz - 2.5 Khz) and 502.5 Khz (500 Khz + 2.5 Khz). Here the Band Width of the transmitted signal would be 5 Khz (502.5 Khz - 497.5 Khz). The portion above the carrier frequency of 500 Khz is called Upper Side Band (USB) and the portion below 500 Khz is called the Lower Side Band (LSB). That is, an AM radio signal from a transmitter consists of carrier frequency and side bands and the applied power in the out put stage of the transmitter is equally divided between the carrier and the Side Bands.

It was John Renshaw Carson, a noted transmission theorist for early communications systems who said that any one side band and the carrier is enough for a successful demodulation. He also found out that transmission of any one of the side bands alone is enough to transmit intelligence because the carrier it misses can quite easily be generated at the receiving point. In effect, transmitting only one side band of the modulated signal will save two third of the out put power. That is, an SSB signal coming out of the transmitter will naturally be three times powerful than an AM signal transmitted from the same transmitter. But what experiments proved was that a SSB signal goes 8 times farther than an AM signal of the same transmission power. There is still one more reason for Amateurs to go for SSB transmitters. That certainly is the limited space they are allowed. SSB signals require only half the space compared to AM.

Now, let us see how an ordinary Super Heterodyne Radio (SHR) Receiver functions (C-2/3). The signal we receive through a tuned circuit is mixed with another signal higher by 455 Khz, which is generated in the receiver itself. What we get in the output of the mixer are a signal that is at the frequency of the difference between the two signals, a signal at the frequency of the added sum of the frequencies and a bundle of harmonics of all these signals including the fundamental. All these signals contain the modulated intelligence without any facelifting. Since the out put is connected to the input of an intermediate frequency tuned circuit of (IF) 455Khz, all other signals except that at 455 Khz are rejected. The following detector stage, further separates the carrier and the content (AF) and we get the original reproduction of the audio signal with which the carrier was modulated. In the AF stages, this audio is further amplified to the desired volume and is finally fed to the Loud Speaker (LS). 

Local Oscillators

Already it was explained why a Single Side Band transmission requires an added carrier for effective demodulation of intelligence. As shown in C-2/3, if a SHR (Super Heterodyne Receiver) is tuned to a SSB signal, the received signal can be mixed with a signal having the frequency of the missing carrier, right in the first stage itself. If the mixing is proposed to happen at the first stage, note that we need to keep changing the carrier frequency every time we change the input signal. Here, what we require is a VFO (Variable Frequency Oscillator). To extract intelligence from the LSB, CW and USB of a 500 Khz signal what we require are generated signals of  497.5 Khz, 500 Khz and 502 Khz respectively.

But, if the mixing is proposed at IF stages, a signal of one definite frequency is enough to do the entire job because all signals received at the antenna are converted into IF, which is a definite frequency. Oscillators used at this stage are called BFOs (Beat frequency Oscillators). In short, for a comfortable listening of LSB, CW and USB we are free to use either a VFO or a BFO. A complete BFO  should have all the below facilities:

1. Switched off at AM signals.
2. 455.7 Khz when receiving CW 
3. 456.4 Khz while receiving USB
4. 453.6 Khz while receiving LSB.

Both these units (VFO or BFO) need not necessarily be mounted inside the receiver. Since VFOs are comparatively big, they are set as separate units outside the receiver and since BFOs are small, they are generally fixed inside the Radio Receivers. BFOs require only negligible power, which can be tapped from the receiver power supply itself. All that we need outside is a separate on/off switch to control it.

Universal BFO

C-2/4 is the circuit diagram of a universal BFO with all four features referred above. The required circuit print layout also is given in C-2/5.

Once everything is assembled, adjust C3 at USB mode, tune C1 at LSB mode and C2 at CW mode. Output lead need not be more than 2 inches and it need not also be connected to any circuit too. Whatever be the BFO, it will easily resolve with all these three LSB, CW and USB signals at a particular point of tuning too, certainly at the risk of listening quality. This is the reason why ‘Universal BFO's are not generally seen in Amateur's shacks.

Outboard BFO

C-2/6 shows the circuit of an outboard BFO.

Crystal BFO

In C-2/7 the circuit of a crystal BFO is given. 

In C-2/8, the circuit and print lay out of a simple BFO is given. This works in an voltage range of 3-9. The only specification for this BFO is that it should be fixed inside the Radio as much away as possible from the Speaker, just keeping the 2” long output lead close to the IF stage. No part of it shall be hanging at any condition. That’s it. 

Commonly used  Simple BFO

Tuning the BFO

First tune the receiver to any SSB station and further switch on the BFO and tune the IFT core to a comfortable listening position. Once the BFO is tuned, the only thing we are supposed to do is on and off the device.

Chapter 1                                                                Chapter 3