5.1 Introduction

It is not the purpose of this manual to cover radio theory except in so far as it affects electronic countermeasures. For those wishing to delve into the radio theory in greater depth, there are many excellent textbooks available on the subject.

For countermeasures work, we are looking for illicit transmitters by using receivers, subjects covered in this section.

Most illicit transmitters will either transmit through the air, mostly (but not all) falling in the 80 to 800 MHZ range, or through the electrical supply, falling in the range of 10 to 500 KHz.

Since any sophisticated perpetrator will also be aware of the above, he will attempt to use clandestine transmitters outside these ranges. Transmitters above 1 GHz are becoming easier to construct, and small shortwave transmitters have been with us for a long time. The main difficulty with short wave transmitters is causing them to radiate their power, suitable antennas being the problem. However, even transmitters in the medium wave band below 1 MHZ are possible, employing ferrite rod antennas, capable of transmitting a few yards to an adjoining room or floor. Thus, all frequencies must be considered by the serious countermeasures officer.

5.2 Receiver Theory

5.2.1 General

A receiver processes modulated signals induced into its antenna and delivers a reproduction of the original modulating tone, audio or video. The signal can then be amplified to drive a reproducing device such as a loudspeaker, earphone, tape recorder or video monitor. A receiver must perform four basic functions: reception/amplification, selection, detection, and reproduction.

Reception is the induction of EM waves into the antenna to produce a voltage in that antenna, and amplifying it.

Selection is tuning of one particular frequency from all the signals induced into the antenna. This is called selectivity. The better the receiver is at differentiating between the desired and undesired frequencies, the better the selectivity rating. This is normally accomplished by adding more tuned circuits to reduce the bandwidth of the amplifiers.

Detection is the action of separating the low frequency audio or video signals from the higher frequency carrier. This is also called a demodulator.

Reproduction is the changing of electrical signals into sound waves. The sound waves will be interpreted by the ear as either speech, music, tones, etc., or by viewing on a cathode ray tube.

Perhaps the simplest way to study receiver technology is to keep this in mind: whatever is done to change the modulating signal at the transmitter, it must be undone at the receiver.

5.2.2 TRF Receivers

The first radio receivers were Tuned Radio Frequency (TRF) receivers. These were usually quite bulky because of the technology available at the time: large vacuum tubes, large ganged tuning capacitors and other components. In these receivers, all the RF amplification is carried out at the incoming received frequency. Several stages, maybe 5 or more, of RF amplification are required to receive weak signals, and in order to be able to tune to different signals, all of these RF stages must be tunable in step with each other.
TRF OR TUNED RADIO FREQUENCY RECEIVER Each stage is individually tunable (except for single channel fixed frequency receivers). This is an advantage over the crystal radio set with one tuned circuit because it increased the sensitivity (the ability to make a useful signal out of a weak station) and the selectivity. The major difficulty is the constant tuning necessary to achieve optimum performance. In order to increase sensitivity, "regeneration" or positive feedback is often employed.

An operator makes a first adjustment, then a second adjustment, and a third. But the last adjustment may have changed the characteristic of the first adjustment so the operator often had to readjust the first and second stages again. He could spend quite a lot of time just to tune in a normal AM broadcast station.

5.2.3 Superhetrodyne Receiver

A dramatic improvement was made in receiving efficiency with the discovery and introduction of the superhetrodyne receiver. Basically, the output from a variable "local" oscillator in the receiver is mixed or hetrodyned with the signals from incoming radio transmissions. Super het, or, in full, supersonic hetrodyne, implies that the oscillation is above sonic or audio frequencies, and is mixed or hetrodyned with incoming signals.

In mixing an incoming radio signal with the local oscillator signal, there will be present at the output the original two signals plus the sum and the difference signals of the two, plus harmonics of these sum and difference signals. For instance, in receiving an FM station (Station "A") on 99.7 MHz, the local oscillator could be tuned to 89 MHz. The output would consist of

The original radio station (A)	99.7 MHz.
The local oscillator (O)	89.0
The sum of the two	188.7 MHz
The difference of the two	10.7 MHz
Second harmonic of the sum	377.4 MHz
Second harmonic of the difference	21.4 MHz

The only one we are interested in is the difference frequency, 10.7 MHz, which is called the I.F. or intermediate frequency. 10.7 MHz is the normal FM receiver I.F., and is chosen because it is easy to amplify the wanted signal and because of the selectivity of the circuits, get rid of, or attenuate , all the other frequencies. All the stages of I.F. amplification take place at a fixed (and convenient) frequency, and it is this factor which gives it its superiority over the old TRF receivers.

At the mixer stage, other radio signals will be present. For instance, the next FM channel or 99.9 MHz may well have a station present (Station B). Its difference frequency will be 10.9 MHz, or a difference of 2% (1.87%) in frequency, which puts it right outside the narrow passband of the 10.7 MHz I.F. amplifier. The difference to a TRF operating at 99.7 MHz, would, however, have been only 0.2 %, which at those frequencies and with all stages needing to be individually tuned, poses very great selectivity problems.

Some receivers are "double-superhetrodyne", and this means that the output from the first I.F. of 10.7 MHz is hetrodyned with a second (fixed) local oscillator to produce a second I.F. for further amplification. It is possible to produce substantially greater selectivity by this means.

For instance, a second local oscillator on 10.245 MHz will produce a difference frequency of 0.455 MHz, or 455 KHz, a common second I.F. for VHF radiotelephones operating on NBFM, and requiring the extra selectivity required for the narrower channel spacings.

The channel spacing for wideband FM broadcast stations is 0.2 MHz (200 KHz), such as between station A and B above. For NBFM - narrow band FM - the channel spacing may be 20 KHz, or even 12 1/2 KHz, or less, and it is for this reason that extra selectivity is needed. 20 KHz spacing at 10.7 MHz is 0.2%, difficult to achieve, at 455 KHz it is 4%, which is relatively easy.

Note that 455 KHz is also the normal (single) I.F. for AM receivers.

Finally, note in our first example above, that if we had wanted to tune to station B (99.9 MHz) and not station A (99.7 MHz), all we need to do is change the oscillator from 89.0 MHz to 89.2 MHz, to bring the station B oscillator difference to the 10.7 MHz I.F. frequency. Station B would then pass down the first I.F. amplifier, station A would fall outside its pass band and be attenuated.

Note that the sum, difference, and harmonic frequencies are so widely different to the I.F. that they disappear. However, it would not be possible to go straight to the second I.F. and bypass the first I.F. because of image frequencies which would also be amplified and appear as interference. For instance, if we assumed there was a station C on 99.5 MHz, with an I.F. of 455 KHz, the frequencies would be

Station B or	99.9 MHz
Oscillator or	99.455 MHz
Difference B-O	0.455 MHz

Station C or	89.5 MHZ
Oscillator or	99.455 MHz
Difference C-O	0.545 MHz

This difference is only 90 KHz and sidebands of station C could easily break into the reception of station B.

Good receivers usually contain another refinement. They don't let all frequencies through to the first mixer or hetrodyne stage. There is usually one stage of filtering or RF amplification between the antenna and the first mixer. The reason for this? Well, let us suppose that a small percentage of local oscillator second harmonic is present, as follows:

Local oscillator (O)	89.0 MHz
Second Harmonic 20	198.0 MHz
Radio station D on	167.3 MHz
Difference 20-D	10.7 MHz

This would be amplified by the first I.F. amplifier along with station A and cause interference. However, as D is so far removed from A, a simple 1 stage tuned circuit is enough to get rid of it.

The detector, or demodulator, allows the extraction of the original modulating signal (audio). It essentially pulls the intelligence from the I.F., leaving a usable audio signal by filtering out the I.F. carrier.

The audio signal is then amplified to drive a speaker or other such monitoring device. This takes place in the audio amplifier stages.

In most superhet receivers, especially those in the commercial broadcast range, the I.F. is constant. Commercial AM I.F. is 455 KHz and FM I.F. is 10.7 MHz.

5.2.4 Receiver Characteristics

5.2.4.1 Sensitivity

Sensitivity is the ability to receive weak signals and amplify them to a usable level. Most quality receivers will be able to amplify signals (lower) than 5 microvolts. That is, the smallest discernible signal is 5 uv in amplitude. Increasing the sensitivity in a receiver can be accomplished by adding more stages of amplification prior to demodulation.

The signal-to-noise ratio is a comparison of the signal power to the noise power (or signal voltages divided by noise voltage, quantity squared). Obviously, the result should be high as possible since eventually the signal will sink below the noise level (atmospheric, or produced in the receiver) and be lost.

Noise should be kept to a minimum as it tends to cover up the weaker signals. Although careful circuit design may reduce a large percentage of noise, other factors may increase it. Some of these factors include atmospheric disturbances, electrical machinery and cosmic or solar radiation.

5.2.4.2 Selectivity

Selectivity is the ability of a receiver to tune to a particular station without any other signal interfering with the reception. To Increase selectivity, more tuned circuits are added prior to demodulation in order to narrow the bandwidth of the I.F. or RF amplifiers. Ideally, selectivity should be no wider than the signal bandwidth (sidebands) demands since broader selectivity captures more noise but no more signal, thus lowering the signal to noise ratio.

5.2.4.3 Image Rejection

Selection of a proper I.F. frequency is important to image rejection. An image frequency is an interfering signal, as described earlier.
The desired station and the local oscillator are separated by the amount of the I.F. The image is the amount of the I.F. above the local oscillator. Since the local oscillator produces harmonics, the image frequency of those harmonics may also produce interference. In most cases, the larger the I.F., the better the rejection. In some receivers, the oscillator frequency is higher than the received signal; in some cases, lower: it is the difference that counts. Different bands in the same receiver may alternate from higher to lower oscillator/signal frequency. However, image frequencies are always taken into account in the design of all receivers.

5.2.4.4 Tracking

Since the most important aspect of a superhet receiver is the constant intermediate frequency, tracking is of great concern. Tracking is the ability of the local oscillator to always remain the distance of the I.F. away from the antenna signal across the entire tuning range. Careful design of antenna and oscillator LC circuit becomes essential.

For instance, in tuning the RF stage of an FM receiver from 88 to 108 MHz, the oscillator must faithfully follow 10.7 MHz below it, from 77.3 to 97.3 MHz at all frequencies.

5.2.5 Methods of Demodulation

5.2.5.1 AM Demodulation

Probably the simplest of all methods of demodulation is AM. It usually consists of a single diode and filter. The diode conducts only on half-cycles, and passes the rectified signal to the filter network which shunts the I.F. to ground, leaving a voltage varying at an audio rate, which constitute the intelligence originally transmitted.

5.2.5.2 DSB and SSB Demodulation

For DSB and SSB demodulation, the carrier frequency from an internal oscillator must first be introduced to the signal to replace the suppressed carrier in the received signal. The signal will then appear as a standard AM signal which can be easily detected by a diode-filter arrangement. This oscillator is sometimes known as a beat-frequency oscillator (BFO), and has to be within a few cycles (Hertz) of the original carrier, or carrier as hetrodyned down to I.F. Thus, the local oscillator and BFO have to be extremely stable.

5.2.5.3 FM Demodulation

FM demodulation is considerably more difficult, and this the reason why FM appeared about half a century after the advent of AM. Perhaps explanations will exceed the amount of time allotted. Instead, we will just mention some of the types of circuits that allows us to change varying frequencies into audio.

5.2.5.4 Subcarrier Demodulation

Subcarrier detection, also known as double demodulation, first must pass through a standard AM or FM demodulator. The resulting signal may be an FM, AM-type, or composite signal.

For FM or standard AM detection, the previous methods of demodulation will remove the audio from the signal. For SSB or DSB detection, the carrier must be re-inserted to form an AM signal and then detected.

If there are more than one audio inputs to the transmitter, the signal may be in composite form. The original audio signals are separately modulated and then added together. Then this composite modulates the main carrier. At the receiver, the main carrier is detected and the composite must be divided by filters. Then each subcarrier may be detected as per its mode of modulation.
Basically, a subcarrier signal consists of an audio frequency modulated on to a very low frequency RF carrier (say 30 KHz). This modulates an HF or VHF carrier. One stage of demodulation will leave the 30 KHz carrier (which is super-sonic, or above audio frequencies) plus AF, which of course cannot be heard. A subcarrier station will sound like a normal station with no modulation; i.e., no sound or music. A 30 KHz receiver (or demodulator) will, however, eliminate the 30 KHz carrier and leave the audible audio frequencies. | BACK |  | NEXT |