Receiver design - the fundamentals

Among the first radio receivers ever constructed I suppose must have been the ever so humble crystal set. Just how many have been constructed over the years would be impossible to guess.

It would be fair to say millions of people, especially children had their first contact with electronic construction via the old crystal set.

Without going into a detailed history of radio it is fair to say the modern radio communications receiver (beyond the basic entertainment type) has evolved to the point all of the following characteristics must be considered at length when either purchasing or building a receiver. This discussion is confined to the type referred to as a "communications receiver"

These characteristics (and not in any particular order) are as follows: 

 

1. GENERAL

All receivers of the type being discussed here are for conveying information between 2 or more people but the description could include specialised receivers such as direction finding, radar etc.

2. INPUT CHARACTERISTICS

As silly as it may sound the first requirement of a receiver is to efficiently and with maximum voltage levels possible, transfer electromagnetic energy from the antenna to the input of the first stage of the receiver.

Well that's pretty basic isn't it?.

You would be surprised just how neglected this area becomes when people establish a receiving set up. How many listeners simply hang up as much wire as possible, cross fingers and hope for the best. If nothing is heard on a particular band it must therefore be assumed there is nothing on the air to hear.

That ain't necessarily so.

You could be missing hundreds of good signals!. Why?. Because of a haphazard approach to interfacing your receiver to the real world. Sometimes, and I am presently in this boat myself now, your location will not allow the best antenna set up possible. Maybe you live in an apartment or face some sort of restrictions on what you may be able to erect on the property where you live. Throwing your hands in the air and lowering a few metres of wire out the window is not a terribly scientific approach. No wonder you are likely doomed to disappointment.

You don't need to be a rocket scientist to establish a functional set-up. Certainly you must live within the constraints imposed upon you but you can always strive for the better mouse-trap.

The how-to's I will leave until later. The important thing to remember now is that no matter how classy your receiver is, you just might be choking off all those elusive signals BEFORE they get to the input of the receiver.

The professional receiver designer has no idea what you are going to attach to it. Therefore conventional wisdom dictates it be designed for a 50 ohm (nominal) input. Some receivers also offer an auxilliary 500 ohm input.

2. GAIN, SENSITIVITY AND NOISE FIGURE

Your communications receiver it is hoped will encounter and process a wide range of signals. It must be capable of handling these signals usefully without introducing problems of its own. Consider a signal emanating from your favourite s.w. commercial broadcaster some 10,000 miles (16,000 kM) away.

This signal may originate with a power level of 20 KiloWatts (20Kw). By the time it reaches the input of your receiver the level may only be 1uV (1 micro-volt). The signal has been attenuated (reduced) by 180 dB. That's a one followed by 18 zeros.

For you to usefully and comfortably hear this signal at the output of your speaker, at a quite modest level of say 250 mW (milli-watts), the receiver needs to amplify the signal by about 130 db or have a gain of 130 dB. Now that's a one followed by 13 zero's. I would estimate that about nearly half that gain would come from the audio amplifier section. This would mean about 70 dB of gain  needs to come from the preceding stages.

Now for the moment I am going to deal with an a.m. receiver here. The sensitivity is influenced by the receiver bandwidth so we will assume a bandwidth of 6 Khz. That theoretically means the receiver will not respond to those portions of a signal which are outside plus/minus 3 Khz from the carrier. e.g. a signal on 27.24 Mhz. A good receiver undergoing a test at that frequency would indicate a sensitivity of about 1.5 uV.

Noise Figure is somewhat nebulous and tends to mean different things to different people.

To dispense with any arguments I will quote in part (omitting the later heavy mathematics) Professor Ulrich Rohde from his book "Communications Receivers - Principles and Design" - P68 -ISBN 0-07-053570-1 

 

"Sensitivity measures depend upon specific signal characteristics. NF measures the effects of inherent receiver noise in a different manner.Essentially it compares the total receiver noise with the noise that would be present if the receiver generated no noise. This ratio is sometimes called the noise factor F, and when expressed in dB, the noise figure."

 - bold type is my emphasis alone. 

  

 

3. SELECTIVITY

This simply means the ability of the receiver to separate the signal you want from all the other signals. This selectivity must be sharp enough to differentiate from adjacent channels yet sufficiently wide enough to reproduce the signal at an acceptable fidelity.

Some would say 300 Hz is ideal for C.W. (morse code) while 6 Khz (6,000 Hz) is too wide for serious short wave listening. A T.V. Receiver has a bandwidth of around 7 Mhz (7,000,000 Hz) and F.M. Radio uses 200 Khz channel spacing.

Therefore the selectivity should be consistent with the type of signal you expect to encounter.

4. DYNAMIC RANGE

Here you faithful lecturer jumps on/off high horse.

Just as with noise figure this means different things to different people. Some manufacturers will even omit this figure altogether in their specifications and a lot of people active in radio have never even heard of it.

It is one of the most critical characteristics of a receiver.

It is quite important how it is defined. 

 

Dynamic Range could be defined as:

"The ability of a receiver to survive in the presence of strong signals."

But I feel it should be defined as:

"The ratio of the level of strong out-of-band signals to the level of the weakest acceptable desired signal. The level of strong signal must be such as to cause the weak signal to become unacceptable". 

 

Expressed another way, it means if we are just managing to listen to our favourite elusive signal from far, far away we don't want a nearby channel, occupied by some powerful transmitter situated close by, to swamp out our desired signal and take control of our receiver.

5. GAIN CONTROL

Harking back to our earlier signal of about 1 uV level. In practice this signal level will vary wildly from instant to instant for a variety of reasons but mainly because of the vagaries of propogation.

Obviously it would be unacceptable for the reproduction to vary wildly at the output of your speaker in sympathy with the varying signal input. Also we don't want to continue amplifying the desired signal if it is already a strong signal at our antenna. Hence the need for automatic gain control.

Ideally we would want a constant output from our receiver regardless of the signal level presented at the input. Gain control should generally be logarithmic in response and a range of 120 dB would be ideal. The time constants of the response (i.e. how fast it operates etc.) should depend on the mode of receiving e.g. C.W., S.S.B. or A.M. etc.

6. FREQUENCY ACCURACY AND STABILITY

We all know how difficult sometimes it is to locate a station on a cheap a.m. radio. With a quality communications receiver we should be able to set our frequency of reception with both accuracy and certainty. We should also be able to remain on frequency for any length of time without the need to unduly re-tune the receiver.

The present state-of-the-art is such that these properties are no longer (or should not be) a problem. Even the lower cost receivers offer exceptional accuracy and stability compared say to 20 years ago.

Ref: http://my.integritynet.com.au/purdic/rec_basics.html