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

Simple AM Radio Receiver

Here's a very simple AM Radio circuit I've designed couple of years ago. Don't know whether anybody listen to AM stations anymore. But I still use it(maybe I wanted to listen to my own built radio lol..). The radio section is wired using a single transistor(BF494) and it was so amazing that an audible sound is recovered at the output which is faint though. It doesn't use any external antenna and the sensitivity/selectivity of the receiver is pretty good. However I used an amplifier(TA 7368P Toshiba, Low voltage) which drives an 8ohm/1W 4" speaker inside a box rocks the entire room with a high fidelity audio that is unbelievable and outperforms Superheterodyne ones in this regard . It is a reflex receiver.

The audio recovered at inductor L is rather strong comparing to ZN414 and free from oscillations.(I've never succeeded in building ZN414 which always give me annoying motorboating and chirping crappy I say!)

Using a flat ferrite bar antenna allows local reception for a pocket radio and a big rod antenna captures stations beyond 200miles! So it doesn't require an external wire antenna, adding it only helps in electrical noise catch.

Only critical part in the circuit is inductor L, its optimum value gives excellent results. Make rf parts close to the transistor. I made it on a 1.5" ultra small pcb. 2 x AA battery lasts very long.

Another important thing is that the radio is absolutely silent in between the stations - means no noise at all if no any electrical interferance which is a plus point over Superheterodyne receivers. It was so amazing to tune it during power failure period. So I'll call it a true radio


ref: electro-tech-online.com/electronic-projects/94129-simple-am-radio-receiver.html

Induction Receiver

The induction receiver shown below is very sensitive and can serve a variety of purposes. It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field ("telephone pickup coil").

The receiving coil could be a "telephone pickup coil" if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don't try to wind your own - it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound "tinny" with poor low frequency response and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air core coil with superb results but the 2 inch diameter was just too big for this application.

The other components are not particularly critical. The 2N4401 can be just about any NPN general purpose small-signal transistor. The TL431 is a shunt voltage regulator but it is being used as an audio amplifier in this circuit. In fact, the whole device is nothing more than a low noise, high gain audio amplifier with a pickup coil connected to the input and other amplifiers will work equally well.

The circuit is built into a 8 mm cassette box with the power switch and earphone jack in the back. The circuit board is a piece of pink countertop laminate which looks good against the violet hue of the cassette box. The battery fits nicely into the box and a piece of foam fills in the remaining space. These video cassette boxes make nice project boxes, unlike audio cassette boxes which are too flimsy.

When you first turn on the unit you will probably hear a lot of buzzing from the wiring in the room. Rotate the receiver in a horizontal plane to find a "null" where the hum is minimal. If you can get a reasonable null, you should be able to hear distant lightning crackles or other magnetic noises. If you cannot get a null then go outside away from the building. Try holding the coil near electronic devices like your computer monitor, telephone (when in use), cell phone readout, etc. You can trace power wires behind a wall or ceiling by listening for a sharp increase in hum as the coil passes near the wire. Make sure that current is flowing in the wires to be traced by turning on a lamp or other appliance. (Here is an experiment to try: Build a line voltage lamp flasher that can be connected to the circuit to be traced. The desired wire will now have an on and off buzz - buzz sound that will be easy to distinguish. I wonder if you could even identify a specific breaker or fuse?)

Other wires can be traced if they are carrying alternating current in the audio range or a signal generator can be connected to produce the current. Connect the generator to the wire to be traced and connect the generator's "ground" to the house wiring ground. Also ground the far end of the wire you are tracing so that current flows in the wire. This ground connection can also just be a temporary wire laying on the floor running from the generator ground to the far end of the wire you wish to trace.

For the ambitious: try wrapping one or two turns of wire around the whole house and connect the loop to the output of an audio power amplifier (one channel of a stereo should work). Add a 4 ohm, high wattage resistor in series to protect the amplifier. You should be able to pick up the magnetic field fairly easily anywhere within the loop with the power amplifier supplying just a few watts of power.

ref: techlib.com/electronics/induction.html

AM reciever

Description:
This is a compact three transistor, regenerative receiver with fixed feedback.
It is similar in principle to the ZN414 radio IC which is now no longer available. The design is simple and sensitivity and selectivity of the receiver are good.

Notes: All general purpose transistors should work in this circuit, I used three BC109C transistors in my prototype.The tuned circuit is designed for medium wave. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz. Q1 and Q2 form a compund transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit.

The 120k resistor provides regenerative feedback,between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit. Too much feedback and the circuit will become unstable producing a "howling sound". Insufficient feedback and the receiver
becomes "deaf". If the circuit oscillates,then R1's value may be decreased; try 68k. If there is a lack of sensitivity, then try increasing R1 to around 150k. R1 could also be replaced by a fixed
resisor say 33k and a preset resistor of 100k. This will give adjustment of sensitivity and
selectivity of the receiver.

Transistor Q3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. Audio level varies on the strength of the received station but I had typically 10-40 mV.
This will directly drive high impedance headphones or can be fed into a suitable amplifier.

Construction:
All connections should be short, a veroboard or tagstrip layout are suitable. The tuning capacitor has fixed and moving plates. The moving plates should be connected to the "cold" end of the
tank circuit, this is the base of Q1, and the fixed plates to the "hot end" of the coil, the juction of
R1 and C1. If connections on the capacitor are reversed, then moving your hand near the capacitor will cause unwanted stability and oscillation.

Finally here are some voltage checks from my breadboard prototype. This should help in determining a working circuit:-
All measurements made with a fresh 9volt battery and three BC109C transistors with respect to the battery negative terminal.
Q1 (b) 1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V
Q3 (c) 3.87V