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Chapter 4 Superheterodyne Radio Receivers
4.1.Superheterodyne AM Receivers As far as the professional manufacturers are concerned, the direct (TRF) receiver has "played out his tune". After half-century struggle on the market, it has been replaced by the superheterodyne receiver, that was patented in 1918 by Edwin Armstrong. In that time, commercially speaking, its main advantage was its substantially easier tuning to the station. It requires only one button for this, comparing to the TRF receiver that needs two buttons to be intermittently adjusted for optimal reception, and also it requires much of the knowledge, skill and patience, which the average buyer does not have. The superheterodyne receiver is, however, also more complex than the TRF, and setting of its stages during its production requires some special instruments, that the average radio amateur does not possess. Nevertheless, it is not impossible to build such device in the amateur environment, and when the operating principles are known, the necessary adjustments can be done "by hearing". 4.1. Superheterodyne AM Receivers On Pic.4.1 you can see the block diagram of
a radio-broadcast superheterodyne receiver The input circuit (UK) refines
the signal of the tuned station from all the voltages created in the antenna
(A) by various radio transmitters and sources of disturbances. In our
example, it's an AM signal that has the carrier frequency fs, and is
modulated by a single tone, as seen in the rectangle above its label. This
signal is being led into the stage called the mixer. Another voltage is also
led into it, the voltage from the local oscillator that has the frequency of
f0, and a constant amplitude. Under the effect of these two signals, the
phenomenon called the outbreak takes place in the mixer, and an AM signal
appears on its output, its frequency being fm=455kHz. This signal is called
the inter-frequency (IF) signal, and its frequency fm the interfrequency.
The IF signal has the same envelope as the station signal entering the
mixer. That means, that the information from the transmitter to the mixer is
carried by the signal frequency fs, and in the mixer it is being assumed by
a new carrier, that has the frequency fm. When transferring to another
station, the user changes the capacitance of the variable capacitor C by
turning the knob, setting up the resonance frequency of the input circuit to
be equal to that station's one. Another variable capacitor, Co, is located
on the same shaft as C, so its capacitance changes simultaneously to that of
C. This capacitor is located in the local oscillator and that is how it gets
the new oscillating frequency, having such value that the difference of the
oscillator and station frequencies is again equal to the inter-frequency
value. Their frequencies would be 999 kHz and 1017
kHz. The ordinary TRF receiver would in this case be totally incapable of
suppressing those signals, which is not the case with the superheterodyne
receiver. These 3 signals are entering the mixer, which gets the 1463 kHz
voltage from the oscillator. The outbreak occurs, and 3 AM signals are
exiting the stage, their frequencies being 455 kHz, 464 kHz and 446 kHz. All
3 signals go to the IF amplifier (MFP), which has several amplifying stages
with oscillatory circuits set to 455 kHz, making it very selective, so it
amplifies only the 455 kHz signal and suppresses the others enough not to
disturb the reception. 4.1.1. The Simplest Superheterodyne AM Receiver The author presumes that most of the
readers, especially those just entering the world of radio with this book,
are somewhat scared by the block diagram from Pic.4.1. Their question
probably is: Can an amateur build such a receiver? Yes, he can. The author
has a friend that succeeded in this some 40 years ago, when all had been
done with the electronic tubes, making the practical realization of a
receiver much harder than it is today, with semiconductors (its radio
amateur call sign is YT1FA, and those who doubt it may contact him).
However, he was doing this in the premises of YU1EXY Radio Club, in the
attic of the Electrotechnical Faculty in Belgrade, using the club (more less
trophy) instruments and, more important, he had help of Sasa Piosijan,
Radivoje Karakasevic and Kiro Stojcevski, who knew all about the radios,
especially Sasa. More will be discussed in the chapter
dedicated to NE612 IC, and the reader should pick one of these, or make the
receiver that suits him best by combining these diagrams with earlier
described HF amplifiers and input circuits. Let us assume that we have (only) 3 MW
signals in the antenna, having the frequencies of fS1=711 kHz (Nis), fS2=855
kHz (Bucharest) and fS3=1008 kHz (Belgrade 2). The IF transformer frequency
could be fm=455 kHz. If we set the frequency of our oscillator on fm=1166
kHz (with CO), the following signals, modulated by the radio stations'
programs, will exit the mixer: * the MFT is also being called the inter-frequency transformer. It is a
special type component that is hard to find in the ordinary electronic
shops, therefore the radio amateurs are usually obtaining them from disused
factory-made devices. The IF transformer is shown on Pics.4.3-a,b,c & d. As
you can see it on 4.3-a, the MFT is, in fact, a parallel oscillatory circuit
with a leg on its coil. The coil body has a ferrite core (symbolically shown
with single upward straight dashed line) that can be moved (with
screwdriver), which allows for the setting of the resonance frequency of the
circuit, being mostly fm=455 kHz. The same body contains another coil, with
less quirks in it. Together with the bigger one it comprises the HF
transformer that takes the signal from the oscillatory circuit into the next
stage of the receiver. Both the coil and the capacitor C are placed in the
square-shaped metal housing that measures 10x10x11 mm (Pic.4.3-b). From the
bottom side of the housing you can see 5 pins emerging from the plastic
stopper, that link the MFT to the PCB, being connected inside the MFT as on
Pic.4.3-a. Besides them, there are also two noses located on the bottom
side, that are to be soldered and connected with the device ground. Japanese
MFT's have the capacitor C placed in the cavity of the plastic stopper, as
shown on Pic.4.3-c. The part of the core that can be moved with the
screwdriver can be seen through the eye on the top side of the housing,
Pic.4.3-d. This part is coloured in order to distinguish the MFT's between
themselves, since there are usually at least 3 of them in an AM receiver.
The colours are white, yellow and black (the coil of the local oscillator is
also being placed in such housing, but is being painted in red, to
distinguish it from the MFT). * Pics.4.3-a, b, c & d almost fully apply for the oscillator coil as well (LO). The only difference is that LO doesn't have the capacitor C. looking from the outside, LO and MFT can be distinguished only by the marking colour, until they're lifted from the PCB. LO's have red colour, while MFT's (IFT’s)
are white, black or yellow. During the PCB design, absolute care must be
taken that pins 1 & 4, as well as 2 & 3, do not permute. If that would
happen, the feedback would be negative (instead of positive) and the
oscillator wouldn't function. However, if you conclude during the design
phase that it would be more convenient to connect pin 4 to Gnd (instead of
pin 1), do have in mind that it can be done only if you connect also pin 2
to Gnd (instead of pin 3). * Fine tuning (if necessary) of the LO's and MFT's inductance values is done
by adjusting the position of the ferrite core with screwdriver. * With CO and CtO, variable capacitor and the trimmer capacitor in the
oscillator are labelled. Acc. to Pic.4.3-e & f, which shows the capacitor we
spoke about in the connection with Pic.3.7, the abovementioned capacitors
are connected with the circuitry over the legs O and G (Ca and Cta are not
used), with G connected to Gnd. * The receiver from Pic.4.2 can be utilized for the reception of AM stations
in the SW waveband. All there is to be done is to make a new oscillator
coil, acc. to Pic.4.3-g & h. It is being made of 0.4 mm CuL wire (a thicker
one can also be used), on the 32 mm diam. carton body, the same one used for
making coils on Pics.3.6 & 3.28. Number of quirks on the picture is 9, but
other combinations should also be tried, say, 12 quirks, or somewhat less
than 9. The feedback coil has 3 quirks and is spooled along the oscillator
coil (as shown on picture), or over it. If you have already accomplished the
reception of SW stations with some of the previously described TRF devices,
you will be surprised with much bigger selectivity of the receiver from
Pic.4.2. in the evening hours you'll be able to perform the receipt of huge
number of stations on the radio-broadcast, professional and amateur
wavebands. * In the previous numerical example we saw that tuning is done by setting up
the frequency of the local oscillator and that fm=455 kHz, Radio Nis will be
heard when the oscillator frequency is fO=1166 kHz. The story is not over,
though: What will happen if there is a station that operates on 1621 kHz?
Mixing its signal with the voltage from the local oscillator the modified
signal is made, its frequency being * If the receiver from Pic.4.2 is power-supplied from the battery (or
adaptor) whose voltage is over 6 V, a voltage stabilizer should be inserted
in the plus (+) line of the power supply for NE612, as it was done with the
receivers on Pics. 4.4, 5.7 and 5.9. * The reception can be significantly improved if input circuit (UK) is added
to the receiver. In order to avoid problems with attuning the UK and the LO,
the UK with special variable capacitor can be used, as on Pic.4.3-i. It is
"our" capacitor from Pic.3.7, with all the capacitors connected in parallel,
and "our" coil from Pic.3.6. Station tuning is now being done with two
buttons, which isn't "a job for everyone". The receiver is first roughly
tuned to the station using these two buttons, and then the optimum reception
is carefully searched. * If you omit the amplifier with 386 IC on the Pic.4.2, and connect
high-resistance headphones instead of R1, it is the truly the simplest
superheterodyne receiver in the world. |
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