![]() |
Radio Receivers on-line FREE! |
![]() |
Index |
![]() |
|||
![]() |
Development systems | ||||||
![]() |
Contact us | ||||||
|
3.7. Receiver with the HF Amplifier In HF amplifier the signal coming from the radio station is being
amplified in its original form. In our case, this means that AM signal is
led at input of the HF amplifier, and on its output the same shaped signal
is obtained, only with bigger amplitude. This device got its name because it
is used to amplify HF signals, although more precise term for it is the
Selective Voltage Amplifier (that's how it is called in professional books).
It also has the same role, therefore HF
amplifier from pic.3.24 having the same selectivity as the input circuit in
all the receivers described so far, with addition of extra amplification.
This is being accomplished in the following manner: under the simultaneous
load of 4 voltages that are coming to emitter from the antenna, their
frequencies being fS1, fS2, fS3 and fS4, four currents flow simultaneously
through the transistor. They share the same circuit: from positive battery
pole, through P1, then transistor (in direction emitter-collector), over the
LC circuit to the minus battery pole. All of them therefore flow
simultaneously through the LC circuit as well. The resonance frequency of
this circuit is set (by C) to be equal to the frequency of one of the
currents and it acts upon it as a huge resistor (200 kOhms, as on
pic.3.2-b). According to Ohm’s Law, this current creates voltage on the
oscillatory circuit. For other 3 currents the circuit acts as a resistor
with much smaller resistance (less than 20 kOhms, as on pic.3.2-b) and they
create much smaller voltage in the circuitry (10x smaller, as in our
example). The important difference in operation of circuits from pics. 3.1
and 3.24 is that all currents are much smaller in the latter case (because
of the amplifying effect of transistor), therefore the voltages on the LC
circuit being much bigger. * With P1 potentiometer the signal
amplitude from antenna to the input of HF amplifier is regulated. If, on
your device, you find the slider for all stations to be in rightmost
position, put a resistor instead of potentiometer, and connect the antenna
with emitter. * As with all input circuits, when
connecting the capacitor C care should be taken to connect the rotor to the
ground (G-point on pic.3.7-a). * The R3 resistor comprises with C2 and C3
capacitors the LF filter which prevents the feedback (that would lead to
unstable operation) between the LF circuitry and HF amplifier. If the
feedback still occurs, the R3 resistance should be increased. * In LF part of the receiver the audio
amplifier with LM386 IC is used. That is by no means necessary, any audio
receiver will do. * There is also a better variation of HF amplifier, with increased selectivity. Its electronic diagram is shown on pic.3.29-b. 3.8. The Audion - Direct Receiver with Drain Detector During the experiments with this receiver, the Author had decided to name
this chapter “The BEST Direct (TRF) Receiver”, which he gave up on later,
having in mind the old Latin saying: DE GUSTIBUS NON DISPUTANDUM EST (Tastes
should not be discussed). It is, however, very hard to make something better
with so little components. Anyway, the Author leaves to the readers of this
chapter to name their own Best Receiver candidates, picking one of these
described in this book, having in mind their own criteria for concepts of
the beautiful, simple, cheap and useful. You can mail your voices to me on
the address: ETŠ "Nikola Tesla" (Praktièna ELEKTRONIKA), Narodnog Fronta 31,
Beograd, or by E-mail: tesla@drenik.net, Subject: Pe9. he shape shown with the dashed line, marked as Q2.When the FET is connected to the oscillatory circuit (as on Pic.3.25) there is practically no damping, and the bandpass curve remains as shown in solid line. This is, clearly, much better, since all the other station voltages are more suppressed (reduced), comparing to the voltage of the tuned station. Considering the curve marked as Q3, we will be discussing it more in context with the Pic.3.29-a.
The FET, together with R1, R2, C2, C3 and C4 forms the so-called Drain Detector (its analogous circuit with bipolar transistors is the Collector Detector, and with the electronic tubes - the Anode Detector. The popular name for the anode detector was - the Audion). The LF signal being detected is received on the drain (D). It has the same shape as the LF signal obtained on the output of the diode detector, but is significantly bigger than it, since the drain detector also amplifies the signal. The LF signal is then led on the volume regulation potentiometer, over the filter that is used to suppress the remains of the HF signal carrier (R3 and C7). After that it goes to the audio - receiver.
* Since FETs have very different characteristics compared to each other, it
might be necessary to change the value of R1 resistor. The simplest way to
do it is to place a 50 kOhm variable resistor instead, tune the receiver to
some radio station and then achieve the best possible reception by moving
the slider. The resistor is then removed from the circuit, its resistance
measured, and the (fixed value) resistor of similar resistivity soldered in
the circuit. The same goes for R2. * The filter (R3 and C7) used to suppress the remains of the signal carrier
affects the colour of the tone of the LF signal. If you wish more bass tones
you should increase the C7 capacitance. Similarly, if you wish more high
pitch tones, C7 should then be decreased. * The receiver will not start operating the very same moment the switch S is
engaged. That is due to a fact that the FET doesn’t work under small supply
voltages. Its supply voltage is the one on the C5 capacitor and the detector
won’t work until C5 doesn’t fill. This is being achieved through R4. Since
this resistance is quite big and so is the capacitance of C5, the filling
time is rather long. If, however, you just need the receiver that will have
a “late start”, you should be increasing the capacitance and the resistance
until reaching the desired delay time. * This receiver works well also with the ferrite antenna. On Pic.3.27-a you can see the symbol for it, and on Pic.3.27-b its shape and dimensions are given. The simplest thing to do isusing the antenna from an old pocket radio, probably the same one you took the variable capacitor from. During the dismount, you should by no means cut the coil ends or shorten them later. Instead, you should carefully un-solder and unhook them from the PCB (the coil is made of the “litz wire”, consisting of a dozen very thin lacquer - isolated copper wires, wrapped together with the thread. If you cut this cable, you will find very hard to re-solder it, since it is difficult to remove the lacquer from all the wires without damaging (some of) them). Such an antenna, as seen on picture, has four ends. We shall be using the coil L, therefore only significant ends for us are those marked as 1 and 2. The end No.1 is easy to identify, it is the single one, which is not the case with the end No.2. To detect it in the group of three you will need an ohm-meter of some other conductivity tester, which you should connect to the end No.1, and then search for No.2 by touching those 3 remaining ends.
* With this receiver the reception of the SW band stations can also be
achieved. All that is to be done is making a new coil. For these purposes we
utilized a piece of carton cylinder already used for building our coil L
(described in previous projects). On it, we bended tightly 6 bends of 0.6 mm
copper wire. The wire diameter isn’t critical, practically any can be used.
This coil is shown on Pic.3.28-a, together with attachment plates. This coil
should replace the coil L, as seen on Pic.3.25. With 6 m long antenna and
the antenna capacitor C1=12 pF, the reception bandwidth should be app. from
fd=7 MHz till fg=10 MHz. This can be changed by changing the number of bends
on the coil and /or C1 capacitance. I such a way you can “take a peek”
what’s happening in the civil area, what are the radio - amateurs doing,
some professional links etc. You can even make a multiple - legged coil,
such as on Pic.3.28-b (number of bends isn’t critical, it may be useful
trying out some other values, too), and to be able to choose SW1, SW2, SW3
with a selector switch. Please do have in mind that the reception quality of
the SW stations isn’t the same during the day. It is good in the afternoon
hours, during the night and in the morning, the weakest reception quality is
around noon. But, that isn’t all. It also depends on the season, solar
activity etc. Anyway, you should see it for yourself.
|
© Copyright 2003. mikroElektronika. A l l R i g h t s R e s e r v e d . F o r a n y c o m m e n t s c o n t a c t webmaster. |