FM Receiver using TDA7088
I used TDA7088 instead of TDA7000
because it is SMT,it has digital tunning.TDA7088 requires 2200uF
capacitor for power supply decoupling at pin 4.
The electronic diagram of the monophonic FM
receiver made with TDA7088T is shown on Pic.4.12. If built with SMD
components it can be placed in a matchbox, altogether with two button-type
batteries. The operating principle of this device is given in the previous
chapter. The only thing new is a very simple audio amplifier made with BC547
transistor, which is loaded by cheap 16-Ohm headphones. The telescopic
antenna is used, as on Pic.4.8.
Small mishap of this receiver is that it has no indication of station
tuning. This problem can be solved by adding a small voltmeter in parallel
to the BB909, whose scale is graduated in MHz, as described in the Appendix.
This solution is not appropriate for the miniature receiver, since the
voltmeter that has the scale that is big enough takes too much space. It is
in this case better using a manual tuning instead of automatic. Such
solution is given on Pic.4.13.
The tuning is done via the variable capacitor C with numbers written on its
button, similar to that on Pic.3.11. It is most simple to use numbers from 1
to 10. The variable capacitor is like the one on Pic.4.8. Some experimenting
is to be done with capacitances of Cx and Cy, in order to cover the entire
reception bandwidth, from 88 till 108 MHz.
The AFC (Automatic Frequency Control) of
the local oscillator is accomplished with BA483 diode, obtaining that
station’s position on scale does not “walk” over the scale.
The complete radio receiver should still have a loudspeaker. Electronic
diagram of such receiver made with TDA7088T is given on Pic.4.14. As one can
see, that is a receiver from Pic.4.12 with an audio receiver made with LM386
Maximum value of the DC supply voltage for the TDA7088T is 5V, therefore if
using a 4.5 V battery the LM386 will work with reduced output power, the D2
diode and C15 capacitor should be omitted, and R4 should be short-circuited.
If higher voltage battery is used, the
voltage stabilizer, comprised by the aforementioned components, has to be
activated. D2 is a Zener diode with 3 V Zener voltage. The optimum value of
R4 is found experimentally: in order to make the power consumption as low as
possible it should have the resistance as big as possible, while
simultaneously keeping the voltage on Pin 4 about 3 V and the device working
well within the entire reception bandwidth (One should start with, say,
R4=1.5 kOhm, and if the receiver operates well bigger resistance should be
tried out, and if not smaller one, until the optimum value is found).
Pic.4.15. shows the PCB for the HF part of the receiver with TDA7088T, that
is realized with ordinary components, instead of the SMD’s. Pic.4.15-a shows
the board layout from the soldering (copper) side. All the components apart
from TDA7088T are mounted on the opposite side of the board, their pins are
put through the holes and soldered through the holes. The TDA is soldered on
the copper side, directly onto the copper contacts. That is why it is being
drawn in dashed line on pic.4.15-b, where the board layout on the component
side is given.
* Pic.4.16-a shows 3x enlarged picture of
the IC and the surrounding lines. The soldering procedure for SMD is as
A thin tin layer is applied on the copper contacts where IC legs are to be
soldered to. The firs legs to be soldered are the diagonally opposite ones,
in this case No.1 and 9. A small cushion-shaped amount of tin (not profuse)
is applied on the contacts where these pins are to be soldered (pic.4.17-a).
The IC is placed in its position, with all the pins properly laid. Pin No.1
is pressed against the tin pillow with a top of a bodkin, with iron head
simultaneously touching both the tin and the pin end. The tin gets melted,
and the pin lies down on its place with the aid of the bodkin, and gets
It is now time to check out the positioning of the chip. If it needs to be
corrected, the tin surrounding the pin No.1 is melted with iron tip and the
chip position is quickly and carefully adjusted, in order not to overheat
the pin. Soldering the pin No.9 is shown on Pic.4.17-b. First, the iron tip
is simultaneously put on the top of the leg and the copper below it, so that
both of them are heated. After app. half a second, the iron is slightly
removed from the leg but remains on the copper contact. The tip of the tinol
wire is then approached to touch the iron, the pin top and the copper
contact at the same time. The wire gets melted and adheres to the copper and
the pin, so it has to be constantly moved downwards. When enough tin is
applied, the tinol wire is removed first, then the iron, and the pin No.9 is
soldered. Once again you have to check whether all the pins are properly
placed, and then they too are soldered as it was just described. The solders
are OK if they look app. as on Pic.4.17-c.
* Pic.4.15-b contains the PCB component side layout. The pushbuttons we used
here are Siemens, type BO2AMAP-2. The common housing contains, as one can
see, two button switches, one of which is being used by this device. Any
other pushbutton switches can also be used. In that case small modifications
on the PCB lines would probably be necessary. The board is mounted fairly
close to the box edge, so that the switch shafts are passing through the
panel, and that the buttons can be mounted on the outside. The panel-mount
switches can also be used, in which case they are connected to the board by
* Any audio amplifier described so far can
be used, e.g. the one with LM386, as on pic.4.8.
* Instead of the antenna, a 20 cm piece of
wire can also be utilized.
Receiver Built with TDA7088T
Stereophonic radio broadcast is performed
in the ultra short waveband, from 88 MHz till 108 MHz. All radio
transmitters operating in this range are stereophonic, but their signal is
designed so that monophonic receivers can also read it, performing the
compatibility. The readers that wish to get acquainted in more details with
the stereophonic broadcast basics can refer to the “Radio Receivers”
textbook, for the IV grade of the Electrotechnical Highschool.
Making an introduction to this part, a
operating principle of the stereophonic radio receiver shall be considered,
its block diagram shown on pic.4.18. Comparing this diagram with the one of
the monophonic receiver given on pic.4.6, one may notice that they are
identical, up to the block called "The Decoder". It means that, as already
described, exiting the FM detector the LF signal is obtained, i.e. the
information that was used to perform the frequency modulation in the
transmitter. However, this is not an ordinary LF signal, but the one, called
the "composed" (KS) or "multiplexed" (Mpx) signal. Besides the full-scale LF
signal used by the monophonic receiver,
it also contains the so-called auxiliary
signal which allows the separation of left (L) and right (R) channels in the
stereophonic receiver. E.g. if a direct broadcast of some band music is
performed, the left part of performers is being recorded with one microphone
(the signal marked as L), whilst the right side is recorded with the other
one (it’s a R signal). These two signals are being led in the FM transmitter
in the stage called “the coder”. Exiting the coder we have the multiplexed
signal Mpx which contains, in an indirect manner, both left (L) and right
(R) signal. Frequency modulation of the transmitter is being performed with
the Mpx signal. In the receiver, Mpx signal is obtained on the output of the
FM Detector and is then led to the decoder. This stage plays a role
complementary to the one of the coder in the transmitter, therefore two
signals are exiting it, the L and D signal. They are being amplified over
two identical audio amplifiers, then reproduced over two same loudspeakers.
The listener can now hear the left half of the performers from the
loudspeaker placed on its left, and the right half from the loudspeaker that
is placed on its right. The performers that are situated in the middle of
the orchestra are being equally reproduced from both loudspeakers, making an
impression to the listener as if there’s a third loudspeaker, located in the
middle, between the left and right one. Based on all this, the listener has
a picture about the layout of the performers in space, which significantly
improves the total musical impression.
Electronic circuit of a portable stereophonic radio receiver with headphones
reproduction, made with TDA7088T is shown on pic.4.19. It is a receiver
whose practical realization was described in the previous project, with
decoder with TDA7040T and dual audio amplifier with TDA7050T blocks added,
the latter was discussed in PE5.
* L3, L4 and L5 are HF chokes that allow for the headphones cable to be used
as a reception antenna. This is accomplished by connecting one of the
headphones’ contacts from the plug-in, over the 10 pF capacitor, to the
point where, acc. to pic.4.14, the outside antenna is connected. The coils
represent big resistance to the station signals, preventing them to “go to
ground” over the 47 mF capacitor or over the TDA7050T output. Each coil has
3 quirks of the 0.2 mm CuL wire, threaded through ferrite pearls, as shown
on detail in the right corner of the pic.4.19. If telescopic antenna is to
be used, these coils should be omitted.