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DiY FM Micro Receiver | Open Electronics

DiY FM Micro Receiver | Open Electronics


A small built-in SMD provides life to a superb moveable FM stereo radio receiver which may be simply created as a result of it doesn’t require the manufacturing of coils or the calibration of tuned circuits.

The semiconductor business constantly churns out new integrated circuits for all software areas and in addition for shoppers and audio; among these, ICs capable of creating gear that when required extra built-in devoted to particular person levels and lots of digital elements. Examples we see within the progress that involved the radio receivers, once realized with transistors and at a later time facilitated by the supply of integrated units that each carried out a stage: antenna amplifier and front-end, superheterodyne (native oscillator and mixer) frequency conversion AF like the well-known SO-42P from Siemens), medium frequency amplifier and demodulator

(for instance, the Philips TDA1200 or the LM3089 of the previous Nationwide Semiconductors), FM stereo decoder (when required) just like the Motorola MC13010P and output amplifier. As we speak we now have built-in units capable of carry out the tasks of all these elements on their very own and, moreover, without requiring the creation and calibration of native oscillator antenna coils, which have been as soon as an actual impediment, both because of the availability of the adjustable ferrite cores, both for the essential issues related to respecting the form of the reels themselves, to be realized with enamelled or silver-plated copper wire.

In fact, single-chip FM radios had existed because the time of the revolutionary TDA7000 introduced by Philips some thirty years ago, but they nonetheless required the manufacturing of reels. As an alternative, for a while now there have been out there elements able to performing every little thing internally and tuning radio alerts, requiring at most quartz or a ceramic resonator and a discrete inductance or on PCB.

An example is the Hex3653, a custom in SMD encapsulated in an SOP container with eight ft per aspect, used in many miniature radio receivers and in ready-made cards which might be available on the market.

In these pages, we propose the undertaking of a digitally managed FM stereo receiver, realized with this built-in circuit and powered by two easy stylus batteries, subsequently appropriate for moveable low consumption units.



So let’s check out the circuit diagram, which seems very simple, since virtually every little thing is “accomplished” inside the integrated U1, or Hex3653; in truth we discover very few external elements (Fig. 1), which are the load resistances of the audio outputs R3 and R4, their decoupling capacitors (the electrolytes C1 and C2), the quartz that gives the clock for the interior levels of the chip (Y1) and the transistor Q1, used to drive the LED by way of the R2 resistor, as a way to acquire a type of peak meter that signifies the level of the audio sign (the extra intensely the LED pulses, the higher the sign degree).

In fact, the LED acts as both a power-on mild (as a result of the base of the Q1 is polarized even in the absence of a sign by advantage of the continual element current on the LOUT output of the integrated circuit) that provides a sign meter, provided that the audio fluctuations involve a change in the base polarization of the transistor.

Within the circuit we even have a collection of normally open buttons terminating in mass, read immediately from the strains of the U1 that internally have a pull-up resistor; these buttons permit the quantity to be adjusted (the VOL- and VOL + pair, which intuitively lower and raise the sound degree to the stereo outputs by one step for every press, respectively), SEEK- and SEEK + for station search (the first returns back of 1 step, that’s sometimes 50 kHz, at every strain, whereas the second also advances the tuning at every press) and eventually PWS, which allows to modify the gadget on or to standby. The entire scan takes lower than 5 seconds.


Fig. 1

Standby is a situation that permits the built-in Hex3653 to be put to sleep in order that, whereas remaining fed between pins 6-10 and ground (2, 3, 5, 11, 14), it absorbs a present of only 10 microamps, which does not significantly affect battery life.

The PWS button is, actually, related to the PDN line, which corresponds to the built-in energy down; word that the on / off change is related to the PDN pin by way of a resistor and that PDN has a capacitor in parallel, to make sure that the ignition signal is dependable and does not endure from the bounce of the button contact. In reality, R1 creates, with C3, a time constant (100 kohm x 100 nF = 0.01 seconds) that permits bringing pin 1 of the built-in provided that the button is actually closed for a certain time and to not make it change if the contact is uncertain.

The tuning ranges between 76 and 108 MHz in steps of sometimes 50 kHz, which is the space between the channels in “slender” FM (in FM, usually the step between the channels quantities to 75 kHz); the extension of the tuneable FM band has been designed to cover all of the frequency-modulated radio bands on the planet. In Italy and usually in Europe the FM vary sometimes ranges between 87.6 and 108 MHz.

The antenna stage of the circuit is extraordinarily simplified and we do not find the normal varicap diodes or the more conventional variable capacitors, but slightly a hard and fast anti-resonant circuit; the stage in question is related to the pin-strip shown in the wiring plan, ASW: the RF input line of the built-in unit is related to contact 3, which, related by a jumper with a 2.54 mm pitch to pin 4, connects it to the antenna pitch itself (sometimes a bit of copper wire with sheath, about eighty centimetres long). In parallel to the antenna input (FMIN pin, or four of the Hex3653) there is a pair of diodes related in antiparallel, which serves to protect the chip from any electrical discharges picked up by the antenna or introduced by mistake by the wiring (it may possibly occur when you use an exterior antenna related with a coaxial cable to the wall socket in your home) as well as a quite simple anti-resonant circuit shaped by the capacitor C4 and the inductance L1; being the latter of 10 µH and C4 of 33 pF, the ensuing band centre frequency (excluding the parasitic elements) is about 87 MHz.



The tuneable signal falls in a fairly large band since in actuality this value is altered by the built-in circuit of the input and by the truth that we’re within the presence of a low Q anti-resonant circuit, provided that the inductance is low Q and the U1 front-end has a non-negligible load resistance.

The signal that passes inside the band is then amplified contained in the chip, tuned and transformed by a superheterodyne circuit synchronized by the quartz Y1, then demodulated in frequency till obtaining the audio sign; the AGC (automated volume management) stage allows the audio degree to be stabilized, preventing it from being affected by the power of the antenna radio signal. A special FM stereo decoder stage that completes the built-in, extracts the alerts of the two channels, that are then preamplified and despatched to the LOUT and ROUT outputs referred to the widespread mass. The two attain, by means of the decoupling capacitors C1 and C5, the three.5 mm stereo jack connector for listening in headphones and in addition the EC pin-strip, which allows connection to an exterior stereo audio amplifier. The same EC connector allows the primary energy to be taken to the surface since it is parallel to the + and – PW contacts, to which the battery voltage is utilized.

As regards to audio outputs, it is value explaining the position of the ASW pin-strip, as a result of having among its connections both the widespread headphone jack and the antenna, one may marvel what connection there may be. Nicely, the concept is that the circuit, being born as a transportable stereo radio, can use as an antenna each the headphone or earphone wire (precisely as it occurs with the FM radio constructed into cell phones and smartphones) and a separate antenna, be it wire or stylus, that’s, of another type; if you wish to use the headphone wire, which allows you to avoid the encumbrance of the antenna, it’s essential leap contacts 2 and three, which brings the widespread contact of the jack on the antenna input. Now two issues have to be made: the primary is that the mass connection is used since in the coaxial cables for headphones or earphones is the display and subsequently the outer wire; if a sign wire have been used, it will be shielded by the display mesh and would not be capable of decide up the radio sign. The second consideration is that so far as one might assume that lifting the widespread of the jack from mass prevents the operation in stereo, in reality, it’s attainable and it’s assured because the inductance of 10 µH to the audio frequencies is practically a brief circuit (the impedance that it presents at 10 kHz is simply 62.eight hundredths of an ohm, towards a couple of tens of ohms impedance of the earphones) and subsequently it’s as if the widespread was truly grounded. The identical would continue to be the case.

If the wire antenna is used, the purpose 3 have to be jumped with the 4 (thus becoming a member of the antenna pad with the FMIN pin of the built-in) and the 1 with the 2 (thus connecting the widespread of the stereo jack).

In the event you plan to integrate the radio receiver in one other gadget containing an audio amplifier, or to connect the circuit to a stereo amplifier that permits listening in the loudspeaker, you may also not bridge the ASW contact 2 neither with the ground nor with the antenna input, as a result of you’ll not use it, until you additionally need to have a headphone output..

We conclude the outline of the circuit with the facility provide, which is filtered by the C5 capacitor and which may be provided by a pair of collection batteries, or by a lithium-ion cell or lithium polymer cell; the absorption is sort of contained and at 3 volts it doesn’t exceed 30 milliamps with a stereo headphone related to the OUT jack.



From openstore

HEX3653 FM 76-108 MHz Radio Receiver


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