Medium-Wave Modulator

If you insist on using a valve radio and listening to medium-wave stations, you have a problem: the existing broadcasters have only a limited number of records. Here there’s only one remedy, which is to build your own medium-wave transmitter. After that, you can play your own CDs via the radio.

The transmitter frequency is stabilised using a 976-kHz ceramic resonator taken from a TV remote control unit. Fine tuning is provided by the trimmer capacitor. If there’s another station in the background, which will probably be weak, you can tune it to a heterodyne null, such as 981 kHz. As an operator of a medium-wave transmitter, that’s your obligation with respect to the frequency allocations. And that’s despite the fact that the range of the transmitter is quite modest. The small ferrite coil in the transmitter couples directly into the ferrite rod antenna in the radio.

Medium-Wave Modulator Circuit Diagram

 

The modulator is designed as an emitter follower that modulates the supply voltage of the output amplifier. As the medium-wave band is still mono, the two input channels are merged. The potentiometer can be adjusted to obtain the least distortion and the best sound. The RF amplifier stage has intentionally been kept modest to prevent any undesired radiation. The quality of the output signal can also be checked using an oscilloscope. Clean amplitude modulation should be clearly visible.

The medium-wave modulator can simply be placed on top of the radio. A signal from a CD player or other source can be fed in via a cable. Now you have a new, strong station on the radio in the medium-wave band, which is distinguished by good sound quality and the fact that it always plays what you want to hear.

 

Author: Burkhard Kainka - Copyright: Elektor

Read more

FM Stereo Transmitter

You'll find that this is a very easy project to build. It will transmit good quality sound in the FM band ( 88 - 108 mhz ). One inportant item is that the IC chip operates on 3 volts DC. The chip will get destroyed if it is operated on any voltage higher than 3.5 volts. The antenna can be a standard telescopic antenna or a 2 foot length of wire. The input is in the millivolt range and you may need to add additional pots for the inputs. I was able to use this circuit for a walkman and a portable CD player in my car. I used the headphone jack on both and varied the signal with the volume control.

FM Stereo Transmitter Circuit Diagram

To adjust the circuit tune your FM radio to a quite spot then adjust the trimmer capacitor C8 until you hear the signal that you are transmiting. When you have a strong signal adjust the resistor R4 until the stereo signal indicator lights. If the input is to high of a signal you may over drive the IC chip. Use two 15 turn pots on the input signals to bring the level down. You can balance the signal by using headphones. The inductor L1 is 3 turns of .5 mm wire on a 5 mm ferrite core.

Read more

Simple RF Noise Source

A noise generator with a wideband output signal is always handy to have around when you're adjusting receivers and other types of HF equipment.

Simple RF Noise Source Circuit Diagram:

The noise generator circuit described here uses the base-emitter junction of a transistor (in this case a 8Fl99) operating under reverse bias. As a result, it acts as a Zener diode and generates a wideband noise signal. The noise signal passes through a 1-nF capacitor to the

output connector (female BNC), which means that its low-frequency components do not appear at the output. The 47-Q resistor gives the noise generator an  output impedance of nearly 50 Q.  You can easily fit the entire noise generator in a small metal enclosure equipped with a BNC connector. The supply voltage is not critical; anything in the range of 8 to 15 V will do.

 

Author : Fred  Brand - Copyright : Elektor

Read more

40 MHZ-Wide Band RF Amplifier

The sensitivity of receiver it is possible to increase itself considerably, if is interfered, between this and his aerial, a amplifier RF.

40 MHZ-Wide Band RF Amplifier Circuit Diagram :

The amplifier of circuit, does not use in resonant circuits and for this is suitable, so much for mid waves, what for the low waves, up to the 40 MHZ. The gain his it is the order 20db and it consumes 7mA, when it is supplied with 12 until 15V dc. His entry and the exit are adapted with coaxial cable, complex resistance 75ohm.

Source : users.otenet.gr

Read more

Simple Long Range FM Transmitter

The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometres.

Simple Long Range FM Transmitter Circuit Diagram :

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below:

• L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
• L2 - 2 turns of 24 SWG wire near top end of L1.
  (Note: No core (i.e. air core) is used for the above coils)
• L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.
• L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal in India.

WARNING: Transmitting on the UK Commercial FM band is also illegal in the UK, please see the general disclaimer. This circuit is shown for educational purposes only.

Read more

Simple Mini FM Receiver

This is the most simple fm radio receiver with good performances that works great even if the sensitivity is not too high. The working principle of this fm receiver may seem a little unusual. It is made of an oscillator (T2 and T3) that is synchronized with the received frequency of T1. This transistor works as a broadband preamplifier in VHF range.

Mini FM Receiver Circuit Diagram :

The oscillator is adjusted between 87 … 108 MHz with C5. Because of the synchronization, the oscillator output will have the same frequency deviation as the received signal from the fm antenna. This deviations are caused by the broadcasted audio informations. The frequency modulated signal show up on P1 + R5. Low pass filter R6/C6 extracts the audio signal and then is amplifier by T4 … T6 and transmitted at the output through C9 capacitor.

The coil details are presented in the fm receiver circuit diagram. The radio receiver is adjusted on different stations with the help of C5. P1 potentiometer is adjusted untill the best reception is obtained. If we attach an audio amplifier and a speaker then this fm receiver can be made very compact as a pocket radio.

Read more

High Level Wideband RF Preamplifier

A linear RF amplifier can be made in two ways: (1) with the aid of a linear active element, or (2) with a non-linear element operating with negative feed-back. This circuit is of the second kind, using an RF power transistor as the active element. Feedback is also required to ensure correct termination (50 Q) of the aerial, since bipolar transistors normally exhibit a low input impedance. Also, the noise figure is not increased because virtually no signal is lost.

Circuit diagram :

High Level Wideband RF Preamplifier Circuit Diagram

The common-base amplifier is based on a UHF class A power transistor Type 2N5109 from Motorola. The feedback circuit is formed by RF transformer Th. The input and output impedance of the preamplifier is 50 4 for optimum perform-ance. Network R3-C5  may have to be added to  preclude oscillation outside the pass-band, which  ranges from about 100 kHz to 50 MHz. The gain is  approximately 9.5 dB, the noise figure is between 2  and 3 dB, and the third-order output intercept point  is at least 50 dBm.

The input/output transformer is wound on a Type FT37-75 ferrite core from Micrometals. The input winding is 1 turn, the output winding 5 turns with a tap at 3 turns.

Read more

FM Tuning Indicator Circuit

Here is an add on circuit to your FM radio for precise tuning of stations. Usually an LED indicator is provided in FM radio to see whether the station is tuned or not. But it is difficult to see the precise tuning points since the variation in the LED brightness cannot be detected easily. This circuit solves the problem.

Circuit diagram :

Precise  FM Tuning Indicator Circuit Diagram

The circuit uses the Op Amp IC CA3140 as a differential amplifier to sense the voltage level between the terminals of the tuning LED. The output of IC1 drives two LEDs one Red and one Green to indicate whether the station is precisely tuned or not. If both LEDs remain off, it indicates precise tuning. If anyone LED is on, it indicates that tuning is not precise. If both LEDs remain on, it indicates that there is no signal.

First tune the FM receiver in a station having strong signal. The tuning LED will light brilliantly. Then connect point A and B to the soldering points of the tuning LED observing polarity. Adjust VR till both the Red and Green LEDs turn off. Slowly change the tuning knob position. Any one LED will light up. So the circuit is working. Now glue the wiper of VR using adhesive to prevent its position change. Disconnect the tuning LED from the board. Now the circuit is ready to use. Power for the circuit can be obtained from the power supply of FM radio.

Read more

Vest Pocket VHF FM Test Generator

After licensing restrictions were relaxed in many countries for VHF FM band transmitters with 50 nW transmit power, several small, inexpensive FM transmitter modules appeared on the  market. In the author's view, such a module forms a good basis for a small FM-band test generator. This only requires a sine-wave modulation signal, which can come from an  existing audio generator. lf you don't have a suitable audio generator available, you can build the Wien bridge oscillator described here.

Circuit diagram :

Vest Pocket VHF FM Test Generator Circuit Diagram

FET T1 provides amplitude stabilisation in order to keep the distortion low.The generated signal is fed to the transmitter module via a 3.5-mm stereo headset socket, which mates with the usual 3.5-mm stereo plug of the FM transmitter (the left and right terminals of the socket are wired together). Adjust the output level of the audio oscillator with potentiometer Pl to avoid overdriving the transmitter.

ln the transmitter module used by the author, the HF stage is built  around a Rohm BH1418FV lC. You can easily find the data sheet for this lC with a Google search, and it can help you identify the HF output on the transmitter circuit board. You can then use a length of coax cable to tap off the FM signal and feed it to the antenna connector of the receiver under test. Here you must pay attention to the maximum rated input level of the receiver and impedance matching, and if necessary you should use an attenuator at the receiver input. You can use an oscilloscope to trace the signal in the receiver and analyse the receiver output signal.

Author : Kai Riedel  - Copyright : Elektor

Read more

Simple Low Power AM Transmitter

This transmitter is basic but allows transmission of audio to an AM radio. It consists of an RF oscillator operating in the AM broadcast band, together with a modulator stage, which mixes the incoming audio and the RF. A signal appears on the output, which has an AM component that can be picked up on a nearby AM radio receiver.

Circuit diagram :

Basic Low Power AM Transmitter Circuit Diagram

The transmitter consists of oscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedback to the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a short-wire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input  along with the carrier signal.

An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output. Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a well modulated AM signal.

Read more

Intruder Radio Alert System

Consider a situation where a burglar has entered your house and snapped the telephone wires, leaving you with no means of communication with the outside world. In such an emergency, you will find this intruder alarm to be very handy. It transmits a prerecorded emergency message repeatedly for reception by an FM receiver.

The message containing address, geographical location, name, etc is recorded onto a chip. The prerecorded message can then be transmitted repeatedly with the help of an FM transmitter, in the hope that some noble soul will hear it and inform the police about the incident.

 

Fig. 1: Block diagram of the intruder radio alert system

The circuit comprises a sound recording-and-playback chip (UM5506BH). This chip consists of a 96kbit SRAM and can record up to six seconds of audio. (For details, refer ‘Mini Voice Processor’ circuit published in April 2000 issue of EFY.) After the required message has been recorded, it is passed to a low-power, VHF FM transmitter wired around BC547 and 2N2369 transistors. The range of this transmitter is 60 to 100 metres using a 40-70cm long wire as an antenna.

Fig. 2: Circuit diagram of intruder radio alert system

The major advantage of this circuit is its low power consumption. The author operated it on 3V button cells (Maxell CR 2032, CR 2025, etc used in digital diaries). To transmit the prerecorded message, the play button is pressed repeatedly. The transmitted message can be heard over the FM receiver.

A possible modification, though it has legal complications, is to vary the coil inductance such that the transmission is on police band, thus alerting the police for quick help. Even the need of repeatedly pressing play button can be obviated by configuring an astable multiviberator (using IC 555 timer) to trigger IC UM5506BH every six seconds so that the message is played repeatedly.

 

Author : David Nash Pious – Copyright : EFY

Read more

Wide-band Receiver for Spark Transmissions

In the early years of radio technology spark transmissions ruled the (air) waves. They  occupied a relatively wide bandwidth, in what  came to be known as the long waveband. The receivers used had a corresponding band-width, as ‘wide open’ as the proverbial ‘barn door’. Most were simple detectors without an amplifier stage.

Circuit diagram :

Wide-band Receiver for Spark Transmissions Circuit Diagram

Today when you operate an electric light switch you produce a wideband spark that’s  audible on some radios as a crackle from long through to short waves. The same occurs with intermittent breaks in cables, high voltage strikes, defective transformers, poorly sup-pressed electric motors and all kinds of contacts that open and close. With a suitable receiver it’s possible to trackdown the source of these problems. Tests  using normal radios are largely unsuccessful, simply because they display restricted bandwidth and are too effective at suppressing short interference pulses. After some research the best results were obtained with a wideband Audion receiver.

The requirements for this kind of receiver are totally different from normal radio reception: the receiver must have bandwidth as wide as possible, with maximum sensitivity in the long wave region. A further special request: since the wave packets of a single spark are often extremely short, the receiver should integrate them into a longer pulse whose spectrum should lie well inside the audible range.

As for the circuit, the audion stage in the collector circuitry detunes the input circuit. To prevent self-oscillation we need to add a 10 kΩ resistor. Using an oscilloscope you can see extremely short pulses on the emitter of the BC557 being broadened. The amplitude is frequently sufficient to drive the final amplifier into limiting. A1 μs long input pulse  results in a circa 1 ms long audio pulse in the loudspeaker.

 

Author :  Burkhard Kainka  Copyright  : Elektor

Read more

Multiband CW Transmitter

A radio frequency oscillator is at the heart of all radio transmitters and receivers. It generates high frequency oscillations, which are known as carrier waves. Here’s a continuous wave (CW) transmitter for transmitting.Morse code signals in the shortwave band (see Fig. 1). It is basically a variable frequency oscillator (VFO) whose frequency can be varied from 5.2 MHz to 15 MHz. The signal can be received in the shortwave band by any radio receiver. The circuit works off a 9V battery.

Circuit diagram :

Fig. 1 : Multiband CW Transmitter Circuit Diagram

Connect the Morse key (S1) across capacitor C5 as shown in the figure. Attach a telescopic antenna (capable of transmitting over a short distance) at the output terminal.The coil and gang capacitor C2 form the tank circuit. The coil (L) has a total of 60 turns. Winding details tails are given in Fig.2. Tappings on the coil allow selection of the required band. The frequency can be varied using C2 (main tuning).

Fig.2 : Multiband CW Transmitter

On reducing turns of the coil (using selector switch S2), the oscillator’s frequency increases because frequency is inversely proportional to inductance. Capacitor C1 couples the signal from the tank circuit to the base of transistor T1 (2N2222). Transistor T1 provides the required positive feedback for oscillation and transistor T2 (BC547) functions as the emitter follower. The output is taken from the emitter of T2.

For stable oscillations, use a polystyrene capacitor as C1. All other capacitors may be ceramic disk type. Enclose the circuit in a metal box for better shielding.

 

Source : EFY

Read more

Short-Wave Superregenerative Receiver

Superregenerative receivers are characterised by their high sensitivity. The purpose of this experiment is to deter-mine whether they are also suitable for short-wave radio. Superregenerative receivers are relatively easy to build. You start by building a RF oscillator for the desired frequency. The only difference between a superregenerative receiver and an oscillator is in the base circuit. Instead of using a voltage divider, here we use a single, relatively high-resistance base resistor (100 kΩ to 1MΩ).

Superregenerative oscillation occurs when the amplitude of the oscillation is sufficient to cause a strong negative charge to be applied repeatedly to the base. If the regeneration frequency is audible, adjust the values of the resistors and capacitors until it lies somewhere above 20 kHz. The optimum setting is when you hear a strong hissing sound. The subsequent audio amplifier should have a low upper cutoff frequency to strongly attenuate the regeneration signal at its output while allowing signals in the audio band to pass through. This experimental circuit uses two transistors. A Walkman headphone with two 32-Ω earphones forms a suitable output device.

Circuit diagram :

Short-Wave Superregenerative Receiver Circuit Diagram

 

The component values shown in the schematic diagram have proven to be suitable for the 10–20 MHz region. The coil consists of 27 turns wound on an AA battery serving as a winding form. The circuit produces a strong hissing sound, which diminishes when a station is received. The radio is so sensitive that it does not require any antenna to be connected. The tuned circuit by itself is enough to receive a large number of European stations. The circuit is usable with a supply voltage of 3 V or more, although the audio volume is greater at 9 V.

One of the major advantages of a superregenerative receiver is that weak and strong stations generate the same audio level, with the only difference being in the signal to noise ratio. That makes a volume control entirely unnecessary. However, there is also a specific drawback in the short-wave bands: interference occurs fairly often if there is an adjacent station separated from the desired station by some-thing close to the regeneration frequency. The sound quality is often worse than with a simple regenerative receiver. However, this is offset by the absence of the need for manual feedback adjustment, which can be difficult.

Author :Burkhard Kainka  - Copyright : Elektor

Read more

Long Range FM Transmitter

The use of transmitters which have a more powerful output than the ‘flea-power’ are sometimes required when there are many obstacles in the path of the surveillance transmitter and monitoring station receiver, or the distance between them is too far so as to make a low powered device feasible. Whereas a typical micro transmitter will produce an RF power in the order of just a few milliwatts, i.e. a few thousandths of a watt, the VHF-FM transmitter described has a power output of between around a half and 2 watts, depending on the power source, which may be anywhere between 6 volts and 30 volts d.c. .

Circuit diagram :

Long Range FM Transmitter Circuit diagram

The battery or batteries should be of the alkaline high power type, since the current drain will be found to be relatively higher when compared to microtransmitter current drain The power output of this device is somewhat proportional to the current drain and so therefore both may be decreased by altering the value of R6 to a higher resistance, or a variable resistor with a value of around 1k may be introduced in series with the existing R6, so as to give a variable power output. The variable resistormust not be a wirewound device because this would act as an inductor which will cause feedback problems.

The audio input to the power oscillator, which incidentally is formed by TR2 and associated components, is derived from a piezoelectric microphone which drives the simple audio frequency amplifier TR1. The input of the audio amplifier is controlled by the gain pot R1, which selects the correct amount of voltage that is generated by the piezoelectric microphone, then connects this signal to the base of audio amplifier TR1 via C7. It may be found that there is insufficient housing space for a bulky piezoelectric microphone, so with a slight modification to the circuit, it is possible to employ an electret microphone insert as shown.

Since the RF field that is generated by this transmitter is relatively large, the problem of RF feedback may very well be encountered. This may be overcome by placing the transmitter inside a metal enclosure, keeping all internal wiring as short as possible and the aerial wire.

Component listing for 1 watt transmitter

Resistors Semiconductors R1 = 27k TR1 = BC547 R2 = 330k TR2 = 2N2219 fitted with heat sink R3 = 5k6 MIC = piezoelectric microphone R4, 5 = 10k R6 = 100R

L = 6 turns 22 gauge enameled wire wound on 3⁄16″ former

Capacitors

C1, 2, 3 = 330 pF C4 = 2–10 pF trimmer C5 = 4p7 C6 = 1 nF

C7,C8 = 40uF/25V Electrolytic

Source : Circuittoday

Read more

Minimalist Dip Meter

In days gone by a radio amateur always had a dip meter close to hand in his ‘shack’. Now that people can afford oscilloscopes, the poor old dip meter has lost its importance and is  frequently no longer to be seen. Actually this is a shame because many tasks are much easier to carry out with a dip meter. Anyone who’s interested (perhaps the second time around) can easily build one rapidly with this very simple but adequate circuit. The interesting question is namely what do you actually need from a dip meter?

Minimalist Dip Meter Circuit Diagram

• A visual display of the dip? Nope, the ‘scope can handle that task.
• A large frequency scale? Not necessary, as you can connect a frequency counter for this.
• A selection of coils? We don’t need these because we can use a jumper to change range (no coils to lose any more!).

The sensor coil L1 has ten turns and is wound  using an AA-size battery as a former. This coil will allow us to over the range from 6 MHz to 30 MHz. With jumper JP1 open an additional fixed inductance of 10 μH comes into circuit. The frequency measurement range is then from 2.5 MHz to 10 MHz. The switch may be replaced by a jumper.

To take measurements you hold a resonant circuit close to the sensor coil. Tune the rotary capacitor C1 slowly to and fro in order to find the resonant frequency, at which the oscillator amplitude decreases somewhat. The frequency can then be read directly off the oscilloscope.

To obtain a very accurate measurement you can additionally connect your frequency counter to the second output.

Author : Burkhard Kainka - Copyright : Elektor

Read more

Four-Stage FM Transmitter

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected at the input of the oscillator.

Circuits diagram :

Four-Stage FM Transmitter Circuits Diagram

Working of the circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna. Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range.

Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the frequency generated. You can also use a 12V battery to power the circuit.
Assemble the circuit on a general-purpose PCB. Install the antenna properly for maximum range. Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively. 

EFY note. This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.

Author : Pradee G.  - Copyright : EFY

Read more

Short-Wave Converter

This short-wave converter, which doesn’t have a single coil requiring alignment, is intended to enable simple medium-wave receivers to be used to listen to short-wave signals. The converter transforms the 49-m short-wave band to the medium-wave frequency of 1.6 MHz. At the upper end of the medium-wave band, select an unoccupied frequency that you want to use for listening to the converted short-wave signals. Good reception performance can be obtained using a wire antenna with a length of one to two metres.

Circuit diagram :

Short-Wave Converter Circuit Diagram

The converter contains a free-running oscillator with a frequency of around 4.4 MHz, which is tuned using two LEDs (which act as variable-capacitance diodes!) and a normal potentiometer. The frequency range is set by adjusting the emitter current using a 1-kΩ trimpot. The oscillator frequency depends strongly on the operating point. This is due to the combination of using an audio transistor and the extremely low supply voltage. Under these conditions, the transistor capacitances are relatively large and strongly dependent on the operating point.

The second transistor forms the mixer stage. If you calculate the resonant frequencies of the tuned circuits, you will obtain 6.7 MHz for the antenna circuit and 1.7 MHz for the output circuit. Additional transistor capacitances and the effects of the coupling capacitors shift each of the resonant frequencies down-ward. The tuned circuits are relatively heavily damped to obtain bandwidths that are large enough to allow the circuit to be used without any specific alignment. The results are good despite the low collector–emitter voltage of around only 0.6 V, due to the fact that only a modest amount of mixer gain is necessary. The entire circuit also draws less than 1 mA.

 

Author :Burkhard Kainka - Copyright : Elektor

Read more

Tuned Radio Frequency (TRF) Receiver

Super heterodyne receivers have been mass-produced since around 1924, but for reasons of cost did not become successful until the 1930s. Before the second world war other, simpler receiver technologies such as the TRF receiver and the regenerative receiver were still widespread.

 

Circuit diagram :

 

Tuned Radio Frequency (TRF) Receiver Circuit Diagram

 

The circuit described here is based on the old technology, but brought up-to-date a The most important part of the circuit is the input stage, where positive feedback is used to achieve good sensitivity and selectivity. The first stage is adjusted so that it is not quite at the point of oscillation. This increases the gain and the selectivity, giving a narrow bandwidth. To achieve this, the potentiometer connected to the drain of the FET must be adjusted very carefully: optimal performance of the receiver depends on its setting. In ideal conditions several strong stations should be obtainable during the day using a 50 cm antenna. At night, several times this number should be obtainable.

 

The frequency range of the receiver runs from 6 MHz to 8 MHz. This range covers the 49 m and the 41 m shortwave bands in which many European stations broad-cast. Not bad for such a simple circuit! The circuit employs six transistors. The first stage is a selective amplifier, followed by a transistor detector. Two low-frequency amplifier stages complete the circuit. The final stage is a push-pull arrangement for optimal drive of the low-impedance loud-speaker. This circuit arrangement is some-times called a ‘1V2 receiver’ (one preamplifier, one detector and two audio frequency stages).

 

Setting-up is straightforward. Adjust P1 until the point is reached where the circuit starts to oscillate: a whistle will be heard from the loudspeaker. Now back off the potentiometer until the whistle stops. The receiver can now be tuned to a broad-caster. Occasional further adjustment of the potentiometer may be required after the station is tuned in.  The receiver operates from a supply volt-age of between 5 V and 12 V and uses very little current. A 9 V PP3 (6F22) battery should give a very long life.

 

Author : Gert Baars - Copyright :  Elektor

Read more

SW Converter for Digital AM Car Radio

This circuit is purposely presented with many loose ends (not literally, of course) to stimulate experimenting with RF circuitry at a small outlay. Looking at the circuit diagram you may recognize a modified version of the SW Converter for AM Radios described elsewhere in this issue. The modifications were necessary to make the circuit compatible with a digital rather than analogue AM car radio. The main difference between digital AM radios and their all-analogue predecessors is that tuning is in 9 kHz (some-times 4.5 kHz steps) in compliance with the international frequency allocation for the band. Obviously, that particular step size, desirable as it may be on MW, is a stumbling block if you want to use a digital AM receiver in combination with a frequency step-up converter for SW, where chaos reigns and there is no fixed step size. The first attempt was to make the crystal oscillator variable by about 5 kHz each way.

 

Circuit diagram :

SW Converter for Digital AM Car Radio Circuit Diagram

 

Unfortunately, despite serious efforts, the crystal could not be pulled more than 1 or 2 kHz so another solution had to be found. After studying the NE/SA602/612 datasheet, it was found that a variable LC based oscillator was the best alternative. The circuit worked after winding a resonant LC circuit and adding a 0.1 µF series capacitor to block the DC component on pin 6 of the NE602 (612). When the tuning was found to be a bit sharp with the original capacitor, a simple bandspread (or fine tuning) feature was added by shunting the LC resonant circuit with a lightly loaded 365 pF tuning capacitor (C10) which, like the main tuning counterpart, C8, was ratted from an old transistor radio. The tuning coil, L1, consists of 8 to 10 turns of 0.6-0.8mm dia. enamelled copper wire (ECW) on a 6-8 mm dia. former without a core. With this coil, frequency coverage will be from about 4 MHz to 12 MHz or so. Details on Tr1 may be found in the referring article.

 

Note that no tuning capacitor is used on the secondary — the input stray capacitance of the NE602 (612) does the trick. A BFO (beat frequency oscillator) was added to enable SSB (single sideband) signals to be received. The BFO built around T1 is simple, has a heap of output and is stable enough to hold an SSB signal for a few minutes without adjustment. The BFO frequency is tuned with C3. Tr2 is a ready-made 455 kHz IF transformer whose internal capacitor was first crushed and then removed with pliers. When S2 is closed the BFO output signal is simply superimposed on the NE602 (612) IF output to the MW radio. The converter should be built into a metal box for shielding. If you find that the BFO gives too much output, disconnect it as suggested in the circuit diagram and let stray coupling do the work. Sensitivity, even on a 1-metre length of car radio aerial, is quite amazing. Bearing in mind that most of the major international SW broadcasting stations like Radio NHK Japan, Moscow, BBC etc.) generate enough power to make sure that you will hear them, it is still quite exciting to hear such signals for the first time on your car radio.

 

Author : P. Laughton, VK2XAN – Copyright : Elektor Electronics

Read more