Extended VHF Extended Range Digital Radios

Ten...twelve years ago, amateur radio magazines often published articles on converting imported receivers from the FM band (88...108 MHz) to the VHF-1 band (65.8...75.0 MHz). At that time, broadcasting was carried out exclusively in the VHF-1 range.

Now the situation has changed dramatically. The airwaves in the range of 100...108 MHz are almost everywhere filled. There are many imported and domestic radio receivers on sale with the VHF-2 range or with general ones (VHF-1 and VHF-2).

Since the VHF-1 range was virtually “orphaned,” a gigantic fleet of old radios and tape recorders remained “out of use.” They can be given a second life through a relatively simple modification of the VHF units of these receivers. The following points should be noted. Conversion of inexpensive portable receivers ("VEF", "Sport", "Sokol", "Ocean", etc.) should be minimal and provide reception of 3...7 VHF-2 radio broadcasting stations in a given region. For stationary devices of a higher class with an external VHF antenna, it is desirable to preserve all its technical parameters (sensitivity, local oscillator stability, wide scale, etc.).

Typically, the VHF radio receiver unit contains an input circuit, 1-2 UHF stages, a local oscillator, a mixer, and IF stages. As a rule, these are 4 (less commonly 5) LC circuits. Having a basic (even better, wiring) diagram of the radio receiver, it is easy to determine all the necessary components (inductors, capacitors, etc.). The first circuit of the amplifier and all subsequent cascades do not need modification.

It is clear that for the range of 100...108 MHz, the capacitances and inductances of all LC circuits of the VHF-1 unit must be reduced. Theory and practice state that the capacitance of the circuit varies in proportion to the wavelength, and the number of turns of the inductor varies with the square root of this value.

When moving from the VHF-1 range to the VHF-2 range and with constant inductances (the number of turns of the inductors does not change) - this is an option for portable receivers for the mid-frequency ranges (69.0 MHz and 104.0 MHz) - we obtain the following ratio for capacities:

With UKV-2 = 0.44*With UKV-1.

Taking this into account, in practice the following ratio of capacities is more suitable:

With UKV-2 = (0.3...0.35)*With UKV-1.

In addition, in VHF units it is possible to change the inductance of the loop coils within certain limits by rotating the tuning cores. Typically, the local oscillator of the VHF-2 block for the range of 100...108 MHz should be tuned within 110...119 MHz (with a margin) at IF = 10.7 MHz, and within 106...115 MHz at IF = 6, 5 MHz, i.e. higher than the signal frequency. On the schematic diagram of the UKV-1 unit, we mark those containers that will be completely soldered out of the circuit, as well as those containers that will be replaced with others with a lower rating. Typically these are miniature disk ceramic capacitors.

Capacitors must be selected in advance, cleaned and tinned leads, shortening them to a minimum. If there is no device for accurately measuring capacitance, the table below will partially help solve the problem. Table 1, where the size and color of the capacitor will indicate the limits of the nominal capacitance.

Table 1

For clarity, you can compare the capacitance ratings in the radio receivers "VEF-221" and "VEF-222", which are built according to the same circuits with the same inductors ("VEF-221" has a range of 87.5...108 MHz, " VEF-222" - 65.8...74.0 MHz). These data are taken from the factory instruction manual (Table 2). The capacitance ratings are given in picofarads.

table 2

The VEF-215 radio receiver and the VEF RMD-287S radio receiver have similar VHF block diagrams, so the data in Table 2 is also suitable for converting the VHF blocks of these devices.

Another example is a removable auto receiver of the "Ural-auto-2" type (input circuit, two UHF stages on GT322A transistors, a local oscillator on a 224-series microcircuit with the index ZHA1 or XA1). In the input circuit in the capacitive divider C1-C2, we change C1=22 pF by 5.1...6.8 pF, C2=33 pF by 10...12 pF. We change capacitors C5, C7 and C14 of 33 pF each (series capacitances with the KPI of the 1st, 2nd stages of the UHF and local oscillator) to 12... 13 pF. In the local oscillator circuit, we replace the tuning core made of ferrite (0 2.88 mm) with a brass threaded one (diameter 3 mm). Another example is the “Radiotechnika T-101-stereo” tuner (VHF unit on KT368A and KT339A transistors, tuned with KVS111A varicaps). Parallel capacitances SZ = 15 pF (input circuit), C14 = 15 pF (UHF), C18 = 9.1 pF (heterodyne) are dismantled. Series capacitances C4 = 130 pF, C13 = 130 pF (input circuit and UHF) are changed to 43...47 pF, and C15 = 82 pF (heterodyne) - to 27...33 pF. To stretch the scale, carefully unsolder the local oscillator coil and unwind 1.5 turns from the top of the coil, 1 turn from the bottom (the tap is from 0.9...1.2 turns as it was). Then carefully solder the coil into place.

It is convenient to divide the process of altering VHF receiver units into several stages.

We provide access to the VHF unit both from the parts side and from the side of the printed conductors by removing the covers of the receiver and the VHF unit.
We determine the LC circuits of the input circuit, UHF, local oscillator, mixer, and the first circuit of the amplifier (the latter is not affected by the alteration).
Carefully solder the containers that need to be replaced and dismantled.
We solder new containers, prepared in advance (with cut and tinned leads) for each individual circuit of the VHF unit.
Having made sure that there are no errors and the circuit is not broken (there are no bad solders, short circuits in printed circuits, etc.), we turn on the power to the receiver and try to hear at least one powerful (in a given location) VHF station. At the same time, we rotate the receiver tuning knob and the local oscillator core. It is very useful to have an industrial VHF-2 receiver nearby. This will help you immediately identify the desired station in the receiver you are tuning. Having heard at least barely a station, we achieve loud reception of this station using the trimming coil cores and trimming capacitors of the input circuit, UHF and mixer. At this stage, you can determine whether it is necessary to change cores from ferrite to brass and vice versa.
By rotating the core of the local oscillator coil, we set the required location of this station on the receiver scale (focusing on an industrial receiver with the VHF-2 range). Typically, the section of the receiver's tunable scale, where stations in the 100...108 MHz range are located, occupies a very small part of the receiver's design scale (about one third).
We pair the circuits of the input circuit, UHF and local oscillator of the tuned VHF unit. In the area near 100 MHz, we achieve the highest volume of stations by rotating the tuning cores of the input circuit, UHF and mixer, and in the area near 108 MHz - by rotating the rotors of the tuning capacitors of the same cascades (in this case, you need to monitor the position of the receiver tuning knobs - the maximum capacity of the KPI or varicaps at the beginning of the range and their minimum capacity at the end). We repeat this operation 2-3 times. In conclusion, it is necessary to reduce the capacitance in the AFC circuit by 2...2.2 times (if its nominal value exceeds 5...6 pF). The last stage must be carried out in the assembled VHF unit through the holes in the covers to adjust the capacitances and inductances with a dielectric screwdriver.

These general rules for altering VHF units should be followed for various schemes and designs of units. Briefly about receiving antennas. Obviously, directional antennas provide excellent reception quality, but they need to be rotated. For the rebuilt T-101-stereo tuner, the author uses a single square (two parallel copper wires with a diameter of 1.8 mm with a distance between them = 15 mm and a perimeter of slightly less than 3 m). The characteristic impedance of the square is about 110 Ohms, so it is powered by a PRPPM cable - 2 x 1.2 (the characteristic impedance is about 135 Ohms). The height of the mast on a five-story building is approximately 9 m. The plane of the square is perpendicular to the line Chisinau - Bendery - Tiraspol - Odessa. As a result, more than 10 stations in Chisinau and 3-4 powerful stations in Odessa can be heard.

Sources

A short reference book for the REA designer (edited by R.G Varlamov). -M.: Sov. Radio, 1972, pp. 275,286.
V.T. Polyakov "Direct conversion transceivers". - M.: 1984, P.99.
P.M. Tereshchuk et al. Amateur Radio Handbook, part 1. Kyiv: Tekhnika, 1971, S.Z0.
"VEF-221", "VEF-222". Manual.
Radiotechnika (tuner T-101-stereo). Manual.
A.N. Maltisky, A.G. Podolsky. Broadcast reception in a car. - M.: Radio and Communications, 1982, P.72.
V. Kolesnikov "Antenna for FM reception." - Radiomir, 2001, N11, P.9.

This article describes a simple and economical receiver that allows you to receive wideband and narrowband FM stations in the range of 30...130 MHz. This receiver is useful for those who repair and assemble radiotelephones. An article was published about a simple radiotelephone operating in the 65...108 MHz range. The choice of this range is due to the ease of setting up the radiotelephone using factory receivers. But if you wish, you can configure this radiotelephone outside this range, since the TDA7021 chip remains operational in the frequency range 30...130 MHz, and the proposed VHF receiver will help with this. The circuit is characterized by high sensitivity, simplicity and good characteristics, does not contain scarce parts, and is easy to manufacture and set up.

Operating principle and configuration of the VHF receiver

The basis of the receiver (Fig. 1) is the DA1TDA7021 microcircuit, which is a superheterodyne with one frequency conversion and a low intermediate frequency (IF). This microcircuit contains a UHF, mixer, local oscillator, amplifier, amplifier-limiter, FM detector, BSN system and buffer amplifier 34.

The signal from the antenna, which

Specifications

Received frequency range, MHz………………………….. 30…130

1st subband, MHz………………………………………….. 30…50

2nd subband, MHz…………………………………………………………….. 50…70

3rd subband, MHz………………………………………………………70…90

4 subband, MHz…………………………………………… 90…110

5th subband, MHz…………………………………………. 110…130

6 subband, MHz……………………………………. 130…150

7 subband, MHz…………………………. 150…170

Sensitivity, µV……………………………………………………. 1

Current consumption, mA……………………………………………………12

Supply voltage, V………………………………………………………………. 3…6

Output power, W…………………………………………… 0.1

Load resistance, Ohm……………………………………. 16…64

The swarm is the wire from the headphones, which is supplied through the capacitor C12 to an external UHF, made on the VT1 KT368 transistor. The amplified high-frequency signal and the local oscillator signal, the frequency-setting circuit of which is inductors L1 ... L5 and capacitor C2, are supplied to the internal mixer of the microcircuit. The IF signal (about 70 kHz) from the output of the mixer is separated by bandpass filters, the correction elements of which are capacitors C4, C5, and is fed to the input of the limiting amplifier. The amplified and clipped IF signal is fed to the FM detector. The demodulated signal, having passed through a low-pass correction filter, the external element of which is capacitor C1, is sent to a silent tuning device (SNT). Connecting resistor R1 helps increase the sensitivity of the receiver by turning off the BSN device. From the output of the disconnected BSN device, a low-frequency signal is supplied to a buffer amplifier. Connecting blocking capacitor C7 helps to increase the low-frequency output voltage and more stable operation of the buffer amplifier. The low frequency signal from the output of the buffer amplifier is supplied through capacitor C6 and the volume control R2 to the input of the low frequency power amplifier on the DA2 TDA7050 chip. Chokes L6, L7 are used to decouple high-frequency and low-frequency signals when using headphones.

The receiver is tuned to the radio station by changing the resonant frequency of the local oscillator circuit. Range switching is carried out by switch SA1, which connects one of five inductors to the local oscillator of the DA1 TDA7021 microcircuit. Adjustment in each range is performed by variable capacitor C2. Inductors L1 ... L5 determine the setting of the required overlap of the corresponding range. The desired receiver volume is selected using variable resistor R2. This completes the receiver setup.

The TDA7021 chip can be replaced with its domestic analog K174XA34. But it should be noted that not all domestic analogues can operate in an extended range. Instead of the TDA7050 microcircuit, any low-voltage operational amplifier will do, but with the appropriate switching circuit. The KT368 transistor can be replaced with any low-noise RF transistor with a cutoff frequency of at least 600 MHz. The maximum capacitance of the variable capacitor C2 should not exceed 25 pF. If the capacitance is large, an additional “stretching” capacitor should be connected in series with this capacitor, reducing the total capacitance to the specified limits. Chokes L6, L7 can be used with any inductance of 20 μH.

The performance of the TDA7021 chip is not limited to the range of 30…130 MHz. Experiments with this chip have shown that it can operate stably in the frequency range 30...170 MHz. This opens up even greater receiver capabilities. Obtaining such a wide range is possible thanks to the good margin for excitation of the local oscillator on the TDA7021 chip.

The table (see below) shows the data of coils for the range 30...170 MHz. The entire range is divided into seven subranges. Five subranges are left the same, only two have been added. Since coils L* and L** are not

Coil data for the range 30… 170 MHz

Designation

Range, MHz

Coil data

10 turns PEV 0.6 mm 0 5 mm with brass trimmer

8 turns PEV 0.6 mm 0 5 mm with brass trimmer

6 turns PEV 0.6 mm 0 5 mm with brass trimmer

4 turns PEV 0.6 mm 0 5 mm with brass trimmer

2 turns PEV 0.6 mm 0 5 mm with brass trimmer

3 turns PEV 0.8 mm 0 5 mm

2 turns PEV 0.8 mm 0 5 mm

The number of turns of the coils is indicated approximately, since their inductance depends on many factors, so selection of turns cannot be avoided. The contour trimmer can be made of brass or ferrite. If desired, you can turn on the silent tuning system (SNT) by replacing resistor R1 with a resistance of 10 kOhm with a capacitor with a capacity of 0.1 μF, but in this case the sensitivity of the receiver will deteriorate by about one and a half times. In stationary conditions, it is better to use a telescopic antenna up to 1 meter long instead of a headphone wire, while chokes L6 and L7 should be excluded.

The modified receiver allows you to receive signals from home radiotelephones, broadcast VHF FM radio stations, aviation services, amateur radio stations, extended-range radiotelephones such as "SONY", "NOKIA", etc. Thus, the receiver has a wide range of capabilities that can satisfy most radio amateurs, operating in the VHF range.

Literature

1. Shumilov A. Simple radiotelephone // Radio amateur. 2001. No. 7.

2. Shumilov A. Returning to what was printed // Radio amateur. 2001.

3. ShumilovA. Returning to what was printed // Radio amateur. 2002

Rarely do people, when they hear an advertisement about FM radio, think about what the phrase means. According to accepted conventions, the term FM implies broadcasting on a carrier frequency falling within the range from 87.5 to 108 MHz, with FM modulation. But this does not exhaust the variety of methods for transmitting entertainment programs. Extended range digital radios are designed to fill the gap.

More often than not, we are talking about increased VHF boundaries. Most products receive at frequencies from 64 to 108 MHz, selected models, for example, Mason R411, extend their hand to 233 MHz. Such a broad framework covers the broadcasting of entertainment radio stations and fully covers the standard values accepted in aviation for negotiations.

Let us mention that within the Commonwealth countries the described equipment capabilities are hardly useful - transmissions are not carried out above 137 MHz - but on the territory of other states the option will be very useful.

Origin of the terms FM and AM

Each country has its own broadcasting standards. FM is considered the accepted name in Western countries for the VHF-2 and VHF-3 bands. AM refers to long waves (LW), while SW1-SW11 covers all short wave bands (SW).

The term FM comes from the English designation for a type of modulation called frequency modulation. The information is included in the deviation - the deviation of the frequency from the carrier value. In contrast, AM implies a change in another parameter of the electromagnetic wave - amplitude.

To summarize, let's say that in the upper region of the VHF range FM (FM) modulation is used, and in HF, MW and LW - AM. This is the origin of their English names. To distinguish SW and DV from HF, the latter are called SW.

It remains to add that SW is divided into 11 subbands, below FM there is an area designated OIRT (VHF and VHF-1), named after the modulation method - polar.

The main principles of expanding the received range

All-wave digital radio receiver works with most broadcast stations. This quality is ensured by a number of special measures.

To what has already been said, we add that the design of the antenna depends on the frequency of the received wave. For HF (3-30 MHz), the use of ferrite rod varieties is optimal; for VHF, a telescopic design is more appropriate.

Portable radios

The receiver preselector is adjusted to the carrier by changing the value of the capacitance, or less often the inductance, of the input filter. Naturally, a single resonant circuit cannot cover the entire spectrum; to solve the problem, a range switching knob is useful. It transfers the antenna input signal between circuits with various areas of action.

To better understand what has been described, let's take a look at a bandpass filter. There are two main characteristics:

Resonant frequency.
Bandwidth.

The filter's action is like a gate through which only the necessary part of the signal can pass, and the gate can move in different directions, allowing stations to exit one by one. The knob allows for smooth adjustment and adjustable movement.

For a long time there has been a struggle to reduce the size and cost of equipment, but how to expand the range of a radio receiver without sacrifice is still unclear. The technology of transferring the received signal between filters is considered generally accepted.

The bandwidth of such a filter is equal to the width of the spectrum of the useful signal emitted by the radio station, and the resonant frequency - the center of the gate - is tuned to the carrier. If the specified conditions are strictly observed, the quality of reception is the best.

Continuing the analogy, let's say that the AM and FM stations are located too “far” from each other, so the device that regulates the position of the gate does not “reach” there. The resonant circuits of an electrical circuit operate in a similar way. Switching bands allows another circuit to “reach” the station that the current one cannot reach.

At the same time, the type of receiving antenna changes. In this way, extended functionality is achieved.

The matter is not limited to combined antennas and modification of input filters - each band uses its own type of signal modulation. The electrical circuit that separates sound from wave vibrations is different for a specific case.

Modulation is a change in a carrier parameter according to the law that describes the transmitted message. On the receiving side, the opposite action occurs - detection. The types of modulation most commonly used in radio broadcasting are:

amplitude;
frequency

In the first case, the amplitude of the carrier is subject to change, and in the second, the frequency. The peculiarities of wave propagation in the air and the functioning of electronic components, for reasons of efficiency, force the use of known types of modulation.

The variety of technical solutions is not limited to the described options; the terms single-sideband and polar modulation are distinguished. The need for sophisticated methods arises when it is necessary to transmit stereo sound over a channel of normal width, to save transmitter energy and reduce the level of factors harmful to human health.

A digital radio receiver with a VHF range for working with HF must provide for switching the type of detector from frequency (FM) to amplitude (AM).

Technically there are no difficulties in this. To receive all radio stations, you must:

Have a range of antennas and input filters for different frequencies.
Include detectors for different types of modulation in the circuit.
Switch between specified items appropriately.

Radio receiving equipment Grundik

The use of several antennas and the modification of the electronic filling described above make it possible to receive extended range waves. Here is how this principle is implemented by Grundig digital radio receivers (Satellit 750) for professional use:

the digital tuner covers all possible ranges of broadcasting and negotiations at permitted frequencies;
100 preset channels ensure instant selection of the desired station;
the impact-resistant case, borrowed from measuring instruments, with protective handles reliably protects the device from damage;
the ability to work with a pilot signal and single-sideband modulation is implemented for professional use;
digital signal processors provide maximum sensitivity with minimal distortion;
a remote antenna with the ability to rotate 360 degrees is installed in the place of best reception;
An additional increase in sensitivity is achieved by reducing the resistance on the gold-plated connector of the external antenna.

The more modest digital pocket radio receiver G6 Aviator differs from the described model in its small size, lack of a shockproof case and remote antenna, and lower sensitivity. However, the device is located in the upper segment of compact household products. To avoid accidentally pressing an extra key, there is a HOLD lock button.

Grundig digital radios are equipped with digital keys for dialing frequencies from the keyboard, line outputs for speakers and headphones, as well as several antennas for reliable reception in all bands. All products are aimed at high-quality radio reception and are not entertainment equipment.

Applicability of extended range devices

From the above, it is clear that extended range digital radios are of limited use. The explanation is simple: most popular stations are located in the FM range.

However, long waves over long distances are better received, especially in bad weather, and there is a demand for all-wave digital radio receivers. Tourists, residents of remote villages, workers of projects under construction - these people are interested in the operation of HF and lower frequency stations.

Operating principle and configuration of the VHF receiver

The signal from the antenna, which

Specifications

Received frequency range, MHz………………………….. 30…130

1st subband, MHz………………………………………….. 30…50

2nd subband, MHz…………………………………………………………….. 50…70

3rd subband, MHz………………………………………………………70…90

4 subband, MHz…………………………………………… 90…110

5th subband, MHz…………………………………………. 110…130

6 subband, MHz……………………………………. 130…150

7 subband, MHz…………………………. 150…170

Sensitivity, µV……………………………………………………. 1

Current consumption, mA……………………………………………………12

Supply voltage, V………………………………………………………………. 3…6

Output power, W…………………………………………… 0.1

Load resistance, Ohm……………………………………. 16…64

Coil data for the range 30… 170 MHz

Designation

Range, MHz

Coil data

10 turns PEV 0.6 mm 0 5 mm with brass trimmer

8 turns PEV 0.6 mm 0 5 mm with brass trimmer

6 turns PEV 0.6 mm 0 5 mm with brass trimmer

4 turns PEV 0.6 mm 0 5 mm with brass trimmer

2 turns PEV 0.6 mm 0 5 mm with brass trimmer

3 turns PEV 0.8 mm 0 5 mm

2 turns PEV 0.8 mm 0 5 mm

Literature

1. Shumilov A. Simple radiotelephone // Radio amateur. 2001. No. 7. Manufacturing technology of parabolic antennas for Satellite TV

Having become interested in receiving STV, radio amateurs, as a rule, purchase a ready-made set of equipment for this purpose. It usually includes a parabolic antenna (PA) of small diameter (0.9...1.2 m). One of the first steps to modernize the system is…….

AM FIELD TRANSISTOR DEMODULATOR Fig. 12.1 A field effect transistor demodulator, assembled according to the above circuit, operates at a frequency of at least 100 MHz. Demodulation in this circuit is not carried out in the same way.......

LOW PASS FILTER FOR ANTENNA M. Steyer, Funkamateur, Berlin, No. 7/97, p. 820-823 The device uses a dual operational amplifier with a bandwidth of 160 MHz. The 143/60.4 Ohm divider reduces…….

PHASE/FREQUENCY COMPARATOR ON THREE TRIGGERS L’Electronique par le Schema, Dunod, vol. 3, p. 177 Fig. 8.1 This device uses the first trigger (A) of one of the four-stage dividers of the CD4520 chip…….

1. DETERMINE HOW WE WILL REBUILD THE RECEIVER.

So, using reasonable caution, we open the device. Let's see what the frequency setting knob is connected to. This could be a variometer (a metal thing, several centimeters long, usually two or one double, with longitudinal holes into which a pair of cores slides in or out.) This option has often been used before. For now I won’t write about it.() And it could be a plastic cube several centimeters in size (2...3). It contains several capacitors that change their capacity at our whim. (There is also a method of tuning with varicaps. In this case, the tuning control is very similar to the volume control. I have not come across such an option).

2. LET'S FIND A HETERODYNE COIL AND CAPACITORS CONNECTED TO IT.

So, you have KPE! Let's move on. We are looking for copper coils around it (yellow, brown spirals of several turns. Usually they are not even, but crumpled and toppled awry. And this is correct, this is how they are configured.). We can see one, two, three or more coils. Don't be alarmed. Everything is very simple. We turn on your device disassembled (don’t forget to connect a longer antenna) and tune it to any radio station (preferably not the loudest one). After this, we touch with a metal screwdriver or just a finger (contact is not necessary, just pass something near the coil. The reaction of the receiver will be different. The signal may become louder or interference may appear, but the coil that we are looking for will give the strongest effect. It will jump in front of us immediately several stations and reception will be completely disrupted. This means that this is what a LOTERODYNE coil is. The local oscillator frequency is determined by a circuit consisting of this very coil and capacitors connected in parallel to it. There are several of them - one of them is located in the control unit and is in charge of frequency tuning (we use it to catch different stations), the second is also located in the KPI cube, or rather on its surface. Two or four small screws on the back surface of the KPI (usually it faces us) are two or four trimming capacitors. One of them is used to adjust the local oscillator. Usually these capacitors consist of two plates that collide with each other when the screw rotates.When the top plate is exactly above the bottom, then capacity maximum. Touch these screws with a screwdriver. Move them back and forth a few (as little as possible) degrees. You can mark their starting position with a marker to insure against troubles. Which one affects the setting? Found it? We will need it in the near future.

3. LET'S DETERMINE AGAIN WHERE WE ARE REBUILDING AND ACTING.

What range does your receiver have and what is needed. Do we lower the frequency or increase it? To lower the frequency, it is enough to add 1...2 turns to the heterodyne coil. As a rule, it contains 5...10 turns. Take a piece of bare tinned wire (for example, a lead from some long-legged element) and install a small prosthesis. After this build-up, the coil needs to be adjusted. We turn on the receiver and catch some station. No stations? Nonsense, let's take a longer antenna and tweak the setting. Look, I caught something. What is this. You'll have to wait until they tell you or take another receiver and catch the same thing. Look how this station is located. At that end of the range. Need to move even lower? Easily. Let's move the coil turns closer together. Let's catch this station again. Good now? It just catches poorly (you need a long antenna). Right. Now let's find the antenna coil. She's somewhere nearby. Wires from the control unit must be suitable for it. Let's try, turning on the receiver, insert it into it or simply bring some ferrite core to it (you can take the DM choke by removing the winding from it). Has the reception volume increased? That's right, it's her. To reduce the frequency, it is necessary to increase the coil by 2...3 turns. A piece of stiff copper wire will do. You can simply replace the old coils with new ones containing 20% more turns. The turns of these coils should not lie tightly. By changing the stretch of the coil and bending it, we change the inductance. The tighter the coil is wound and the more turns it has, the higher its inductance and the operating range will be lower. Do not forget that the actual inductance of the circuit is higher than the inductance of a single coil, since it is added to the inductance of the conductors that make up the circuit.

For the best reception of a radio signal, it is necessary that the difference in the resonant frequencies of the heterodyne and antenna circuits be 10.7 MHz - this is the frequency of the intermediate frequency filter. This is called correct pairing of the input and local oscillator circuits. How to ensure it? Read on.

CONFIGURING (CONNECTING) INPUT AND HETERODYNE CIRCUITS.

FIGURE 1. High-frequency part of the VHF-FM radio receiver board. It can be clearly seen that the input circuit trimmer capacitor (CA-P) is set to the minimum capacitance position (unlike the heterodyne trimmer capacitor CG-P). The accuracy of installation of the rotors of the trimming capacitors is 10 degrees.

The local oscillator (LG) coil has a large gap in the winding, which reduces its inductance. This hole appeared during the setup process.

Another coil is visible at the top of the photo. This is the input antenna circuit. It is broadband and does not change lanes. The telescopic antenna is connected precisely to this circuit (via a transition capacitor). The purpose of this circuit is to remove gross interference at frequencies significantly lower than operating frequencies.

AND ONE MORE ACTION, SINCE WE ARE ALREADY HERE.

Tune to your favorite station, then shorten the antenna to the minimum when interference is already appearing and adjust the IF filter, which you look like a metal square with a purple circle (in the middle left of the photo). Fine tuning of this circuit is very important for clear and loud reception. The slot installation accuracy is 10 degrees.

Thematic materials: