Basic parameters of transmitters and receivers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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To understand what a particular device is, you need to know its parameters. Since we are going to build receivers and transmitters, it would be nice to know by what criteria they are classified.

Now everything is in order.

Operating frequency (frequency range)

If the transmitter or receiver is hard-tuned to a certain frequency, then we can talk about one operating frequency. If during operation it is possible to tune the operating frequency, then it is necessary to name range operating frequencies, within which adjustment can be carried out.

It is measured in kilohertz (kHz), megahertz (MHz), or gigahertz (GHz).

Previously, to determine the frequency range, it was more often not the frequency that was used, but the wavelength. From here came the names of the ranges LW (long waves), SV, (medium waves) HF (short waves), VHF (ultrashort waves).

To convert the wavelength to frequency, you need to divide by it the speed of light (300 m/s). That is,

where:

- wavelength (m)

c - speed of light (m/s)

F - frequency (Hz)

Now it is not difficult for you to calculate what our grandfathers called "ultrashort waves". Yes, yes, do not be surprised, the range of 65 ... 75 MHz is no longer just "short" but "ultra-short". But their length is as much as 4 meters! For comparison, the wavelength of a GSM mobile phone is 15 ... 30 cm (depending on the range).

With the development of technology and the development of new frequency ranges, they began to be given unimaginable names like "ultra-short", "hyper-short", etc. Now frequency is more commonly used to designate a range. This is more convenient even if you don't need to recalculate anything and remember the speed of light. Although, the speed of light still does not hurt to remember :)

We will mainly be working on the VHF broadcast bands. There are two of them: VHF-1 - what is popularly called "VHF", and VHF-2 - what is commonly called "FM". The name FM comes from the English Frequency Modulation - Frequency Modulation (we read about modulation below). In fact, to be serious, then calling the frequency range according to the type of modulation is technically illiterate. However, among the people this name is firmly rooted and has become a household name. There's nothing you can do about it.

Modulation type

Two types of modulation are widely used: amplitude (AM) and frequency (FM). In bourgeois it sounds like AM and FM. Actually, everyone's favorite "FM" range got its name precisely because of the frequency modulation with which all radio stations of this range work. There is also phase modulation, abbreviated as FM, but already, in our letters. Please do not get confused with bourgeois FM!

FM, unlike AM, is more protected from impulse noise. Generally speaking, at the frequencies at which VHF radio stations are located, the use of FM is more convenient than AM, which is why it is used there. Although, the television signal is still transmitted with amplitude modulation, regardless of frequency. But that's a completely different story.

Frequency modulation is narrowband and wideband. Broadband radio stations use broadband FM - its deviation is 75 kHz.

In communication radio stations and other non-broadcast radio equipment, narrow-band FM is more often used, with a deviation of about 3 kHz. It is more immune to interference because it allows a sharper tuning of the receiver to the carrier.

So our ranges are:

VHF-1 - 65,0...74,0 MHz, modulation - frequency

VHF-2 ("FM") - 88,0...108,0 MHz, modulation - frequency

Output power

The more powerful the transmitter - the farther it can transmit the signal, the easier it will be to receive this signal.

In almost every description of a bug, its range is written. Usually - starting from 50 m and ending with three kilometers ... This information cannot be taken seriously. Never take advantage of a range of 1 km in a city, or don't get upset at fifty meters in an open area - after all, the authors never give the parameters of the receiver with which this bug was tested. Namely - they do not name the sensitivity of this receiver. But a lot depends on it. You can test a powerful transmitter with a receiver with lousy sensitivity - and get a short range as a result. Or vice versa, listen to a low-power transmitter through a sensitive receiver - and get a longer range. Therefore, when considering a bug scheme, first of all, pay attention not to big words, but to bare facts. Namely - try to estimate the power of the transmitter. Usually, the power is not indicated in the description of the bug (the authors simply do not measure it, considering it sufficient to measure the "range"). Therefore, we can only "by eye" determine what the beetle is capable of.

For this you need to look at:

- Supply voltage. The more - the more power (ceteris paribus)

- The value of the transistor in the final stage (or generator, if the antenna is connected directly to it). If there is some lousy KT315 - you can’t wait for a lot of power from the circuit, you won’t wait. And if you try to raise it, the transyuk, without saying anything, will simply treacherously explode ... It is better if there is a KT6xx or KT9xx transistor, for example, KT608, KT645, KT904, KT920, etc.

- Resistance of transistors in the collector and emitter circuits of the final stage. The smaller they are, the more power (ppr).

For comparison, I will say this: a power of 1 W is enough in urban conditions for about a kilometer, provided that the sensitivity of the receiver is about 1 μV.

Receiver sensitivity

Well, we've already started talking about sensitivity.

Sensitivity depends on 90 percent of the "noisiness" of the input stage of the receiver. Therefore, to achieve good results, it is necessary to use low-noise transistors. Often used by field workers - they make less noise.

For VHF receivers, the sensitivity is usually in the range of 0,1 ... 10 μV. The given values ​​are extremes. To get a sensitivity of 0,1 - you have to sweat a lot. Just like you have to disrespect yourself very much in order to make a receiver with a sensitivity of 10 μV. The truth is somewhere in the middle. About 1 ... 3 μV is the optimal sensitivity value.

Transmitter output impedance

This is very important to know, because you can make a very fine powerful transmitter and not get even a tenth of the rated power from it due to improper matching with the antenna.

So, the antenna has a resistance R, say 100 ohms. In order to radiate power P with this antenna, let's say - 4 watts, you need to apply a voltage U to it, which is calculated according to Ohm's law:

U2=PR

U2=100*4=400 U = 20 V

Got 20 volts.

At a voltage of 20 volts, the output stage of the transmitter must hold a power of 4 watts, while current will flow through it

I = P / U = 0,2A = 200mA

Thus, this transmitter at a resistance of 100 ohms develops a power of 4 watts.

And if instead of a 100 ohm antenna you connect a 200 ohm antenna? (And the voltage is the same - 20 V)

We believe:

P = UI = U(U/R) = 20(20/200) = 2W

Twice smaller! That is, physically, the output stage is ready to pump 4 watts, but it cannot, since it is limited by a voltage of 20 volts.

Another situation: the antenna resistance is 50 ohms, that is, 2 times less. What happens? Double power will go to it, double current will flow through the final stage - and the transistor in the final stage will be significantly covered with a copper basin ...

In short, why am I all this? And to the fact that it is necessary to know what kind of load we have the right to connect to the output of the transmitter, and which one - not in the right. That is, you need to know the output impedance of the transmitter.

But we also need to know the resistance of the antenna. But here it is more difficult: it is very difficult to measure. You can, of course, calculate, but the calculation will not give an exact value. Theory is always at odds with practice. How to be?

Very simple. There are special circuits that allow you to change the output impedance. They are called "matching schemes". Two types are most common: based on a transformer and based on a P-filter. Matching circuits are usually placed on the output stage of the amplifier, and look something like this (on the left - transformer, on the right - based on the P-filter):

(click to enlarge)

To adjust the output impedance of the transformer circuit, it is necessary to change the number of turns of the II winding.

To set up a circuit with a P-filter, you need to adjust the inductance L 1 and capacitance C 3.

Tuning is done with the transmitter turned on and the standard antenna connected. At the same time, the power of the signal emitted by the antenna is measured using a special device - a wave meter (this is such a receiver with a millivoltmeter). During the tuning process, the maximum value of the radiated power is achieved. It is highly recommended not to tune powerful transmitters while in close proximity to the antenna. Unless, of course, your mother wants to have grandchildren ... :)

Receiver input impedance

Almost the same. Except grandchildren. The received signal is too weak to do any harm to the domestic gene pool.

Resistance matching is performed using the input oscillatory circuit. The antenna is connected either to part of the turns of the circuit, or through a coupling coil, or through a capacitor. The diagrams are here:

(click to enlarge)

The signal from the circuit can also be taken either directly, as shown in the diagrams, or through the coupling coil, or from part of the turns. In general, it depends on the will of the designer and specific conditions.

Harmonic coefficient

Tells us how "sinusoidal" the signal emitted by the transmitter is. The less k.g. - the more the signal looks like a sine. Although, it also happens that visually - it seems to be a sine, and harmonics - darkness. So, after all - not a sine. Humans tend to make mistakes. Technique is more objective in its evaluation.

This is what a "pure" sine looks like (the sine wave is generated by the WaveLab sound generator):

Harmonics arise, as we know, due to non-linear distortion of the signal. Distortions can occur for various reasons. For example, if the amplifying transistor operates in a non-linear section of the transfer characteristic. In other words, if the base current changes are equal, the collector current changes are not equal. This can be in two cases:

1. Insufficient bias current is applied to the transistor. That is, in the absence of a signal, it is completely closed, and begins to open only with an increase in the signal level. At the same time, the bottoms of the output sinusoid are "cut down": 

   

   By the way, the output stages of most transmitters operate in mode C. This mode does not imply the presence of a base shift.

   That is, at the outputs of such cascades there will always be a signal with cut bottoms. But they put up with this due to the high efficiency of such cascades. Harmonics are cut out by filters after the cascade. By the way, the cascades shown in the matching diagrams work just in C mode.

2. The input signal amplitude is too high and the required collector current cannot be provided.

   For example:

   There is a 100 ohm resistor in the collector circuit of the transistor,

   supply voltage - 25 V.

   Accordingly, with a fully open transistor, the collector current will be 25/100 = 0,25 A = 250 mA.

   The transistor gain is 50, that is, the collector current is 50 times the base current.

   Now this situation: a current of 10 mA was applied to the base. What will be the collector current?

   What? 500 mA? Nothing like that! We just said that with a FULLY open transistor, the collector current is 250 mA. So, more than this value, it cannot be under any sauce. If we increase the base current from zero to 10 mA, then the collector current will only increase until it becomes 250 mA. After that, it will not increase, no matter how much we increase the base current. This transistor mode is called " saturation mode ". At the moment the collector current reaches 250 mA, the base current is 250/50 - 5 mA. That is, for the correct operation of this stage, a current of more than 5 mA cannot be applied to its input. The same thing happens with the signal. If the current signal "goes off scale" beyond a certain value, then the transistor goes into saturation. On the oscillogram, this manifests itself in the form of "cut off" tops of the sinusoid:

   

In addition to such characteristic distortions, there are also various other non-linear distortions of the signal. Frequency filters are designed to deal with all these distortions. Usually, low-pass filters (LPF) are used, since, as mentioned earlier, harmonic frequencies are usually higher than the frequency of the desired signal. The LPF passes the fundamental frequency and "cuts" all frequencies that are higher than the fundamental. At the same time, the signal, as if by magic, turns into a sine of pure beauty.

Receiver selectivity

This parameter indicates how well the receiver can separate the signal of the required frequency from signals of other frequencies. Measured in decibels (dB) relative to an adjacent frequency channel or image channel (in heterodyne receivers).

The fact is that thousands of all kinds of electromagnetic oscillations are constantly flying in the air: from radio stations, television transmitters, our favorite "mobile friends", etc. and so on. They differ only in power and frequency. True, they do not have to differ in power - this is not a selection criterion. Tuning to any radio station, whether it be the MTV channel or the base of your home radiotelephone, occurs precisely in frequency. At the same time, the receiver is responsible: to choose from thousands of frequencies - the one, the one and only, which we want to receive. If there are no signs of intelligent life at close frequencies, good. And if somewhere in half a megahertz from our radio station, there is a signal from another radio station? This is not very good. This is where good receiver selectivity is needed.

The selectivity of the receiver depends mainly on the quality factor of the oscillatory circuits. In more detail, we will deal with selectivity when considering specific receiver circuits.

The remaining four parameters refer to the low-frequency path of the receiver and transmitter.

Sensitivity on the low-frequency input of the transmitter

The more sensitive the transmitter input, the weaker the signal can be applied to it. This parameter is especially important in bugs, where the signal is taken from the microphone, and has a very low power. If necessary, the sensitivity is increased by additional amplification stages.

Receiver LF output power

The signal strength that the receiver outputs. You need to know it in order to choose the right power amplifier for further amplification.

THD (Total Harmonic Distortion)

Well, in general, we have already figured out what non-linear distortions are and where they come from. But! If it is enough to put a filter on the HF path - and everything will be fine, then in the audio path it is much more difficult to "treat" non-linear distortions. More precisely, it is simply impossible. Therefore, with an audio or any other modulating signal, it is necessary to handle it very carefully so that it has as little non-linear distortion as possible.

Publication: radiokot.ru

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