Basics of mixer operation, mf amplifiers and detectors
These electronic circuits occur in almost all types of communication devices and play a crucial role in their good function. A cursory glance at their printed circuit boards says nothing about their activities and real properties; it is therefore best to study the block diagram and then focus on the associated circuits and their wiring. So we can get an idea, how devices and their accessories work, understand the data in advertisements and equipment specifications and much more. What are mixers and detectors for??
Electrical signals, corresponding to the normal voice spectrum with a maximum frequency in kHz, it is not possible to transmit to the ether as electromagnetic signals directly in low frequency form; the antenna is able to radiate signals with a much higher frequency with sufficient efficiency – in the case of short waves up to MHz units and higher. Low frequency signal corresponding to e.g.. AC voltage from the microphone must therefore somehow be tied to the high frequency signal, which only allows long distance communication. Such a signal is called a high frequency carrier (vf) signal and this process is called modulation. Several types of modulation have been developed gradually. To illustrate, let's use a mid-wave radio receiver for amplitude modulation. It transmits a carrier signal with some defined frequency, in our example, need 610 kHz. Low frequency (nf) the microphone signal is amplified to the required level and used for control – modulation – output carrier RF signal of the transmitter. In contrast to a pure RF carrier signal, in the case of an amplitude-modulated signal, further signals appear in the vicinity of the carrier frequency., which we call sidebands. The sidebands are mirrored above and below the carrier frequency depending on the modulation frequency (width 1 kHz for nf frequency modulation 1000 Hz, width 2 kHz pro 2000 Hz apod.). Because there are many radio transmitters, they would interfere with each other, they are allowed to occupy sections in the frequency spectrum side by side, a maximum of approx 10 kHz. Highest nf frequency, which their carrier frequencies can be modulated, is therefore 5 kHz. As in the receiver we get at the end of the whole signal processing chain antenna again nf signal for speaker or headphones? The receiver input is tuned to some RF carrier frequency, modulated by nf signal. The original nf signal is retrieved from it at the receiver, by procedure, which is called detection. The resulting very weak nf signal should be amplified before being fed to the headphones or speaker, to be audible. After the crystals and direct-amplifying receivers, further stages began to be inserted between the input amplifier and the detector: mixers and IF amplifiers. To understand the principle of their operation, we will deal with the change of frequency in mixers in the next part.
Basic concept of frequency change
At the beginning of the radio broadcast, there were only a few stations on the air and it was easy to tune in to the crystal or feedback double lamp.. As the number of stations began to increase, their mutual interference manifested itself, because the circuits of simple receivers were not able to select and further process only relatively narrow (10 kHz) the band corresponding to the modulated signal of a single desired station. This would make the circuits more complicated, but it was not possible to ensure at the same time a good ability to select only the required frequency band (this property is called selectivity) while still adjusting the position of the received carrier signal as required (carrying nf modulation information in its sidebands) so, so that we can choose to receive different stations by retuning the whole band.
It looked like that, that you will have to choose: either we will be able to receive different quality stations, or we use complex circuits, but the receiver will not be able to tune to different carrier frequencies and so we will be able to better receive only one station on the frequency, given by the fixed tuning of the circuits in the receiver. Of course, none of these variants is suitable for communication. The problem was solved that way, that circuits have been included in the receiver, allowing good processing and amplification of a single fixed frequency (in fact, a narrow frequency band around one fixed RF carrier), thus circuits with good selectivity for this solid – intermediate frequency (mf) frequency. By the way, in the original concept of such a receiver, the mf frequency lay somewhere between the received hf frequency and the modulating nf frequency, hence its name intermediate frequency. The reception of modulated signals of different stations with different carrier frequencies was thus ensured, that the signal of the selected station was – including modulation – in the receiver always converted to a fixed mf frequency, which could then be processed in a well-tuned intermediate frequency amplifier, amplify, etc.. After detection, we finally received the nf signal again. To make this principle work, we have to include other circuits in addition to the mf stages in the receiver: mixer, in which we obtain from the signal on the original carrier frequency a similarly modulated signal on the intermediate frequency, local oscillator, whose signal after mixing with the original carrier frequency just in the mixer will allow the creation of the necessary modulated signal on the mf frequency for further processing in the mf amplifier, detector and nf amplifier. We want e.g.. tune to the station, whose carrier frequency is 610 kHz. One RF input of the mixer is tuned to approximately 610 kHz, we bring the frequency from the local oscillator to its next input 1065 kHz; so we have to tune the local oscillator in the same way, to generate this frequency. A conventional mixer then has four frequency signals at its output 610, 1065, 1675 a455 kHz. Two of them are frequencies, which we brought to the inputs of the mixer and which passed through the mixer, in addition to them, however, a sum signal was generated in the mixer 1675 kHz (1065 + 610) and differential 455 kHz (1065 – 610). The following mf stages are tuned to the frequency 455 kHz and suppresses the remaining three frequencies from the mixer. Signal 455 kHz, which still contains sidebands carrying information about the modulation of the original RF carrier frequency, is fed to the detector after amplification in the VF amplifier, in which the nf signal is recovered, which is audible after the next amplification in the nf amplifier. If we want to receive another radio station, e.g.. with frequency 2400 kHz (see second example), we tune the rf input of the mixer to 2400 kHz local frequency oscillator 2855 kHz. The frequencies now appear at the mixer output 2400, 2855, 5255 a455 kHz, mf stages again amplifies only the frequency 455 kHz and do not respond to other signals. It's visible, that to receive the desired station we must set the local oscillator to the frequency, which, when mixed with the desired received signal, produces an mf signal. The next mf stage ensures selectivity, needed to receive only the desired signal. The traditional single-mix method is not too complicated, but the result is also not completely perfect: the reception can still be disturbed by other carrier signals on the so-called. mirror frequencies. Let's explain the situation for an explanation, when we receive a station on the frequency 610 kHz and our receiver has an mf frequency 455 kHz. If it happens to be broadcast by another station on the frequency 1520 kHz, the RF input of the mixer can also receive its signal from the antenna and at the frequency of the local oscillator 1065 creates a kHz mixer at the same time – among others – at its output from the signals of both stations one signal on the mf frequency. The result will be it, that the originally completely separate signal of the second station will interfere with the reception of the first station. Similarly, when receiving a shortwave station on 2400 kHz (second example) and the mf frequency of our receiver 455 kHz we may encounter interference from a completely different station, which simultaneously operates on the frequency 3310 kHz. In the past, this problem was not very serious, today, however, the HF bands are crowded with a number of strong stations. The situation is different.
So what is the concept of modern receivers?
In the past, the receivers used mechanically coupled oscillator and RF circuits in front of the mixer, or mixer circuits. Nowadays, there are RF degrees, possibly in the input circuits of the mixer, usually untuned (broadband). Bandpass filters are inserted in front of them, transmitting different frequency ranges. The local oscillator ensures the stability and accuracy of the frequency setting, while mf degrees ensure selectivity. We will better understand these facts, when we look at the solution of modern shortwave receivers e.g.. for receiving SSB signals (SSB is commonly used to refer to another type of modulation, used almost exclusively in shortwave phonic operation; The abbreviation is derived from the words Single Side Band and means, that instead of both sidebands, only one sideband is transmitted, which is sufficient for the transmission of nf modulation information). Antenna, unless it is explicitly narrowband, can supply all kinds of signals of comparable strength in the range from 1 until maybe 50 MHz. These are first introduced in the receiver into the appropriate bandpass filter passing e.g.. signals 14,0 to 14,5 MHz (for 20m band). If we involve the so-called receivers. preamplifier, we can make them even stronger. Frequencies from the whole frequency range 14,0 to 14,5 The MHz is then fed to one input of the mixer and the signal of the local oscillator is fed to its other input. For mf frequency 9 MHz we need to tune the local oscillator from 23,0 MHz (for signal reception 14,0 MHz) until 23,5 MHz (for signal reception 14,5 MHz). At the output of the mixer we then get (among others) and a signal with a differential mf frequency 9 MHz, whose frequency does not change when retuning the frequency of the local oscillator. Only the input frequency changes, which we actually choose from the whole frequency range, which are supplied to the receiver input by an antenna. Bandwidth, which is passed by the mf amplifier, is in fact about 20 or 30 kHz, with medium frequency 9 MHz. To this extent 8,985 MHz to 9,015 MHz but in the original band 14 MHz can operate more than half a dozen SSB stations at the same time, and the signals of all of them would pass through the mf amplifier, they would be amplified and eventually converted to an nf acoustic signal. So we would hear several stations at the same time, which transmit on close frequencies. Therefore, we include a special component in front of the mf amplifier in the signal path – crystal filter, which passes signals only in range 9000,3 to 9002,8 MHz, its bandwidth is therefore only 2,5 kHz. This already corresponds to the width of only one SSB signal. No other frequencies pass through the crystal filter – they are therefore filtered out – and in the signal, which is then processed by an mf amplifier, does not disturb. The resulting signal 9 MHz, wide 2,5 kHz, it is then amplified and fed to a second mixer. A signal is also fed to this mixer 9 MHz from the second local oscillator and the difference of these frequencies (nf signal between 300 to 2800 Hz) is amplified and brought to the speaker. This second mixer works as an SSB or CW signal detector. We then have an nf signal available at its output.
Double and triple mixing
During the development of amateur broadcasting and with the ever-increasing occupancy of HF bands, the selectivity of the receiver has become its very important parameter.. Because the best selectivity can be obtained in mf degrees, was a logical solution, that another has joined the receiving chain – second – mf amplifiers operating on another mf frequency, again with its own crystal filter. This, of course, again requires a second local oscillator and mixer. The advantageous properties of both crystal filters are then actually added- and may result in even better selectivity and less interference. This concept is called double mixing. Assume the same tuning example in the 20m band, when signals in the range 14,0 to 14,5 MHz pass through the input filter, they are amplified and brought to the input 1 of the first mixer. At the entrance 2 mixers 1 the frequency of the local oscillator is applied 23 MHz (the frequencies given are chosen as an example, are rounded; you can, of course, substitute other numbers here, according to the parameters of a particular device). Output signal from the mixer (9 MHz) then it passes through the first mf stage through a crystal filter 9 MHz per input 1 mixers 2. At the entrance 2 mixers 2 a constant frequency signal is applied at the same time 9445 kHz from local oscillator 2 (therefore, this oscillator is no longer tuned). Output signal 455 to 458 kHz then passes through a second MF amplifier, where the crystal filter 455 kHz filters out interference, which the crystal filter did not remove 9 MHz of the first mf amplifier. Output signal (455 to 458 kHz) it then mixes with the fixed frequency signal 455 kHz from local oscillator 3 and an nf signal is generated 0 to 3000 Hz. This signal is amplified and fed to the speaker. Triple and quadruple mixing were gradually used to achieve greater selectivity. Frequency combinations in such cases are more complicated, but if you understood the previous interpretation, you can also calculate them without any problems. try it!
"Up" mixing
In one of the previous paragraphs, we talked about mirror frequency interference, originating from out-of-band signals, where we work. This problem has grown with a growing number of different strengths, often non-amateur stations on HF bands. The best solution, used in modern HF equipment, is "up" mixing. The principle is, that a high mf frequency is used, which lies above the actual received frequencies; any interference would then cause up to (mirror) VHF signals, which are far apart from the receiver's own operating range. These can be removed more easily in the input circuits of the receiver just behind the antenna, so the receivers do not penetrate to the next stages and cannot cause interference. "Up" mixing works the same as double or triple mixing with the difference, that the first mf is chosen e.g.. ranging 60 up to 75MHz. The frequencies are again only approximate. So what follows from the foregoing considerations? First, they provide you with the basics for understanding some of the technical terms in advertisements and equipment descriptions. They will show you further, that today's transceivers contain many technical means in a small space, which you can buy for your money. Understanding the function of filters, concepts such as bandwidth, etc.. it will also help you orient yourself in terms such as "excellent selectivity", etc..
According to CQ 11/2000 translated by Jan Kučera, OK1NR
