Superheterodyne receiver

The superheterodyne receiver (also superheterodyne or superhet short, Super) is an electronic circuit for receiving and processing of high-frequency electromagnetic radio signals. It is characterized by the reduction of the input frequency to a constant, much lower intermediate frequency before the actual demodulation because filter lower frequencies easier but can amplify. It is used in many devices of radio transmission, telecommunications and RF test equipment, from simple radio and television to the GPS.

The term " overlay " is a mathematical name by the addition theorems of the theory of oscillations. " Superposition " is a synonym for superposition. This principle is used for problems in many areas of physics and provides for linear filtering conditions and reasonable good results.

In recent years, not only auxiliary functions such as operation or LO frequency generation are digitized in the receiver technology, but increasingly larger parts of the signal processing. This development led to the area of ​​Software Defined Radio ( SDR).

• 4.10.1 HF-Verstärker/Vorselektion
• 4.10.2 mixer and first IF filter
• 4.10.3 tuning oscillator (VCO )

• 7.1 Double and multiple superheterodyne receiver
• 7.2 converter, frequency converter
• 7.3 Measuring Receiver

Basics

The first radio receiver were straight receiver to very low frequencies (often after amplification), the demodulated signal. The increasing number of transmitters forced a reduction in bandwidth, to prevent that some stations could be received simultaneously. The filter costs increase so dramatically that a modified concept had to be invented with increasing frequency: In a superheterodyne receiver, the frequency of the RF signal is reduced at least once before it is demodulated. For this purpose it is mixed with the signal from a receiver located in the ligand so-called local oscillator (LO ) signal to the same modulation content to receive a fixed, usually lower intermediate frequency (IF ) as the RF signal. The frequency of the local oscillator determines together with the intermediate frequency two receive frequencies, one of which can pass the input filter.

Only the most significant reduction in the frequency, the necessary high gain and filtering of the signal can be achieved - the receiver is more sensitive and more selective. The signal filtering is carried out at a constant and low IF (intermediate frequency), and therefore - can be made of fixed frequency filters - in contrast to the tunable RF filter required in direct detection receiver. This results in a simplified structure, much higher selectivity (selection ) and thus significantly improved reception quality. Receiver for very high frequencies such as radar or radio astronomy can achieve good sensitivity only in this way.

This received principle makes sense only applicable to about 1010 Hz, because at higher frequencies the strong phase noise of the mixer oscillator sensitivity at very reduced.

No heterodyne receiver with the described features is Homodynverfahren in the LO and RF signal have approximately the same frequency. Here, the amplitude- modulated received signal is directly converted to the low-frequency range ( without scope ), is a direct beneficiary or direct mixer. Their main problem, the strong 1/f-noise, does not allow high sensitivity, which is why it is only used for mixing of optical frequencies, where the strong phase noise of the oscillators makes it impossible superheterodyne receiver according to the IF principle.

History

The name heterodyne or superheterodyne is a neologism composed of the Latin word super = " on " and the Greek words heterosexual = "different" and dynamis = " force ", and describes the mixing of two signals of different frequency. In contrast, the Greek word homόs = " equal " is used for the name of the homodyne receiver. The term local oscillator means that this oscillator is so even in the superheterodyne receiver at location ( lat locus = place ). Superhet or just super are common in amateur radio shorthand for superheterodyne receiver according to the heterodyne principle.

Other sources indicate that in 1918 Armstrong had the idea for it when he was stationed in France. U.S. Patent No. 1,342,885 by Edwin Armstrong describes the superposition principle. Armstrong has this patent pending in early 1919 in the U.S. and receive the mid-1920s.

Almost simultaneously, but also to Lucien Lévy (1917 ) in France and Walter Schottky ( 1918) have developed in Germany this operating principle. Lucien Lévy received in 1919 and 1920 in France a patent (No. 493,660 and No. 506,297 ) for his circuit design (IF) worked with an intermediate frequency.

One of the first commercially built superhets was the Radiola AR -812 by RCA, which was sold over 140,000 times from 1924 to about 1927. The German company DeTeWe developed in the years 1924/1925 the " ULTRADYN ". In France, to have been produced in 1923 by Lucien Lévy's three home recipient company "Radio LL".

In the following decades, the circuit principle did because of its many benefits by more and more. It was designed and built many variants of heterodyne receivers, some with double and multiple overlay ( up to four times ) and mixture at constant instead of variable mixing frequency, so-called converter, such as the LNB in satellite technology.

The overlay is a universal method and is also used in transmitters. Virtually all currently available on the market wireless transmitting and receiving devices operate on the principle of superposition (radio, walkie talkie, mobile phone, base station, relay, Television, Satellite ).

Principle of operation

First, a filter limits the bandwidth of the antenna signal to a narrow range around the desired frequency. This reduces the voltage level of all signals generated by transmitters in other frequency ranges, thus less unwanted mixer products arise and need to process the input amplifier or mixer lower voltage. In particular, the image frequency must be suppressed. High-frequency amplifiers are needed above the shortwave range, so that weak signals over the noise of the mixer are raised, they also prevent the LO frequency is radiated via the antenna. Medium wave receivers have hardly ever RF preamplifier.

The band-limited, and if necessary amplified antenna signal reaches the mixer where it is mixed with the LO signal of the tuning oscillator, and a new set of frequencies is generated. The LO frequency is selected by a fixed amount above or below the desired reception frequency. The composite signal after the mixer contains, inter alia, the sum and the difference of the input and LO frequencies, the modulation of the input signal is preserved. A bandpass constant frequency can pass one of the mixed products, only this is amplified in the following intermediate frequency amplifier and then demodulated. In this case, the useful signal (ie voice or music to radio reception) obtained from the IF signal.

Circuit stages in detail

Abbreviations used:

HF-Verstärker/Vorselektion

The high-frequency amplifier has several functions:

• It adapts the impedance of the antenna to the circuit at the following ( it is to be passed on a maximum of the power taken from the antenna to the subsequent stage ).
• He reinforced the weak antenna signals so that they are above the inherent noise of the mixer. Thus, the input sensitivity of the receiver is increased.
• It prevents the LO frequency passes from the mixer to the antenna and is radiated there ( jammer ).
• In this stage, a pre-selection is made ​​so that only frequencies from the reception area ( passband ) can pass through the amplifier.

The pre-selection can either run with the input frequency ( typical of the tube technique ) or in the form of mostly switchable bandpass filters are realized. The pre-selection has several responsibilities:

• Suppression of the reception at the image frequency.
• Reduction of the maximum RF voltage of the following active components, because stress components of all channels are suppressed, whose frequency is far enough away. The linear working range of transistors, etc. is limited and non-linear behavior would lead to mixing effects between the input signals. Such large-signal disturbances can create ghost channels and cause a level of noise that can mask the desired signal.
• Suppression of potential signals at the intermediate frequency which would pass unhindered from the antenna to the IF amplifier in the most primitive single-ended mixer circuit.

Mixer

In a superheterodyne receiver, the input frequency (), including its modulation, is converted by a mixer to a different frequency by mixing with the frequency of the tuning oscillator. With an ideal mixer is obtained at the output of only two new sidebands with signals and on, but real mixer generate an entire frequency spectrum.

When superheterodyne receiver from the difference frequency is almost always filtered. This is at low cost receivers in long, medium and short wave 455 kHz, which barely allows image rejection in the shortwave range, because the image frequency is only 910 kHz away. In the normal FM intermediate frequency is 10.7 MHz. In televisions, mobile phones and Mehrfachsuperhets is usually taken as a ZF technical reasons, a much higher frequency.

The oscillation frequency generated by an oscillator circuit. Only in the early days it was generated by the mixer itself, because active components were still very expensive. Main disadvantage of this " self-oscillating " mix stages is the modulation- dependent frequency change.

Since especially in the early days of reception technics mixer actually were only overdriven amplifiers, the IF breakthrough is a major problem. Each unbalanced mixer an antenna signal at the IF flows unhindered and often exacerbated (except diode mixer ) to the IF amplifier and is treated by this as the desired difference frequency on. Both signals, the down-converted received signal and the " sweeping " the transmitter signal at the IF are heard simultaneously, the interfering signal may even predominate. On AM, the two carrier frequencies produce in the demodulator in addition a very disturbing interference pipes. When FM is due to the FM threshold only the stronger of the two stations audible.

Since these highly undesirable side effect can be eliminated by extremely good pre-selection and the use of balanced mixer only partially, it was agreed internationally to the usual IF frequencies to operate at a sufficient distance 455 kHz and 10.7 MHz radio stations. This has not been changed.

With the ( inexpensive ) Availability of multi- grid tubes and later of dual- gate field-effect transistors spread the multiplicative mixture. Here, the two voltages are in each case to a separate input of the control element, such as, for example, the two gates of a dual-gate field-effect transistor or the control grid of a vacuum tube, out. The output signal is controlled by two input signals, thereby forming a mixing effect of the two control signals is generated. At higher frequencies ( in the higher GHz range ) are frequently used - and now (2006 ) still - a diode ring mixer.

The multiplicative mixture has relative to the additive mixing circuit slight advantages, the reactions are lower in the RF amplifier, and there is the possibility of an additional control of the mixer, but this is rarely used. He also produces less intermodulation frequencies and thus less "Phantom receiving agencies".

Advantages and disadvantages of the additive mixture

Advantages:

• The mixing transistor can be used as oscillator charge (the self-oscillating mixer )

Cons:

• Without bridge circuit oscillator frequency and input frequency are difficult to decouple
• Creates many unwanted mixing products

Advantages and disadvantages of the multiplicative mixture

Advantages:

• Creates less unwanted mixing products
• The oscillator and the input frequency are decoupled
• A control of the conversion gain is possible

Cons:

• With transistor technology no self-oscillating mixer is possible while that was indeed realized in tube technology, for example, with a octode so.

Tuning oscillator

The tuning oscillator has the task to generate a constant voltage as possible to the desired frequency with high accuracy. This frequency must be adjustable over a wide range, so that can be tuned to any desired transmitter within the reception range. There are various oscillator circuits, which are suitable for this purpose. Are generally used LC resonant circuits to obtain a sinusoidal oscillator signal, when it comes to a vote on a non-raster area. However, if the grid clearly specified (eg FM with 25 kHz) or CB radio, then LC oscillators are a bad choice and PLL oscillators take their place - they are far more accurate and cheaper, see below

From the tuning oscillator very much depends on the frequency stability (which means that a sender can be received over a longer period without having to readjust the vote must manually ) and the uniqueness of the scale (same scale setting the frequency pointer to same reception frequency supply ) from.

With an RF synthesizer and automatic frequency control (AFC) independence from temperature and aging effects can be achieved. (See VFO, VCO, PLL and DDS).

When using an analog tuning oscillator any frequency can be set within the reception range of the receiver. With digitally tuned oscillators, the input frequency can be set only with a certain step size. In simple short-wave devices such as these are usually at 100 Hz or 1 kHz. High-quality DDS - controlled units now offer but tuning steps of 0.1 Hz, so that you can practically perceive no difference for analog tuning with manual tuning.

For broadcast bands with a fixed channel spacing (FM, TV) such fine increments are not necessary. Since it is not, however, keep all channels to the standard, good FM receivers are manufactured with a step size in half the channel spacing ( the above 25 kHz).

Intermediate-frequency filter

The IF filter is a bandpass for a narrow frequency range, to shut out the signals outside this range and the frequencies pass within unhindered and unchanged. This will be forwarded from the frequency spectrum at the output of the mixer only the desired frequencies to the IF amplifier. The IF filter thus has a major contribution to the selectivity of the receiver. Depending on the frequency band and mode, the IF filters are required with different bandwidths.

In the early days of radio and radio art (see history of radio broadcasting ) existed only large-volume coil filter. These were later supplemented by extremely narrow-band mechanical filters and quartz filters. Ceramic filter ( ceramic resonator ) are inferior to the quartz filters in their properties, but are often used in consumer devices because of their lower price. In modern devices like cell phones can be found, without exception, Surface Acoustic Wave filter.

Typical values ​​for the IF frequency is 10.7 MHz for FM receiver ( FM radio ) and 455 kHz for AM receiver on long, medium and short wave. 38.9 MHz for TV receiver (Analog, IF picture ) and 33.4 MHz and 33.158 MHz for TV audio channels (FM, stereo). These values ​​are not standardized, but spread worldwide.

Intermediate-frequency amplifier

The IF amplifier amplifies the signal, and limits the amplitude at a frequency of modulation. The limitation in the FM can be made by two anti-parallel diode and is needed because changes in amplitude caused by noise on the transmission path, may degrade the reception quality. Amplitude changes transmitted on FM - in contrast to amplitude modulation - no information and can therefore be removed. For this reason, IF amplifier for FM also do not require control.

The gain of each stage in AM or SSB IF amplifiers, however, must be regulated in order to process a large dynamic range can. Otherwise, the receiving volumes of weak and very strong signals would differ too much.

Especially in the tubes and the discrete transistor technology, the IF selection was not arranged as a compact unit from the IF amplifier. Instead served band filter, so usually two magnetically coupled resonant circuits for extracting the signal from the mixer or amplifier output and performance adjustment to the following amplifier input.

There are some receivers on the market, one of the IF stages (usually the niederfrequenteste ) supplement or completely replace by digital technology. The analog signals are supplied to the IF stage, it is converted into digital signals in real time ( see analog to digital converter ), and then processed by a signal processor. This has the advantage that many are difficult or impossible fulfillable in hardware functions can be implemented in software. These include high-quality variable in the IF bandwidth filter or notch filter ( notch filter Sheet ) which automatically follow the interference frequency, to name but a few applications.

Demodulator

The demodulator separates the message content of the high-frequency carrier frequency. The demodulator circuits differ depending on the mode:

• The useful signal amplitude modulated transmissions is recovered with an envelope detector. This is basically a diode followed by a low-pass RC circuit. For modes with suppressed carrier, such as SSB, the carrier frequency must be missing in a mixer - for example, a ring modulator - to be mixed. This is generated in the receiver of a BFO (Beat Frequency Oscillator). In both cases the control signals to the automatic gain control ( AGC) in the demodulator is derived from the demodulated voltage.

• Frequency-modulated signals are usually demodulated by comparing the phase angle of the signal with the phase position of a loosely-coupled resonator. This resonator may be a resonant circuit ( ratio detector or, ratio detector ), a ceramic resonator or a PLL circuit. Votes resonance frequency and the useful signal frequency match, the result is 90 ° phase shift. The useful signal frequency smaller decreases the phase angle at the higher frequency used it rises. As a byproduct of the power to the automatic frequency control (AFC) is generated.

• The numerous special forms of pulse modulation and in particular the frequency spread each require specially adapted demodulators can be realized only through digital signal processing.

LF amplifier

The AF amplifier raises the demodulated signals back so far that so a speaker, headphones or external amplifier ( hi-fi component) can be controlled. (Note:. Traditionally, was connected with a radio the connector for the amplifier to the demodulator diode, hence the name "diode - plug ", " cable " or " socket" for the relevant connection components ) The low-frequency amplifier, the affect sound quality, such as raising or lowering the highs and lows.

Automatic gain control

The automatic gain control, in German with AVR abbreviated (English automatic gain control, AGC) compensates for variations in the received field strength. To this end the control voltage, which is obtained from the demodulator is supplied to the HF-/ZF-Stufen (reverse control) or the low-frequency amplifier (forward control). There, the gain of the stage is increased or decreased accordingly. This makes it possible to reproduce weak and strong stations in the same volume or compensate for the loss of a short wave reception.

Automatic frequency control

The automatic frequency control, in German with AFR abbreviated (English automatic frequency control AFC ) compensates for variations in the receive frequency.

Consideration of the previously discussed steps on a schematic

The illustrated image FM tuner has an adjustable RF amplifier (yellow), a multiplicative mixing stage (green) and a VCO (red). The tuner is equipped with dual - gate FETs, which are characterized by high input resistance and low noise. Main advantage over other types is that two transistors present in these components in the form of a cascade circuit, therefore the capacitive feedback from the output ( drain ) to the gate 1 is so low that the amplifier operates stable even without neutralization.

HF-Verstärker/Vorselektion

To receive as much energy to be transferred from the 75 -Ohm coaxial cable to the first resonant circuit, the antenna impedance through the transformer L1/L2 is coupled to the RF amplifier stage. L2, C2, C3, D1, D2 form the first input circuit ( parallel resonance circuit ) whose frequency can be tuned over the variable capacitance diodes D1, D2. The necessary tuning comes via the series resistor R8. C3 is used to match the first input circuit ( is for the manufacturer or service technician).

The pre-selected input frequency passes through C4 to the gate of one (G1) of Q1 which amplifies the input frequency. Its gain can be changed via the Gate 2 (G2). C7 ensures that "grounded" to G2 wechselspannungsmäßg (ie connected to zero potential ) is because the only way the internal shielding between input and output is protected. The amplified input frequency is inductively transmitted to the next input circuit, which constitutes a further parallel resonant circuit with C9, C10, D3, D4, and can be tuned by D3, D4.

Mixer and first IF filter

The input frequency passes through a tap ( inductive voltage dividers ) of L4 C11 G1 of Q2. Since conductivity of the upper transistor of the cascode circuit by the oscillation voltage Q2 can be changed, there is a mixture in which the intermediate frequency - usually 10.7 MHz - is formed. This is filtered by the first IF bandpass and forwarded via the terminals 5 and 6 to the following intermediate frequency amplifier.

Tuning oscillator (VCO )

The transistor Q3 of the oscillator operates in common base configuration. The operating voltage is applied across R25, L5, R23 to the collector of the transistor. C26, C25 are used to block the operating voltage and the oscillator circuit L5, C24, D6, D7 is not frequency -determining, since they represent only a short-circuit at this frequency (C26 = C25 = 560 pF). C22 causes a positive feedback, so that the oscillator oscillates. Together with the C20 phase condition of 0 ° is fulfilled in this oscillator circuit.

R26, R21 and D5 form the base voltage divider, D5 temperature compensation serves. This is necessary so that the oscillator frequency changes only slightly with temperature fluctuations.

C19 eliminates RF interference on the operating voltage. The same is true for C16, C17 in the tuning. AFC input leads to a variable capacitance diode that can vary the oscillator frequency by a few kilohertz, so that the intermediate frequency is 10.7 MHz observed. Only then generate the band passes the minimum distortion of the modulation content.

The vote

As already mentioned in the explanation of the tuning to the frequency can be adjusted by the user. Possible reception frequencies are always by the amount of IF frequency higher or lower than the frequency of the tuning:

• With upconversion:
• At downconversion:

One of which is desired, and the other is referred to as image frequency and needs to be suppressed by the band filter in front of the mixer, or by the IQ process.

If for example, a station in the frequency range of 800 to 1200 kHz is desired, you can adjust on 1455 kHz. Then these frequencies and their sums and differences are at the output of the mixer available. The IF filter is but by only 455 kHz. The single frequency in the range of which can meet this condition, the receiving frequency is 1000 kHz. An addition to the input frequency using the tuning frequency is always ≥ 2255 kHz; so it remains only the difference:

In practice, one can not only these individual frequency pass through the filter, because in this way the side bands containing the modulation, are cut off. To select an appropriate range of the IF filter, for example, 10 kHz ( which then results in the usable bandwidth of the audio information signal ) for passing all frequencies between approximately 450 and 460 kHz. This corresponds to the total signal of an amplitude- modulated medium-wave transmitter that occupies an area of ​​995-1005 kHz.

In the above example was admitted to only 800 to 1200 kHz. This pre-selection is removed, then is a disadvantage of the superheterodyne and the need in the RF preamplifier (or upstream of the mixer ) to limit the reception bandwidth filter means.

Could reach even higher reception frequencies to the mixer, it would still be a difference frequency and which provides 455 kHz:

In addition to the desired receiving frequency of 1000 kHz will also the frequency 1910 kHz to the IF down-converted, and demodulated to get the IF amplifier. This second, unwanted reception frequency is called the image frequency. It is reflected by the distance of the IF frequency with respect to tuning.

For receivers with insufficient image rejection at any station is received twice (if the frequency of the tuning can be changed far enough ): Once on the actual transmission frequency and a second time as a mirror frequency of this transmitter on the frequency. Although this is ugly, but often do not bother us. The problem of image frequency reception, when the receiving frequency and the image frequency is occupied by a transmitter, which is very common in high band occupancy. Then both channels are demodulated at the same time and there is audible noise.

Pros and Cons

Advantages:

• The superheterodyne is the best way to very high reception frequencies, such as those encountered in FM or satellite, to be processed stable. A direct detection receiver is unsuitable for this purpose because it has low gain and wide bandwidth. A direct conversion receiver is too insensitive because of its high 1/f-Rauschens.
• Frequency modulation can only be demodulate good if the ratio of frequency deviation / frequency is as large as possible. Therefore, the circuit is considerably simplified by a frequency reduction.
• The IF filter is set to a fixed frequency which is usually lower than the reception frequency. Therefore, it is easier to design the filter with a higher Q factor.
• The IF amplifier can be constructed electrically stable at low frequencies than at higher frequencies. A high total gain can be achieved without risk of feedback and a few gain stages, as the amplification of the signal occurs at different frequencies.
• This high overall gain is deliberately so reduced ( automatic gain control ), at the output approximately equal signal strength is measured, even if the antenna voltage changes by several orders of magnitude. See also gain control.
• A filter that works directly on the receiving frequency must be tuned ( in the frequency variable ) to enable different frequencies ( channels) can be received. High quality narrow-band, tunable filters are difficult to implement at high frequencies, they also change their bandwidth with the receive frequency.
• A filter -resistant, low frequency increases the technical production reproducibility of the recipient crucial compared to other concepts such as the straight receiver or the Audion. For a large part of the elaborate calibration work required in a multi- county Audion, enough with the superheterodyne a unique setting in the production.
• If surface acoustic wave filter ( in ancient devices: multiple -band filters ) are used, a nearly rectangular transmission curve is obtained which allows a high degree of separation by high slope without curtailing the high frequencies.
• The oscillator frequency can be - in contrast to the straight receiver - for example with PLL digital adjust and stabilize. Thus, the receiving frequency can also be controlled remotely.
• Ultimately it should be the ease of use ( " one-button operation " ) mentions that brings the superposition principle with him.

Cons:

• By the superposition principle creates a slave reception site ( image frequency ), which can be suppressed either by increased filter complexity before the mixer or with special circuits such as phase method (IQ ) method.
• When using a simple mixer may lead to undesired by-products and " birdies " because of intermodulation.
• The noise floor of the receiver is increased by the additional tuning oscillator and the mixer compared to a straight receiver. At very high frequencies above about 5 GHz, the phase noise of the oscillator is so high that the sensitivity of a heterodyne receiver is markedly reduced. With tricky circuits can be used in special cases, this frequency limit stretch to about 50 GHz.
• At optical frequencies (1014 Hz) no heterodyne receiver can be built because no known oscillator (laser ) has the necessary frequency accuracy and constancy. In this area insensitive direct conversion receiver are used by necessity.
• Parts of the local oscillator signal are transmitted via the receiving antenna, thus permitting the positioning of the receiving system by other receiver with a directional antenna, if the frequency is known. For commercial application, however, this disadvantage is largely irrelevant. However, it may result in extreme cases, interfere with other receivers.

Circuit variations

Simple overlay has the drawback of high frequencies, that at a low intermediate frequency ( 455 kHz), the image frequency is hardly separated from the desired reception frequency. If one chooses a high intermediate frequency ( 10.7 MHz), the bandwidth of the IF filter increases considerably. Therefore, and in response to specific requirements of the superheterodyne receiver types have been developed.

Double and multiple superheterodyne receiver

When Einfachsuperhet the selected intermediate frequency is always a compromise. On one hand, they should be very low, because of low frequencies can be built up steep edges and high quality factor, the IF filter. On the other hand, a low IF exacerbated the problem of image frequency. The lower the IF frequency is, the smaller the distance between the reception frequency of a signal at the image frequency ( distance =).

A low IF therefore requires a narrow-band pre-selection to effectively suppress the image frequency. This is becoming increasingly difficult the higher the receiving frequencies are, as for the filter in the pre-selection at the same bandwidth must have a higher quality.

To circumvent this problem, the dual-conversion works with two intermediate frequencies. In short wave and ham radio receivers, the first IF is often chosen in the range of 40 to 70 MHz and used less than 455 kHz second IF or. The tuning oscillator (VCO in Figure 1) oscillates here to the 1st IF center frequency higher than.

Due to the high first IF the image frequencies are very far away from the received useful frequency in the range 40 .. 100 MHz. Thus presented as a pre-selection on the RF preliminary stage, in principle, a 30 MHz low pass filter. In most cases, however, these recipients have multiple switchable bandpass filters to hide as many strong stations, eg in the middle or low short- wave range.

On this high first IF crystal filters are expensive and have limited selection. Therefore, one used for all modes the same filter ( " Roofing filter") with typical 12 kHz effective bandwidth and sets with a second oscillator signal (quartz oscillator in Figure 1) to a substantially lower second intermediate frequency. The further selection can then, as in a single conversion be realized in, for example, 455 kHz. The roofing filter restricted the frequency window must process the further stages, strong and lasts so much away many strong foreign signals. Weak point: If an undesired transmitter close enough that its signal is indeed passed by the wide crystal filter, but not the narrower filters the 2nd IF, the second mixer can be overridden.

Now a popular variation is to use a still considerably lower second (or third ) IF frequency to digitize the IF signal to an analog- to-digital converter and digitally processing - including demodulation.

It is possible to make more than one oscillator tunable. This principle is, for example, the short- wave receiver Barlow Wadley XCR -30 applied. In this receiver, the desired input signal is up-converted to an adjustable oscillator in the first IF range from 44.5 to 45.5 MHz. This first oscillator used to select the MHz range. The first IF is then mixed with an oscillator signal of 42.5 MHz to the second constant IF range between 2-3 MHz. From the second IF the desired receive frequency is then adjusted in the kilohertz range and mixed down to the third IF of 455 kHz with a normal Einfachsuperhetschaltung. This principle requires two tuning operations: The selection of the MHz - frequency range with the first Abstimmrad (MHz SET), and then the selection of the receiving frequency inside this MHz portion to a second Abstimmrad (kHz SET).

The advantages of this circuit are good for an analog receiver reading and repeatability and quite a high image rejection. This works without PLL, ie without the associated high-frequency potential sources of interference, but suffers from poor large signal behavior. Since the selection is performed only in the fifth stage, the previous stages can be overridden by adjacent stations, without that you can listen to these stations.

Converter, frequency converter

Converter or frequency converter are ballasts which convert one frequency range to another ( convert ). They are usually used to " open up " existing equipment new frequency ranges. For this, the frequency range to be received in the first mixer is mixed with a constant frequency and transferred to a whole frequency band to another frequency range. Within this frequency range is then matched with a single or multiple super to the desired station.

One example is the LNB in satellite technology. This reduces the receive frequency of about 10.7 to 12.7 GHz to about 1-2 GHz and sends it first intermediate frequency over a longer cable to the satellite receiver. Here is the first IF filter is but no fixed frequency filter as in a conventional receiver, but the satellite receiver is itself a superheterodyne, which converts the coming from the LNB frequency range (usually 950 to 2150 MHz) to 480 MHz.

Use find frequency converter nor the reaction of the 70 -cm amateur radio band to the 2 -meter amateur radio band ( historically ) and during conversion of UHF stations in the VHF band ( historically ). For older TVs, there are converters which convert the frequency range of the special cable channels in the UHF range and for car radios, there were converter installed which parts of the HF bands in the MW range.

Measuring Receiver

A measuring receiver is used - similar to a spectrum analyzer - the determination of the magnitude spectrum of an electromagnetic signal. The principle used is that of a spectrum analyzer is not dissimilar. The demodulation is performed here with the detectors with which the signal level will be assessed. However, addition takes place before the mixing of the signal, a pre-selection of the RF signal. A measuring receiver " sweeps " ( engl. sweep ) do not like the analyzer continuously over a frequency range ( engl. span ), but it can be selected discrete frequencies at which the level is to be measured.

( Is called, however, also often called " sweep" ) As a counterpart to "frequency sweep" of the analyzer have advanced measuring receiver via a "frequency scan". Here, a defined period of time is measured for a specific frequency range to a frequency before the device an automatic step (English step) executes the next measurement frequency and measured again. The step size is dependent on the respective resolution bandwidth, which in turn is required in standards. The measurement time or residence time has to be chosen depending on the signal to be measured. For narrowband signals, the time can be selected relatively small, with intermittent transient ( interference ) signals, however, the measurement time of the repetition frequency has to be adapted.

In modern test receivers are the IF filtering, as well as the detectors, partially or fully implemented digitally. Requirements for measurement receivers and their detectors are specified in the CISPR 16-1-1 internationally.

Today There are now more methods that using the fast Fourier transform ( engl. Fast Fourier Transform, FFT) empathize with the functionality and accuracy of a measuring receiver. Mainly want to shorten so long measurement times, as they are necessary for measurements for electromagnetic compatibility. Measurements of this type are (English Time Domain Measurement), known technically as time-domain measurements or time domain methods. In Germany in particular, much research has been in recent years operated, and it created solutions implemented in both commercial test receivers, as well as from individual components ( measuring receiver, digital oscilloscope, PC) to the previous owner.

Terms

Mathematical Appendix

The existence of the two sidebands when mixing ( ideal mixer; multiplier ) can be explained mathematically as:

Is the input signal

The signal of the ideal tuning oscillator is

The output signal of the multiplier is thus

By applying the addition theorem follows

This corresponds to the part

And

Others

Block diagram of a commercially available stereo receiver ( receiver) with VCO, PLL and microcomputer control:

View of the circuit board of a superheterodyne receiver:

The FM tuner ( 1) contains the RF stages, the oscillator (VCO) and the mixing stage. The RF stages and the oscillator via capacitance diodes are matched. The tuner has an input for, inter alia the tuning voltage and an output for the oscillator frequency ( for PLL). In (2) the three 10.7 MHz ceramic filter, the IF can be seen. ZF is the IC (3 ) is supplied, which contains, inter alia, the FM demodulator. The often used 7.1 MHz crystal in (4 ) is responsible for the reference frequency of the PLL. The PLL IC (usually a LM 7000 LM 7001 ) is an SMD component on the back side of the circuit board and not to be seen.

An application of the Heterodynprinzips in the infrared has been realized with the Infrared Spatial interferometer, in which the collected radiation is mixed with the infrared laser and thereby from converted to RF.

In general, the Heterondyn detection principle comes into optical assemblies for use, for example, by a very narrow band monochromatic laser radiation modulated by acousto- optic modulators ( = local oscillator ) and are produced so slightly up and down shifted frequencies of light that are then due to interference filter or Fabry -Perot etalon can be well separated from the output frequency.