A straight-ahead receiver is a relatively simple receiver for radio waves , for example a radio , as it was mainly used in the early days of this technology and is only used in very special situations today. In contrast to the later developed heterodyne receiver , the actions of frequency selection, high frequency amplification and demodulation take place on the same frequency . The frequency to be received reaches the demodulator "straight ahead", ie without mixing .
Depending on the number of oscillating circles, they were also called single-circle , two- circle or three- circle . A designation, for example in the form 0-V-1, was also common. The 0 stands for the number of HF stages, the V for the rectifier stage (valve) and 1 for the number of LF stages.
With the establishment of VHF broadcasting, this concept lost its importance because, on the one hand, high-performance, high-quality VHF radios with straight-ahead receivers cannot be implemented and, on the other hand, the superimposition receivers ("superhets") became more and more affordable, and with them the advantage of simplicity of the straight-ahead receivers stepped into the background.
Today it is the widespread receivers in radio clocks that receive the time signals from DCF77 , in which this reception principle is still used, as this transmitter works with a fixed, very low frequency of only 77.5 kHz and the effort for a superimposition receiver is quite high would be large and would not bring any advantages.
Due to the shutdown of numerous European medium-wave radio stations, fewer radio stations can now be received with straight receivers than 25 years ago.
The advantage of straight-ahead receivers and the reason for using this technology is their simple construction, at least when there is only one oscillating circuit (with a single circuit). If there are several resonant circuits to be tuned, there are no problems with fixed-frequency receivers, but if a variation of the receiving frequency is desired, because then the structure becomes more complicated because of the synchronization problems. In principle, a straight-ahead receiver cannot have any secondary reception frequencies or whistling points from which a heterodyne receiver often suffers.
The desired reception frequency f e is selected from the composite signal coming from the antenna with a filter ( oscillating circuit ) , the high-frequency signal is amplified, possibly filtered again and fed directly to the demodulator . The demodulation can take place in a separate stage with a separate diode or in the last HF stage. For tubes, for example, grid rectification (as with most tube audions) or anode rectification is possible.
The main disadvantages of the straight-ahead receiver can be derived from this description:
- If several frequency filters are provided, all filters must have exactly the same center frequency and be detuned with each other when different reception frequencies are selected (synchronism). This was z. B. achieved with the help of multiple variable capacitors , but can only be done with limited accuracy.
- Because of the synchronization problems, it is almost impossible to build a filter with a well-defined pass characteristic (e.g. bandpass with a steep drop outside the frequency range) with several coupled resonant circuits.
- The bandwidth of frequency filters based on resonant circuits is determined by the quality factor and the center frequency. Especially at high frequencies ( short wave range ) it is almost impossible to achieve a sufficiently narrow bandwidth; the receiver then has too little selectivity . With tunable straight-ahead receivers, the bandwidth also depends on the frequency, but should always be the same for a radio receiver.
- Since the quality factor of tunable oscillating circuits is usually less than 100 and decreases with increasing frequency, straight-ahead receivers for VHF are illusory, because no satisfactory selectivity can be achieved.
- Since the amplified high-frequency signal has the same frequency as the input signal of the receiver, the output signal of the high-frequency amplifier can be picked up again by the antenna and feedback occurs. Since a certain minimum voltage is required for the demodulator, this problem cannot be remedied by reducing the high-frequency gain, but a great deal of effort is required to shield and decouple the amplifier output from the input.
- The overall gain, which is usually too low or nonexistent, does not allow automatic gain control as in the heterodyne receiver .
The problems mentioned can be overcome by a superimposition receiver (“superhet”). This makes use of the fact that when two frequencies are mixed, several mixed products are created. The generated frequencies have the same modulation as the useful signal. If you now generate a frequency with an oscillator at a constant distance from the frequency of the transmitter to be received, you only need bandpass filters after the mixer that are permanently set to this intermediate frequency . The intermediate frequency can be selected so that filters of the desired bandwidth can easily be implemented at this frequency.
Overlay receivers offer no advantages at low reception frequencies if only a single radio service is received and the required bandwidth can also be achieved with a straight-ahead receiver with resonant circuits. Only when temperature and aging independence is required is it used. This applies in particular to the reception of the time signal from DCF77 . Due to its modulation, no bandpass filters are required, the resonance circuits are available as commercially available quartz filters , which ensures adequate independence from temperature and aging. Even very simple receivers for remote controls with a short range are still built as straight-ahead receivers. Another area of application is wireless front door bells and wireless weather stations that work in the 433 MHz range.
An example of a straight-ahead receiver is the integrated circuit ZN414 from Ferranti , which was presented in 1972 and also a complete straight-ahead receiver in a 3-pin TO-92 housing for use with a 1.5 V battery in medium wave and with reduced sensitivity in the long wave range. Due to the simple structure of the externally necessary circuit parts, this IC was popular among radio hobbyists, among others. Another application in the hobby area is the reception of long waves with the PC sound card, whereby the amplifier of the sound card takes on the role of the "high frequency" amplifier.
Since today's analog-to-digital converters and digital signal processors already work fast enough to replace the function of the mixer and the filtering and demodulation of the intermediate frequency, receivers can now be built in which a high-frequency amplifier is only required for the receiving frequency, however the other components of a heterodyne receiver are no longer required, as is the case with software-defined radios . However, these systems are not referred to as straight amplifiers because when converting to digital signals by means of a sample-and-hold circuit, they are converted to a lower frequency.