Let's made a Superheterodyne!
![](pic/demod1.en.print.png)
![nur mit Diode](pic/demod1.en.png)
To create low frequency information from a RF-signal, the received signal has to be demodulated. However, the received RF-signal is extremely weak and the diode needs a certain minimum value to function. (For this reason this type of receiver can only be used close to very large transmitters.)
![](pic/demod3.de.print.png)
![](pic/demod3.de.png)
For this reason a HF- preamplifier, which lowers the minimum discernible signal even further and extends the range, is used. To be able to amplify only the needed frequency the pre-amplifier has to have a very narrow bandwith.
![](pic/demod4.de.print.png)
![](pic/demod4.de.png)
This only allowes us to receive one frequency. Rather limited! Therefore we'll make the RF pre-amplifier tuneable with e.g. a variable capacitor.
![](pic/demod5.de.print.png)
![](pic/demod5.de.png)
Still not really sufficient. We need more amplification.
We don't want to turn two buttons at the same time and thus these two need to be
connected mechanically (so-called: “ganged capacitors”).
Be carefully! All amplifiers need to be well insulated!
In case of high frequency installations every larger wire becomes a transmitting antenna.
Then the transmitted energy is received again by our receiving antenna and the whole
system begins to oscillate. As a result the sensitive receiver becomes a strong transmitter!
Theoretically this can be extended until...
![](pic/demod6.de.print.png)
![](pic/demod6.de.png)
... the tuning mechanics fail and feedback occurs again!
If we abandon tuneability, the following circuit would be possible:
![](pic/demod7.de.print.png)
![](pic/demod7.de.png)
What a pity! We can only receive one frequency but the receiver is very sensitive to it.
What happens if we tune the “One-Frequency-Receiver” into a frequency which is not particularly active and at the same time install a tunable converter, whose output frequency as a difference between the received frequency and the tunable oscillator frequency remains constant?...
![](pic/demod8.en.print.png)
![](pic/demod8.en.png)
... in that case we invented a Superheterodyne!!!
The Super Heterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver) was invented by Edwin Armstrong in 1918.