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I would like to construct the following receiver:
I have a fair knowledge about a receiver building blocks. I have previously build small receivers.
In the description it say:
The local oscillator ... generates an oscillations with a frequency twice lower than the frequency of received signals.
How can that be possible and why it is needed?
The oscillator may really be running at half of the received frequency. Search for Poliakov mixer on the web for an example of this.
Mixing products from the oscillator. Best way is to use the oscillator on a higher frequency. Example: receiving 10 MHz, mf is say 9 MHz, so the oscillator is on 19 MHz.
Demodulation does it´s work on 9 MHz so no [you hope] no mixing problems.
You can also use a oscillator in 1 MHz, mf is 9 MHz bus you see the problem harmonics from the 1 MHz oscillator. 1,2,3,4,5,6,7,8,9,10 <- problem
It is also possible if you use a crystal oscillator that it is a thirtieth overtone crystal so in basic the crystal oscillates on 19 MHz but also on a third of the oscillator frequency 6,33 MHz and 12,66 MHz.
Hope you find the error.
This appears to be a design that uses a harmonic mixer (or a subharmonic mixer, both names are used on the Wikipedia page). While these are more commonly used at microwave or millimeter wave frequencies in contemporary equipment, they were probably reasonable, low-cost solutions for these single-conversion receivers. As described in the text, VD1 and VD2 form the mixer for this radio receiver.
As a consequence of being a harmonic mixer, the mixer uses a harmonic of the local oscillator, making the local oscillator a subharmonic of the radio frequency being received.
A diagram for a microwave-type mixer using this concept shows the anti-parallel diodes seen in the HF mixer in the question's design. This is currently advantageous because generating high-power oscillations >10GHz is difficult and there are applications targeting 60 and 70 GHz with test equipment up to 110 GHz.
How does it work?
Its a good question. I will be using example frequencies and not the actual ones used in a real receiver. The Design is called the Superheterodyne receiver and what happens is that you will get EVERY frequency coming in to the arial(assuming that the arial is tuned for all frequencies) so having a tuned arial starts to reduce the frequencies incoming on the input.
Next you are interested in say 1500khz frequency (AM signal here for simplicity) so you set your local ocillator to either 2400khz or 600khz (difference of 900 khz).
The local ocillator is mixed with the incoming signal from the arial which will for our wanted frequency provide the original signals, the sum and the difference.
1500,600,900,2100
Because your next stage is a Band pass filter tuned to 900khz.
meaning only the difference is passed through to the Audio amplifier stage which uses a diode to remove the radio frequency and pass only the audio signal out to the speakers.
The antiparallel diodes VD1,VD2 of type BAP794 form the subharmonic mixer, this type of mixer is efficient for even order harmonics of the local oscillator, here, harmonic 2 is used.
This type of mixer can be traced to a 1955 patent https://patents.google.com/patent/US2706775A/en one can see on the patent diagram the antiparallel diode valves of type 6AL5, patent text mentions "The converter characteristic requires that the local oscillator employed be operated at one half of the normal oscillation frequency"
A more recent text is "High-Order Subharmonically Pumped Mixers Using Phased Local Oscillators" it is worth reading the Intro paragraph which gives an overview of the topic.
