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/radio/receiver/double polyakov/direct conversion/
NOTE: GIVE THIS SIMULATION SOME TIME TO SETTLE DOWN
This is the business-end or main part of this direct conversion receiver. The important point to note here is that this receiver is 'tuned' by injecting half the desired frequency into the mixer. In this case we can use a square wave signal as the VFO, which is good as this frequency can then easily be generated by any number of low cost sources including low cost micro-controllers such as the Attiny85.
So the difference between the two input generators produces a 100k carrier which is Amplitude Modulated with a 5 KHz audio AM signal. This is done in order to test this product detector. (Thanks to @hurz for providing the simplest method of generating an AM modulated tone in EC.)
The Polyakov detector is interesting, as it uses a frequency of half the frequency that you are tuning into. This is useful as it potentially allows for a far more sensitive receiver. Higher sensitivity is obtained by lowering the risk of undesirable products from the VFO.....
The normal arrangement for the Polyakov is just two anti-parallel switching diodes, and then there is a low value 500 ohm preset pot that does not seem to do anything other than serve as a voltage divider, which I don't like much.
So here, we have two sets of Polyakov detectors back to back. No useless pot! Unlike the original, the input is terminated with 50 ohms and the LO frequency is injected into the centre of the diode arrangement.
A crude filter network removes much of the 50K LO and allows us to see the 5 KHz signal recovered.
Unlike balanced diode mixers this mixer does not require large input signal levels. In fact one has to be quite careful not to overdo it. Note that I mean that high levels are not needed on the RF input side. Whereas, the LO side can be 3 - 5 volt logic triangle wave!
It is suitable for a low cost multi band receiver
Note that in an attempt to prove that this mixer is indeed operating as a switch that works twice every cycle, and not some weird subharmonic heterodyne explanation that floats around the internet, I have used a square wave LO, which is fed into the mixer and a completely acceptable output results.
This mixer is very easy to build, does not require any complex transformers and is able to retrieve SSB CW and AM (so maybe there is some small truth in the heterodyne argument, as the only other simple receiver able to do this is a regenerative receiver - i.e. using the double Polyakov, no insertion of the carrier frequency is needed to recover SSB using this mixer. In a regenerative receiver the carrier insertion is done by the LO which is the regenerative oscillator itself )
Also note that the choice of 100 KHz as the carrier is purely done so that the 5 KHz Audio signal can be seen against the input modulated AM signal in EC.
In reality this mixer can be used for all the HF bands (with appropriate changes in the capacitor values and the LO frequency.
Enjoy
73
Fine business!
P.S I now invite all sorts of comments even if they may be negative, or way off the mark. I even encourage comments from persons who have no idea what I offer here but would like to demonstrate how wrong it all is etc....
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