The passion and interest for the receivers, primarily of super heterodyne type, took me over the years to focus my attention in the mixer stage which is undoubtedly the most critical. The design of the mixer was, and is, a pain and a challenge among the various designers. The supremacy is being fought, in particular, with ever higher IP3 or TOIP values! In the mid / late eighties I tried different types of mixers, a few unpublished, with results that I would say interesting, I resumed the "research", although occasionally, in 2003. Below is described one of those mixers.
 

CONVERSION GAIN IN PASSIVE MIXER

Vincent Italia

Talking about positive conversion gain in passive radio frequency mixers may seems incoherent, but what I'm going to describe is an original and unedited way to obtain it. As we know the passive mixer is superior in manipulating the intense signals in antenna but, having no electronic device powered by DC voltage, cannot produce gain instead causes a loss in the translated frequency signal amplitude. The passive mixer is usually of switching type and then, to obtain the mixing process, interrupts or commutates to ground the signal to translate. The principle on which is based the operation of this new RF mixer is below exposed.
Suppose to apply a DC voltage to an inductor, as shown in Figure 1, an electric current will begin to flow slowly and gradually creating a magnetic field in the inductor, where energy is accumulated. By opening the switch (Fig.2) the current, with a certain delay, will stop to flow and the magnetic field will tend to collapse rapidly. Opposing to the fast changes of the electric current is the main feature of the inductor when it is powered. The change of the magnetic field, due to the closing and, especially, to the opening of the switch, produces an electromotive force (or potential difference) of an opposite sign (reversed polarity) and higher than that of the applied power, this across the inductor.
 

The amount of the electromotive force produced depends on two factors:
1 - More turns in the coil of the inductor, and therefore a high inductance value, equals to a greater magnetic flux and thus higher will be the voltage produced.
2 - The faster the change of the magnetic field (opening and closing the switch) the higher the voltage across the inductor.
Replacing it with an LC circuit, tuned for example to 100kHz, and replacing the DC power supply voltage with an alternating one, for example with a frequency of 900kHz, and assuming to open and close the switch at the rate of 0.001 milliseconds (1000kHz) produces a frequency translation from 900kHz to 100kHz, the latter selected and filtered by the tuned circuit. Thanks to the mechanism of "magnification", as explained before, the 100kHz signal level, across the LC circuit, will be higher than the signal "power", or input signal, of 900 kHz. Substituting the manual switch with one of the electronic type (FET or other) and driving it with a frequency of opening and closing of, in the example, 1000kHz, we would obtain a passive mixer, and then without any DC power supply, with conversion gain (Fig. 3).
It’s evident that, in the example just shown, 100kHz is the intermediate frequency, 900kHz that of the antenna signal and 1000kHz the local oscillator frequency.

This circuit is also well suited for demodulating SSB signals, and thus as a product detector, achieving an excellent signal amplification of the detected low frequency. Anyhow it is suitable where it is required to translate a frequency in another one with a positive conversion gain.

References:

1-  www.radiopharos.it
     SEM - An inedited passive FET Mixer with conversion gain

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