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hello,

I'm building my first reciver. I can't choose what kind of mixers
should I use. I have read that diode ring mixers are far superior
compared to dual gate mosfet mixers.

"Superior" is something of a loaded word. Whether a particular parts is
superior or not depends on your design intent.

Probably the most popular mixer for simple HF receivers is the NE/SA
602/612. This is an active mixer. It has amazing amounts of gain, such
that an RF stage is almost never needed. It is extremely simple to deploy,
and it requires almost no power. Thus, in portable/battery powered circuits
it is almost always the mixer of choice. It has an absolutely horrid TOIP.
There are other, mostly older, even poorer, active mixers, but the 602 is a
very versatile part, so it seems to show up everywhere.

At the other extreme are diode ring mixers. These can have stellar TOIPs,
but take a lot of oscillator power. Further, they need lots of RF as well,
so some sort of RF stage is needed. All this adds up to a need for plenty
of power. The best diode ring mixers will use matched, Schottky diodes, but
good old 1N4148's do work, and quite well. Many designs use packaged diode
ring mixers such as those from Minicircuits.

The dual gate MOSFET falls kind of in the middle. It doesn't have the
horrible TOIP problems of an active mixer, but it's not as power hungry as a
diode ring. The MOSFET seems to have fallen out of favor lately, in spite
of being a "balanced" sort of solution. I suspect most designers are either
going for power consumption or performance, and really, quite good
performance can be had with the active mixers with careful design.

Is this true for both - first (RF / VFO) and second (IF / BFO) stages?
Or is there any real difference at all?

Careful design can manage what the second mixer sees more easily than than
the first. This can make the dynamic range problems of an active mixer less
of an issue. For that reason, balanced designs that tend toward management
of power consumption will sometimes use a diode ring for the first IF and an
active mixer for the second. But a superhet bent all out on power
conservation will almost always use a pair of 602's. Designers who want to
avoid ICs for whatever reason will use a pair of diode rings.

I shouldn't sound so down on the 602. A WELL-DESIGNED 602 receiver can
easily match the performance of the $1000 class rice box rigs. It cannot,
however, come close to the performance of an equally well-designed diode
ring rig. But the diode ring rig will probably consume three times the
power, meaning three times the heat to deal with and the associated
oscillator compensation issues.

So you picks your poison.

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