On Wed, 1 Mar 2006 07:03:01 -0500, "xpyttl"
wrote:

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.

If you really intend to use mixers with such horrible IP3 figures, I
would suggest using a very selective front end ahead of it. For a
single band CW receiver some fixed tuned stages might suffice, but
otherwise some tunable input filters should be used.

In Europe, there are several high power broadcasters starting at 7100
kHz, which would easily overload the 40 m receiver. Assuming loaded Q
of 100 and the front end tuned to 7000 kHz, the -3 dB bandwidth would
be +/-35 kHz from the centre frequency with some usable attenuation at
7100 kHz.

Using fixed tuned octave wide front end filters with the 602 is just
asking for trouble.

Paul OH3LWR