Re: Choosing a toroid for impedance matching from antenna to NE602

Jerry Gaffke

Here's an interesting document regarding impedance matching networks to feed the 602/612 mixers:
Page 3 tells us that the SA602A has the same mixer as the SA605, as described in this app note.
Page 3 includes some other history, but never quite says what an SA612A is (only what the SA612 was).
Page 6 shows that when driven single ended at 10mhz, the input impedance is around 5k
Page 9 reminds us that when driving the mixer differentially, that input impedance is doubled to 10k.

Page 4 of the datasheet:
says   "AC input impedance is approximately 1.5k || 3 pF through 50 MHz."
I'm pretty sure that would be doubled to 3k when driven differentially.
Since this is a later document than AN1994 and is specific to the currently available SA612A,
seems more likely to be correct with regard to the input impedance.  But that's by no means certain.

This blog posts suggests that for all parts currently available, the 602's are the same as the 612's.
The story conflicts a bit with AN1994 page 3 in that it claims the upgraded part was called the 612, not the 602A.
Never says anything about the A variants, though that transition happened around 1990, long before this was written.
Also explains that the NE612 was good only from 0-70C, whereas the SE612 is good from -40 to 85C.
The only 602/612 part I see being sold these days is the SA612AD.

If you don't need to wring every ounce of gain from these mixers
should be ok to drive them from a lower impedance source.
Some successful QRP rigs just drive them from 50 ohms single ended,
as atmospheric noise tends to predominate at HF anyway.

Jerry, KE7ER

On Sun, Dec 3, 2017 at 03:47 am, Kerr Smith wrote:
... I have chosen to use 1500 ohms but Jean-Matthieu calculations suggest 3472 ohms?

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