From: Kevin Purcell (firstname.lastname@example.org)
Jim, WD9EYB, said:
>There are three IC's, an NE612 (equivalent to NE602), an LM358 and an TDA2611A.
I also saw a comment similar to this in Spring 1994 Hambrew (p44) hence
this note. Is Bruce, WA6IVC, on this list? Have you made any quantative
measurements of '602 vs '612 receivers?
Looking at the datasheets the NE612 and NE602 differ in one major way:
third order intercept point. Its about 10dB lower in the '612. I suspect
that '612s are '602s that didn't make the spec but are still fine for less
critical applications. However I don't think a 40m RXes falls into this
With a little fiddling you can get up to 85dB SFDR from a '602 (with its
-10dBm intercept point).
Some design rules to follow are:
* Make sure you have a no amplification before the '602. It has a noise
figure of 5dB so you don't need an RF amplifier for the most h.f. bands.
* Put an RF attenuator in front of the '602 for the lower HF bands. The
external noise is higher than the front end noise. You want to set the gain
so the noise just goes up when you add an antenna.
Note that in the better '602 receivers (for example, the NorCal 40) these
rules are followed and result in a simple RX with good performance and
small power requirements. In fact the NorCal avoids an IF amplifier by
using the gain of two '602 to its advantage. Most of the Neophyte RX
designs never address these issues.
Many moons ago (in QST Technical correspondence) one of the Signetics
application engineers wrote a letter describing the best way to use a '602.
Its someplace in my "ne602" collection of papers.
Kevin Purcell, N7WIM / G8UDP
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