product detector improvement
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Posted by: "w1eat" w1eat@yahoo.com
<mailto:w1eat@yahoo.com?Subject=%20Re%3Aproduct%20detector%20improvement>
w1eat <http://profiles.yahoo.com/w1eat>
Fri Jun 25, 2010 6:05 am (PDT)
The CA3053 product detector (U3 on the IF/AF board) has been the subject
of several mods intending to improve intermodulation distortion in the
audio passband. W3TS has written about this at length.
The CA3028 is the same part as the CA3053, so I did some research and
found a Harris Application Note AN5337 discussing the CA3028 in RF
applications. In studying this info I noticed that Harris always biases
pin 5 at 2/3 of Vcc, as opposed to 1/2 Vcc in the O-V. The datasheets
also show this if you look close enough.
R14 and R15 form a voltage divider to provide this bias, so I put a 10K
resistor in parallel with R14 to get the bias level to 2/3 of Vcc. The
voltage at pin 5 in my radio is now about 5.6v, up from about 3.6v
before the change.
The result is that the distortion goes away. Pile-ups now sound so clean
and clear that it's hard to believe one little resistor can make such a
difference.
Try it! You'll like it!
_____________________________
73, Jerry, KG6TT
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