[R-390] 60 Hz Hum Output from R-390A IF Module

[Jon Schlegel]

Don,

I understand the problem of which you speak.  I've been making all my 
measurements relative to a 30% modulated 1 kHz tone to try and 
prevent those kinds of difficulties.  I've also been pushing the IF 
module into AGC far enough for the carrier meter to read upscale to 
20 dB.  I've been using a real time spectrum analyzer to make the 
audio measurements so measurements really are the level of ONLY the 
the 60 Hz component relative to the 30% modulated tone.

Perhaps I should be using a smaller microscope to look at this stuff 
but it bugs me that I can still see on the oscilloscope the 60 Hz 
riding along on top of the recovered 30% modulated tone.  It's just not right.

The 120 Hz component from the B+ supply doesn't seem to be a real 
issue at this point but may become more prominent if I can beat down 
the 60 Hz some more.

Regards,
Jon



At 08:17 PM 5/20/2009 -0400, 2002tii wrote:
>Jon wrote:
>
> >hum level drops by about 11 dB just by
> >changing from 8 kHz to 16 kHz filters.  For this test, I had the IF
> >module configured for normal operation, mounted firmly in the radio
> >chassis with it's power plug (20 pin connector) plugged directly into
> >the module (i.e. extender cable NOT used).
>
>Do note that the insertion loss of the various filters can vary by
>that much.  I don't know how you are testing, but if you are below
>the AGC threshold (e.g., no antenna), any such differences would be
>apparent in the detected audio output.  The noise should go up and
>down similarly, but this can be confusing to the ear because unlike
>the hum, the bandwidth of the noise changes from one filter to another, too.
>
>As others have suggested -- if you have not done so already,
>determining with certainty whether the hum is 60 or 120 cycles will
>potentially be very helpful.
>
>Best regards,
>
>Don
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>______________________________________________________________
>R-390 mailing list
>Home: http://mailman.qth.net/mailman/listinfo/r-390
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:R-390 at mailman.qth.net
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html