[Heath] SB-303 Noise Blanker Schematic

Brian Wood brianwood at qwestoffice.net
Sun Mar 16 15:40:02 EDT 2014

An RF attenuator is not the same as an RF gain control. RF Gain is actually
IF gain (or even more appropriately, IF attenuation!), and is inside the AGC
control loop. An RF attenuator is outside that loop, at the same place as
the bandpass filters (or preselector) and any RF preamps, i.e. close to the
antenna. It's useful to attenuate strong signals before they get into the


It IS, however, pretty obvious that something was amiss. I also have the '71
catalog, and it clearly shows "NOISE BLANKER" text carefully scratched out
so as not to be all that noticeable. Very cool! A mystery! Having designed a
noise blanker, I can attest to how hard it is to get the dang things to work
the way you want them to. And of course there is the popular misconception
that they reduce broadband noise. They only reduce impulse noise, such as
from electric fences or car ignition. True NR is a DSP thing.


Brian, W0DZ


From: Heath [mailto:heath-bounces at puck.nether.net] On Behalf Of Guy
Sent: Saturday, March 15, 2014 9:07 PM
To: pbrickey at verizon.net; heath at puck.nether.net
Subject: Re: [Heath] SB-303 Noise Blanker Schematic


Coool....now, does anyone have an SB-104 schematic handy so that a
comparison could be made?  I'm betting that Pete's 303 blanker is unique to
the 303'...and now the big question remains, why didn't they develop this on
the 303'?  I know the answer to one question though....The question, is why
does a 303' have an rf attenuator in the first place?  It's kind of stupid,
and it doesn't do anything that an rf gain control can't already do.  Soooo,
I bet this "rf attenuator' was the cheap answer to "What do we fill that
spot with?" that the engineers had to answer when they couldn't get the
noise blanker to work.  At least, that's my theory...


On 3/15/2014 2:30 PM, pbrickey at verizon.net wrote:

  I have had quite a few questions about this, so I thought I would send out
the schematic that is in my SB-303 manual.
It is attached.

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