From: jimbell@pacifier.com (jim bell)
Date: Thu, 16 Nov 1995 06:28:27 +0800
To: Peter Monta <pmonta@qualcomm.com>
Subject: Re: Hardware-random-bits interface
Message-ID: <m0tFpnk-0008yDC@pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


>> > And if one is left with only internal noise, why not simply use a nice
>> > clean source like a Zener diode?
>> 
>> Quite solidly agreed. Internal noise beats external noise, and there
>> is no point in using a radio receiver when what you want is internal
>> noise.
>
>It may have been misleading to refer to a radio receiver chip:  the
>noise generated by the chip will be entirely internal.  It doesn't
>depend on any external signal "sneaking in"; the noise comes from
>the effective resistance of the first amplifier stage.  Nothing
>is being "received", in some sense; it's just a big hunk 'o gain.
>
>A Zener diode would be fine, but the design is a bit more complex,
>especially for wide bandwidths---you need to mess with biasing
>voltages, speedy op-amps, interface to CMOS, and all that jazz.
>You'd need external power (or a DC-DC converter).  I wanted a
>super-simple design that people could just plonk down on a PC
>board.  Two chips and some bypass capacitors.

Why not do both?  A Zener is supposed to be a relatively high-level source
of white noise, up to some upper bandwidth, so you could just feed the Zener
voltage (through a blocking cap, if necessary) to the radio receiver input.

See, you and the chip manufacturer are working at crossed purposes:  a radio
receiver chip is SUPPOSED to be a low-noise device.  For your purposes, you
WANT it to be a high-noise device, to minimize the significance of external
noise.  Since the radio receiver chip does some things you want (gain, maybe
AGC), but doesn't necessarily add an adequate amount of noise, a zener would
be an excellent addition.