[LAD] Smallest, simplest, silliest SDR?

Gene Heskett gene.heskett at verizon.net
Sun Feb 28 03:50:02 UTC 2010

On Saturday 27 February 2010, Folderol wrote:
>On Sun, 28 Feb 2010 00:50:07 +0000
>Gordon JC Pearce <gordonjcp at gjcp.net> wrote:
>> On Sun, 2010-02-28 at 01:41 +0100, fons at kokkinizita.net wrote:
>> > On Sun, Feb 28, 2010 at 12:40:00AM +0000, Gordon JC Pearce wrote:
>> > > I wanted a very simple SDR with jack inputs and outputs for a
>> > > demonstration I was doing.  I had a look at the DSP guts of dttsp and
>> > > quisk, and sat down to code.
>> >
>> > Forgive my ignorance, but what is an SDR ?
>> Software-Defined Radio.  Basically you downmix incoming RF to the audio
>> range with two mixers fed 90 degrees out of phase.  You can then munge
>> this in various different ways to tune and demodulate various different
>> radio signals.
>> Gordon MM0YEQ
>Sounds suspiciously like some form of quadrature demodulator. Rather
>like GEC/Sobel introduced with their 1018 TV chassis in the 1960s.
>Oh how we laughed ...

Why were you laughing?  Zenith did this, with a self excited circuit, using a 
type 6BN6 gated beam tube to recover the audio directly from the 4.5 mhz 
inter carrier frequency, starting in the fall of '51 with the intro of the 
'52 model year.  It worked fairly well too.  Stable, not sensitive to the 
fine tuning setting, so folks out in the fringes could tune for a slightly 
better if not as sharp a pix.

I even made a minor improvement to the circuit that reduced the AM 
sensitivity noticeably that was incorporated into the next 3 years production 
runs.  I was all of 17 at the time and working in the service dept., the only 
one, at a zenith wholesaler.

It still, like all tv's of the day, had some synch buzz, and it took the FCC 
35 damned years to let us broadcasters fix that.  As the CE of a tv station, 
one of the things I did when they allowed us to experimentally reduce the 
aural transmitters power output from 100% of visual to 20%, was to try 10% on 
the sly, which helped even more.  Then a year or so after I came to WV, I 
thought 5% might be worth a try, but couldn't get even a bad final tube that 
low, so I bypassed it one night, saving us many KW a day in electricity since 
the final took 12 volts at 180 amps (2160 watts/hour just to light it up, and 
used about 10KW/hour in high voltage draw) which took it down to about 900 
watts from the driver stage output, compared to 26kw of visual, figuring I 
would hook it back up if the phone started ringing.

18 years later the phone still hadn't rung.  The tubes were out of production 
& tired, and it was down to about 500 watts when we turned it off (NTSC) for 
good.  Nary a complaint in all that time about weak signal, not even from the 
cable folks.

Quadrature detection works, and there are even some integrated circuits that 
do that, they take an fm signal in, need about 6 parts including a tuned coil 
circuit for the frequency of the fm input, work at less than 1% THD at 4.5 
mhz with its +-25khz modulation, or about 1% THD at 10.7 mhz with its wider 
modulation.  If you need any lower THD than that, go find a 50 year old 
McIntosh C8 tuner and restore it.  They were, in their day, the absolute 
cream of the crop and to my extended knowledge, such performance has never 
been even close to duplicated at any price, and that one was con$iderable 
when new in '60.

Yeah, I'll plead guilty to being an old fart, 75 now.  And its been one hell 
of a ride to get to this day.  Mostly enjoyable, I'd do it again almost 
exactly as it happened.

Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)

I did this 'cause Linux gives me a woody.  It doesn't generate revenue.
	-- Dave '-ddt->` Taylor, announcing DOOM for Linux

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