Re: [LAD] Smallest, simplest, silliest SDR?

On Monday 01 March 2010, Folderol wrote:
>On Sat, 27 Feb 2010 22:50:02 -0500
>
>Gene Heskett <gene.heskett@email-addr-hidden> wrote:
>> On Saturday 27 February 2010, Folderol wrote:
>> >On Sun, 28 Feb 2010 00:50:07 +0000
>> >
>> >Gordon JC Pearce <gordonjcp@email-addr-hidden> wrote:
>> >> On Sun, 2010-02-28 at 01:41 +0100, fons@email-addr-hidden 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
>> >> mung 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.
>
>< snip .. very interesting stuff actually :) >
>
>That must have been in the USA I guess, over here the intercarrier is
>6MHz
>
>Us young 'uns laughed on two levels.
>
>The first was that most of us didn't have a clue how the things worked,
>due to pathetic instruction from our employers (I'm looking at you
>Radio Rentals). When asked what, exactly, quadrature demodulation was,
>there was a pregnant pause after which the instructor said 'Well it's
>demod in quadrature of course' then swiftly moved on to the next topic.
>
>Our next cause of merriment was the horrendous EH90 heptode (yes I did
>say heptode) implementation. See, it scarred as all so badly I can
>still remember the precise details!
>
>The screen feed was via an 18k - 5k6 potential divider, only there was
>something very strange about the composition resistors used for this.
>In the first place they were under-rated (especially the 18k. If you did
>the math you could prove it. Secondly, where resistors normally go
>high if they overheat these went low, so a nice little thermal runaway
>ensued. This actually set fire to the (paxolin) PCB, burning a hole
>through it. If you were very unlucky it also cooked up a wirewound
>resistor further along the board, which then de-soldered itself, arced
>and burned another hole in the board.
>
>In those days we were expected to fix these things on site, so after
>scraping out all the carbon, you were left with point-to-point wiring
>across the gaping holes, supporting the replacement components as best
>you could.
>
>Such fun we had!
>
The 6BN6 was what was called a gated beam tube, with the #1 grid being driven
by the somewhat am limited carrier (4.5, 6 or 10.7mhz, I believe it worked
well at all those freqs) Half a volt to a volt of signal IIRC although its
now been 50 years since I probed the circuit with a scope. The 3rd grid,
after the screen grid, was also quite closely spaced so that it had decent
gain also, and a tuned circuit with a Q selected according to the expected
bandwidth was tied from this grid to ground. Space charges drove this grid
by the coupling effect of the electron beam going by, and natural resonance
effects saw to it that its voltage lagged (or led, its been years) the driven
grids voltage by about 90 degrees electrically. The end result was a pulse
duration modulation of the plate current because the tuned circuit lagged the
input signal by an amount dependent on the instantaneous frequency.

Actual plate current in this tube was only 1 to 1.5 milliamps, so there
wasn't really a large amount of power being handled. Screen (grid #2)
current was maybe 3 milliamps, so the developed audio voltage wasn't too
large and they usually had another medium gain small triode to bring it up to
a level suitable for driving a 6V6 or similar output stage that gave about 2
watts of reasonably clean audio to drive the speaker(s).

In solid state, RCA made a CA3089E for several decades that worked on a
similar principle, but mopped the floor with a 6BN6 because of its superior
performance.

-- 
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)
The future is a race between education and catastrophe.
		-- H.G. Wells
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