1 Mar
2010
1 Mar
'10
5:57 p.m.
On Sat, 27 Feb 2010 22:50:02 -0500
Gene Heskett <gene.heskett(a)verizon.net> wrote:
On Saturday 27 February 2010, Folderol wrote:
< 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!
--
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
On Sun, 28 Feb 2010 00:50:07 +0000
Gordon JC Pearce <gordonjcp(a)gjcp.net> wrote:
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. On Sun, 2010-02-28 at 01:41 +0100,
fons(a)kokkinizita.net wrote:
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 ... 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