Hi Roy.

10-Jan-04 12:06:39, Roy J. Tellason wrote to Jasen Betts


 RJT> Jasen Betts wrote in a message to Roy J. Tellason:

 JB>> Hi Roy.

 JB>> 07-Jan-04 20:16:25, Roy J. Tellason wrote to all

 RJT>> One is a chip resembling an op amp,  but apparently
 RJT>> preconfigured as a unity-gain buffer.  I found it easily enough
 RJT>> in my databooks, though I don't know why you'd get these and not
 RJT>> just use a standard op amp and configure it as a unity gain
 RJT>> part. Maybe some difference in the spec that's not apparent to
 RJT>> me, or something...    (Anybody know?)   I suppose I'll think of
 RJT>> something to do with these sooner or later

 RJT>> The other part that jumped out at me as being a little unique
 RJT>> was a 565 PLL chip.  I have heard of this one before,  but have
 RJT>> *no* idea what you'd do with it.  Any of you guys have any
 RJT>> thoughts on the matter

 JB>> ISTR that a PLL is a kind of frequency follower, other than using
 JB>> it in a frequency synthesiser (in a radio tuner) or frequency
 JB>> multiplier etc I don't know what you could do with it,

 RJT> Sounds about right.

 JB>> HMM, maybe you've got a parallel-port radio tuner kit there.

 RJT> Could be...

 RJT> I've been bumping into all sorts of circuits lately,  radio
 RJT> circuits among them.  I'd like to know,  though,  how some stuff
 RJT> (scanners and other things that deal with a *lot* of frequencies)
 RJT> manage it.

AIUI the use a superheterodyne (I can spell it but not really understand
it)

basically what they do is generate a sine wave near the desired frequency
(say 60Khz below it) and use that to modulate in antenna input this results
in a prequency shift of the program signal, then they use a fixed frequency
detector/demodulator setup.

 RJT> It's been quite some time since I read anything on
 RJT> this subject,  but I remember early gear that switched coils,
 RJT> even to the extent of plugging different ones in,

old clunk-clunk VHF TV tuners....

 RJT> and I have my doubts that modern stuff does anything of the sort

more recent TV tuners use varicap diodes .... diodes sepcially designed
have a junction capacitance which varies depending on to their bias
voltage...

but ISTR they still use split the spectrum into 3 bands VL VH and UHF

 RJT> I suspect that this is where things start getting less into
 RJT> ordinary "tech" stuff and more into that "black
art" portion of
 RJT> design that I've been avoiding all these years.  :-

it'd be real hard to design an RF circuit that'd work from 100Khz all the
way up to 1Ghz... ISTM most devices get at most factor of 3
betweens ends of each tunable band, that may be a limitation of tuning
devices or it may also involve limitations in the filters employed in the
receiver.


 -=> Bye <=-

---