Well, IR is RF in the terahertz range, so what was described was
amplitude modulation of a signal using a frequency modulated signal as
input?  ;-)

On Mon, Oct 17, 2016 at 07:42:30PM -0400, Jason White wrote:
> Hello Harold,
>=20
> Thanks for sharing your design, quite interesting! Both the
> application and that style of data transmission is completely new to
> me. For some reason, the idea that it is possible to do RF style
> transmissions over infrared is mind-boggling to me.
>=20
> -Jason White
>=20
>=20
> On Sun, Oct 16, 2016 at 8:45 PM, Harold Hallikainen
> <harold@mai.hallikainen.org> wrote:
> >
> >> I'm quite curious about R4, which appears to be increasing the gain of
> >> the LO transistor. That's odd because (at least in simulations) I have
> >> had the exact opposite problem, too much gain, which yields a LO
> >> amplitude of greater than the specified 300mV maximum. Maybe harmonics
> >> aren't a problem in a direct conversion receiver?
> >
> > Yes, R4 is interesting. It's a 22k that's in parallel with the internal
> > 25k. I don't think it increases the gain since this is an emitter
> > follower, so the gain is limited to a maximum of 1. I'd be concerned th=
at
> > this would mess up the DC bias of the circuit. The theory of operation =
in
> > the schematic does not explain why they deviate from the chip datasheet=
..
> >
> > So far, my SDR experience is limited to listening to websdr. But, I'm v=
ery
> > impressed with what they've done.
> >
> > Somewhat like the NE602, I did a receiver using the SA639. This is an
> > infrared data receiver (
> > http://www.uslinc.com/index.php?option=3Dcom_virtuemart&page=3Dshop.pro=
duct_details&flypage=3Dflypage.tpl&category_id=3D23&product_id=3D67&Itemid=
=3D65&vmcchk=3D1&Itemid=3D65
> > ). It has a bunch of IR photodiodes in parallel. They are reverse biase=
d
> > through a tunable inductor becoming resonant at 1.8MHz. The data is FSK=
 at
> > 10kbps at 1.8MHz. The photodiodes drive an op amp-based preamp, then in=
to
> > the SA639. A crystal based local oscillator converts the signal up to
> > 10.7MHz where it goes through IF amplifiers with ceramic filters. The
> > ceramic based quadrature detector from the datasheet is used to demodul=
ate
> > the FSK. The resulting signal drives a comparator in a PIC, then the UA=
RT
> > in the PIC.
> >
> > That's my recent "RF" design experience.
> >
> > Harold
> >
> >
> >
> > --
> > FCC Rules Updated Daily at http://www.hallikainen.com
> > Not sent from an iPhone.
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
>=20
>=20
>=20
> --=20
> Jason White
> --=20
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--=20
James Cameron
http://quozl.netrek.org/
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