How far exactly can you modulate a led and have a photodiode

[Quazar]

Having some problems getting higher frequencies to go through a led
and down two a photo diode.

My original idea was to build a two stage 90MHz 10.7 IF FM Superhet
complete with antenna filter to pick out a beam of light from an
extreme distance using a photo diode and a modulated led, which would
later be replaced by infrared counterparts to minimize 60Hz light
interference.

Well the superhet worked great, but unfortunately there seems to be a
problem with the frequency range of my photo diode and possibly the
LED.

I changed the frequencies of my superhet to 1MHz 455 KHz IF, to try to
get within range, but there isn't anything present.

It seems at frequencies as low as 20K are almost gone when sent
through, ick!

I did get a little better range using a infrared diode, but not nearly
enough.

Are there high freq. signals there and I'm just not notching them
efficiently enough, or do I need a special semiconductor that I could
find elsewhere?

I have tried coiling the crap out of this thing and amping it; till
all I get is RFI.

Unusually enough, I did have some luck at about 100 MHz after coiling
the crap out of the IR detector diode, but the signal was so faint
that it just barely rode on the discrminator.

Any advice?

Help would be appreciated.

 

[Yukio]

"Quazar" <NOSPAM4ME0@Bots.com> wrote in message
news:qaq1vvsf45m3pqmain7c6dlibtb4gctjb2@4ax.com...

Having some problems getting higher frequencies to go through a led
and down two a photo diode.

My original idea was to build a two stage 90MHz 10.7 IF FM Superhet
complete with antenna filter to pick out a beam of light from an
extreme distance using a photo diode and a modulated led, which would
later be replaced by infrared counterparts to minimize 60Hz light
interference.

Well the superhet worked great, but unfortunately there seems to be a
problem with the frequency range of my photo diode and possibly the
LED.

I changed the frequencies of my superhet to 1MHz 455 KHz IF, to try to
get within range, but there isn't anything present.

It seems at frequencies as low as 20K are almost gone when sent
through, ick!

I did get a little better range using a infrared diode, but not nearly
enough.

Are there high freq. signals there and I'm just not notching them
efficiently enough, or do I need a special semiconductor that I could
find elsewhere?

I have tried coiling the crap out of this thing and amping it; till
all I get is RFI.

Unusually enough, I did have some luck at about 100 MHz after coiling
the crap out of the IR detector diode, but the signal was so faint
that it just barely rode on the discrminator.

Any advice?

Help would be appreciated.

 

[Yukio]

"Quazar" <NOSPAM4ME0@Bots.com> wrote in message
news:qaq1vvsf45m3pqmain7c6dlibtb4gctjb2@4ax.com...

Having some problems getting higher frequencies to go through a led
and down two a photo diode.

My original idea was to build a two stage 90MHz 10.7 IF FM Superhet

Last time I looked ~ 15- 20 years ago best I could do was about 100 Khz.
Fibre Optic Cable was just being introduced and I wondered what in the
world are they using because off the shelf photo-diodes and
photo-transistors and LEDs just don't cut it above ~ 500 Khz. I think, the
brighter the source, the more sensitive the detector, the higher the stray
Capacitance , thus higher the bandwith the closer the coupling between
source and detector, a vicious circle !

Yukio YANO

 

[Quazar]

On Tue, 30 Dec 2003 19:26:59 GMT, "Yukio" <yano@shaw.ca> wrote:

Last time I looked ~ 15- 20 years ago best I could do was about 100 Khz.
Fibre Optic Cable was just being introduced and I wondered what in the
world are they using because off the shelf photo-diodes and
photo-transistors and LEDs just don't cut it above ~ 500 Khz. I think, the
brighter the source, the more sensitive the detector, the higher the stray
Capacitance , thus higher the bandwith the closer the coupling between
source and detector, a vicious circle !

Take a look at this - http://www.i-fiberoptics.com/leds/IFD91.pdf

This photo detector "claims" to be good at bandwidths up to 70MHz.

A look inside shows what looks to be a standard "run of the mill"
infra red photo diode.

I wonder if your correct about the brighter the source theory, or this
is a special order sensor.

Take a look at laser type radar guns, I assume the laser would have to
be modulated in order for it to be selected out, and be able to get a
some what accurate reading back for the counter to determine the
objects speed right?

The signal coming back from them can't be that strong, even if it is a
laser.

It's unfortunate that in cd players they had to integrate the detector
module inside the sensor, or I could cannibalize one and find out;
look at the frequencies there dealing with.

I wonder if I cut the end off a two-dollar IFD91(link above), if that
would be sufficient, and if it is infact the sensor, where can I get
one?