Looking at a better head amp ?

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Looking at a better head amp ?

Postby Harry Dalek » Tue Mar 10, 2015 9:43 pm

I was looking into what was out there as far as a better light sensitive receivers and what others have been experimenting with and came across this ,which i suppose really could be used in our hobby for better head amps bandwidth on the circuits look a bit low for nbtv fine for sstv.
I found the details on different light sensitive devices an eye opener .
http://modulatedlight.org/optical_comms ... l_rx1.html


http://modulatedlight.org/optical_comms ... igher_freq


http://modulatedlight.org/optical_comms ... 3mile.html

The 173 mile sstv light long distance test very interesting.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Steve Anderson » Wed Mar 11, 2015 6:54 pm

Most of the designs on Chris's site will work at NBTV frequencies, the bandwidth was limited to about 300Hz-3kHz simply to get the best signal-to-noise ratio for telephone-quality speech. By opening the bandwidth up to say 10kHz you could expect the s/n ratio to worsen by 6-9db depending on the noise spectra.

But given a vastly shorter distance noise shouldn't be too much of a problem. I didn't have any problems with noise on my laser-link for NBTV up to 500 metres or so. What was a problem was atmospheric disturbances (scintillation) as I was using a laser-diode, not a LED/Luxeon as Chris did.

Steve A.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Wed Mar 11, 2015 11:04 pm

Steve Anderson wrote:Most of the designs on Chris's site will work at NBTV frequencies, the bandwidth was limited to about 300Hz-3kHz simply to get the best signal-to-noise ratio for telephone-quality speech. By opening the bandwidth up to say 10kHz you could expect the s/n ratio to worsen by 6-9db depending on the noise spectra.

But given a vastly shorter distance noise shouldn't be too much of a problem. I didn't have any problems with noise on my laser-link for NBTV up to 500 metres or so. What was a problem was atmospheric disturbances (scintillation) as I was using a laser-diode, not a LED/Luxeon as Chris did.

Steve A.


I was looking at your design as you know i tried the transmitter some time ago ,does the receiver just work on a Pwm signal ? its a very different design.

500 meters is pretty good range ,i did read on their site its better to use a luxeon ,i wonder if theres a point where ones better than the other sounds like the luxeon wins for long range .

I was thinking their light receivers must be better than our head amp versions due to its range ,i am curious as to what one of these would be like compared to our best head amp design .

The light sensor would be the heart of it ,i have tried my head amp with an LDR and solar cell the dome sensor which we now know is a photo diode and 3 photo transistors ,for me the photo transistor seems better it was a touch better than the dome sensor even with its larger surface area .

One thing i don't understand is why don't we use a bank of either photo diodes or what ever bit like in a nbtv monitor we want more light we use more than one luxeon .

May be its not as simple as just hooking up these together ? .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Thu Mar 12, 2015 11:24 am

Looking at all the optical receivers from the web site link posted earlier,this one has a wider bandwidth out of pin 1 U1.

I have none of the correct transistors or ics ,i'd have to substitute, thinking of using the NE5534 for the U1 U2,i have a few fets ,MPSA18 the bc 547 looks close to it .

I would like to do a test as in my easy sstv flying spot test to see the difference in head amps and sensors.

2N5484 and equivalent or cross reference yields the following results: on the FET
2N3819
2N5457
2N5486
2SK104
2SK168
2SK17
2SK37
2SK49
2SK55
BF410A/B
ECG312
FE5484
J305
J5103
K305-18
LS3819
LS5103
LS5246
LS5484
MPF102 (MPF-102)
TIS58
Attachments
ka7oei_optical_rx_ver310a.jpg
ka7oei_optical_rx_ver310a.jpg (81.46 KiB) Viewed 3595 times
Last edited by Harry Dalek on Thu Mar 12, 2015 1:03 pm, edited 1 time in total.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Steve Anderson » Thu Mar 12, 2015 11:43 am

Harry Dalek wrote:One thing i don't understand is why don't we use a bank of either photo diodes or what ever bit like in a nbtv monitor we want more light we use more than one luxeon . May be its not as simple as just hooking up these together ? .

One thing to remember about Chris's work is that he used large Fresnel lenses at both ends of the link, effectively multiplying the surface area of the emitter/detector perhaps thousands of times. Gain without noise or power consumption - nirvana!

There is no reason why you cannot use more than one sensor, you could use two photo-diodes in photo-current mode and get twice the detected current while the non-coherent noise only goes up by 3db, a better s/n ratio of +3db and twice the signal. You could continue this process but with each doubling of sensors the cost/performance ratio suffers.

There is also in the case of a NBTV camera getting all these sensors in one place, i.e. detecting the same small area of the image. Much like a CCD camera (digital still camera). Small pixels = good resolution but poor sensitivity, large pixels = the opposite.

Using Chris's designs may actually be worse, they are very good for what they were designed for but even in the most optically inefficient NBTV camera there's too much light and the pre-amps could overload and saturate. They could be 'dumbed down' I guess.

Steve A.

I have seen reference to the early pioneers using multiple photomultipliers with a flying-spot camera.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Thu Mar 12, 2015 11:54 am

Hi Steve just been out to the shed looking for parts.

I suppose in our case the design is not as critical in some ways due to the distances ,i was thinking more in using the pickup in flying spot scanning as it might be better than what i am using .

I could see i would really need either a larger surface area sensor or more than one of what i was using due to its pick up range .

I will give this one a go ,i will later convert sstv camera into a flying spot camera with a laser it would have a hell of a range from a quick test i did .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Thu Mar 12, 2015 12:09 pm

I forgot about the Fresnel lens sounds good i recall Gary telling me about them on my first camera go, i did have one of those large flat over head projector lens things might still have it some where ,I was thinking the parabolic dish would be perhaps as good .

Perhaps a little test with my photo transistor head amp and the one small mirrored dish out of a side projector might show what increasing the the surface area has to distance with and without thats an easy test .I will look into the focal point those are deep dishes so i know the focal point is close ,it would be the smallest parabolic dish i have ever worked on .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Steve Anderson » Fri Mar 13, 2015 11:35 am

Another issue with these pre-amps is the LF response. They were designed primarily for speech and at the low-frequency end are tailored to roll of at about 300Hz. That figure could be reduced such that they could be used for NBTV but for SSTV a response down to DC is virtually essential. It is possible to 'get away' with a LF roll-off at around 0.1Hz but in many ways it creates more problems than it solves.

The DC response problem was solved in SSTV by using the FM subcarrier system. The original AM system held the promise of providing the same, but use in poor signal conditions (the HF bands) fading was/is a problem.

Steve A.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Fri Mar 13, 2015 9:55 pm

Steve Anderson wrote:Another issue with these pre-amps is the LF response. They were designed primarily for speech and at the low-frequency end are tailored to roll of at about 300Hz. That figure could be reduced such that they could be used for NBTV but for SSTV a response down to DC is virtually essential. It is possible to 'get away' with a LF roll-off at around 0.1Hz but in many ways it creates more problems than it solves.

The DC response problem was solved in SSTV by using the FM subcarrier system. The original AM system held the promise of providing the same, but use in poor signal conditions (the HF bands) fading was/is a problem.

Steve A.


Well perhaps it is easier to stick with low noise wide bandwidth op amps bit like the optical circuit in the pdf below,i have been pretty happy with the simple Ne5534 could do with a second stage .

The idea of using extra sensors on the same circuit is something i have not tried and the passive light collection with lens reflectors and such .
Attachments
LT1797_1100_Mag.pdf
This one has a wide enough bandwidth
(306.53 KiB) Downloaded 186 times
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Looking at a better head amp ?

Postby Harry Dalek » Sat Mar 14, 2015 8:47 pm

At the moment i am also trying out the amplifier part of this receiver circuit ,does not really say what the band width of this is but looks to me at least good enough for SSTV ,i have the ics at least this time so this ones a bit easier .

The light detector head amp part of the circuit ,i'll give it a try but i have a head amp for it already just want to see how it works with the extra amplification .
Attachments
2009622225631459 (1).jpg
2009622225631459 (1).jpg (84.36 KiB) Viewed 3557 times
Misc Optical Stuff 3 (1).pdf
High quality version the 89 optical transmitter receiver part 1 and 2 plus extras
(2.02 MiB) Downloaded 176 times
Light Beam Communicator (2).pdf
just the transmitter receiver part low quality
(1.46 MiB) Downloaded 173 times
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Harry Dalek
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