IR Receiver/Demodulator [Design]

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From: John Larkin on 17 Mar 2010 16:59

On Wed, 17 Mar 2010 12:35:09 -0500, "eeboy"
<jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote:

>
>>3. Dump the photodiode current into a tuned LC tank and amplify that
>>with a very low-noise device, probably a jfet. The 4069 will be about
>>the noisiest and least gain-predictable amplifier you can buy for a
>>reasonable amount of money. An LC to ground allows a lot of ambient
>>light signal to get dumped without saturating any amplifier stages.
>>
>
>This sounds interesting to me. So, as I understand it with ambient light
>(DC) I'll have a low impedance path to ground. However, when I am wiggling
>close to the point of resonance I'll begin to see a voltage develop across
>the LC tank as the impedance increases. Correct?

Yes. The bandpass response of the LC reduces the system noise
bandwidth. But optical bandpass filtering will help a lot, as
background light will make wideband shot noise, some of which will
sneak through the LC with the signal.

>
>How is the Q of the tuned circuit adjusted?

Usually, for low Q situations, with a shunt resistor. You could
reasonably have an LC with a native Q of 100 maybe, and kill that down
to 10 or 20 maybe with a resistor.

How much capacitance does your photodiode have at its operating bias?

John

From: John Larkin on 17 Mar 2010 17:01

On Wed, 17 Mar 2010 11:18:07 -0700, Joerg <invalid(a)invalid.invalid>
wrote:

>eeboy wrote:
>>>> This sounds interesting to me. So, as I understand it with ambient light
>>>> (DC) I'll have a low impedance path to ground. However, when I am
>> wiggling
>>>> close to the point of resonance I'll begin to see a voltage develop
>> across
>>>> the LC tank as the impedance increases. Correct?
>>>>
>>>> How is the Q of the tuned circuit adjusted?
>>>>
>>
>>> How much can your 32kHz vary? Modulation rate (pulse widths that need
>>> to be discerned)?
>>>
>>
>> I wasn't exactly sure what you were specifically asking so hopefully the
>> answer is in here somewhere... :)
>>
>> First off, I am actually free to choose any carrier frequency I want. I
>> chose 32kHz because it was lower than what most of the consumer remotes use
>> (36-40kHz from what I can tell). Presumably that will make life easier on
>> me. The transmitter is controlled by a dirt cheap microcontroller which
>> utilizes a 8MHz RC oscillator as a time base. It's fairly easy to adjust
>> the transmitter carrier. Given that it is RC based the timing can vary by a
>> few percent with temperature. I've been doing some tests (heating
>> up/freezing the transmitter) and can say that at the extremes I am within
>> 3% of the base (room temperature). There will also be a variation from
>> device to device of course. Although my testing of the variation is not
>> complete, given the above, I'd like to have a 2kHz window or so.
>>
>
>2kHz may be a bit much for this application. Can't you use a watch
>crystal to get the uC stable? Modern ones have internal digital loops to
>give you 8MHz or thereabouts of clock.
>
>Then use a watch crystal on the receive end as filter. Cheap, but may
>not work well in really frosty temps. And the crystals have to be very
>accurate because the bandwidth will be in the order of 10-20Hz. Anyhow,
>just as an idea to play with in case all else fails.

A crystal filter in the receive path will have too much Q and not let
his pulses through.

John

From: eeboy on 17 Mar 2010 17:15

>
>How much capacitance does your photodiode have at its operating bias?
>

The terminal capacitance is stated as 70pF with the conditions of Vr=0V and
f=1MHz. Obviously neither of those conditions apply. There really is not
much to the datasheet (PNZ323B)
http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+SC+4+BFA7002+PNZ323B+8+WW
..

---------------------------------------
Posted through http://www.Electronics-Related.com

From: Joerg on 17 Mar 2010 17:23

John Larkin wrote:
> On Wed, 17 Mar 2010 11:18:07 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>> eeboy wrote:
>>>>> This sounds interesting to me. So, as I understand it with ambient light
>>>>> (DC) I'll have a low impedance path to ground. However, when I am
>>> wiggling
>>>>> close to the point of resonance I'll begin to see a voltage develop
>>> across
>>>>> the LC tank as the impedance increases. Correct?
>>>>>
>>>>> How is the Q of the tuned circuit adjusted?
>>>>>
>>>> How much can your 32kHz vary? Modulation rate (pulse widths that need
>>>> to be discerned)?
>>>>
>>> I wasn't exactly sure what you were specifically asking so hopefully the
>>> answer is in here somewhere... :)
>>>
>>> First off, I am actually free to choose any carrier frequency I want. I
>>> chose 32kHz because it was lower than what most of the consumer remotes use
>>> (36-40kHz from what I can tell). Presumably that will make life easier on
>>> me. The transmitter is controlled by a dirt cheap microcontroller which
>>> utilizes a 8MHz RC oscillator as a time base. It's fairly easy to adjust
>>> the transmitter carrier. Given that it is RC based the timing can vary by a
>>> few percent with temperature. I've been doing some tests (heating
>>> up/freezing the transmitter) and can say that at the extremes I am within
>>> 3% of the base (room temperature). There will also be a variation from
>>> device to device of course. Although my testing of the variation is not
>>> complete, given the above, I'd like to have a 2kHz window or so.
>>>
>> 2kHz may be a bit much for this application. Can't you use a watch
>> crystal to get the uC stable? Modern ones have internal digital loops to
>> give you 8MHz or thereabouts of clock.
>>
>> Then use a watch crystal on the receive end as filter. Cheap, but may
>> not work well in really frosty temps. And the crystals have to be very
>> accurate because the bandwidth will be in the order of 10-20Hz. Anyhow,
>> just as an idea to play with in case all else fails.
>
> A crystal filter in the receive path will have too much Q and not let
> his pulses through.
>

Yeah, true, 20-80msec could be a tight squeeze. But LC can also be
pretty steep and stable there at 30-some kHz.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

From: mpm on 17 Mar 2010 18:30

On Mar 17, 11:24 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 17 Mar 2010 05:58:20 -0500, "eeboy"
>
>
>
>
>
> <jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote:
> >I need to design an IR receiver which will demodulate IR input at a given
> >carrier frequency (~32kHz) over distances of 60-80 feet. I am not
> >transmitting data, simply pulses of varying length (20ms-80ms). These
> >pulses occur at the rate of approximately 1Hz. It is important that the
> >pulse length is preserved from transmitter to receiver. In other words, if
> >I send a 20ms pulse I expect it to be within 5% on the receiving side. The
> >sensor will be used outdoors so it must discriminate against ambient IR and
> >other sources (street lights?). I'd like to have a fairly high Q such that
> >'normal household remotes' operating around 38kHz are significantly
> >attenuated. Finally, given the device is battery powered (5V regulated) it
> >should consume little power (sub 1mA if possible)
>
> >I've tried using a simple IR remote receiver module (Vishay PN
> >TSOP85238TR). They work great indoors (~50') but are lousy outside (~8')..
>
> >Now, I am not an analog guru which is why I am here to seek guidance. My
> >initial thoughts were to amplify and filter (bandpass) using op-amps.
> >However, I started googling to see what I could find on the subject and
> >nearly everything I found was contrary to my initial thoughts. So,
> >obviously I am not thinking correctly! :) However, the types of circuits I
> >was finding are quite puzzling to me. For example, take this circuit:
> >http://www.discovercircuits.com/PDF-FILES/4069rvr.pdf. Seems simple (based
> >on component count) although I don't quite understand how the center
> >frequency is set using the inductor and resistor. But why CD4069UB? I've
> >never seen them used as linear amplifiers before. And 2H isn't really that
> >practical eh?
>
> It's a bizarre circuit.
>
> Some things that would help you:
>
> 1. Maximize optical path gain, which means using lenses or equivalant
> to focus the transmitter and receiver at one another and exclude as
> much ambient light as possible from entering the receiver.
>
> 2. Put a narrowband optical filter in front of the receiver to pass
> the optical signal and reject wideband ambient light.
>
> 3. Dump the photodiode current into a tuned LC tank and amplify that
> with a very low-noise device, probably a jfet. The 4069 will be about
> the noisiest and least gain-predictable amplifier you can buy for a
> reasonable amount of money. An LC to ground allows a lot of ambient
> light signal to get dumped without saturating any amplifier stages.
>
> 4. Increase transmit power.
>
> 5. Buy Phil Hobbs' book
>
> The problem with using a high-Q resonator is making sure it's on
> frequency, and not over-doing Q to the point that the data rate is
> compromised.
>
> What's the application?
>
> John- Hide quoted text -
>
> - Show quoted text -

#2 was the first thing I thought about too.

Mainly, because earlier today, I purchased a new camera lens (a 500mm
mirrror), and I always make it a point to grab a couple Roscolux
swatchbooks while I'm at it.
Such as this one from BHPhoto:
http://www.bhphotovideo.com/c/product/45189-REG/Rosco_950SBLUX0103_Roscolux_Swatchbook.html

Hard to beat at $1.95 each.
I wonder if there's a gel in there that will do the job?