[sci.electronics] Sensitive Receivers

tom@syssoft.com (Rodentia) (04/04/91)

Greetings:

    I have been experimenting with an IR receiver I modified
from an article.  It is a two stage amplifier, meant to get high
gain, but I am trying to receive square waves, and it does not
appear to be sufficient for my needs.

    At this time, I am sending a 25KHz 25% duty cycle square wave
(at least that's how it looks on the transmitter).  On the reciever, 
it is very smoothed out, and resembles a sine wave of 50KHz.  I 
suppose this is due to the harmonics present in the assymetrical
waveform.

    Adjusting the gain is very touchy.  There is little latitude 
from no signal to "chaos to the rails".  In addition, it is very
sensitive to distance and alignment (when tuned for poor reception,
if I point it directly, it goes wild).

    My first goal is to clean up the received wave.  It can be
adjusted such that the beginning of each wave form generates a 
scopeable spike (at 25KHz, not the 50KHz harmonic) and this
differential signal seems that it may be enough to reconstruct
a good approximation of the transmitted wave.  A similar spike
on the falling edge may be there, and it would be a plus to use
that information as well, but it is not necessary.  I can assume
the pulse width.

    My second goal is to get the bloody 60Hz and 120Hz out of there.
This is complicated by the fact that these sources of interference
(flourescent lights, televisions) may have fast rise and fall times
fooling the circuitry I am considering to reconstruct the wave.

    One bit of information I don't want to lose is the pulse 
position (or perhaps frequency).  I am attempting to transmit
analog signals, but I am pushing the transmitter to the rails, 
so all of my modulation is in the time domain.  This would be
interesting to try on digital, but just using two tone detectors
won't cut it, I need continuous values.  Maybe I could threshhold
them after this stage for digital reception.

    If I can't generically reconstruct received waves (through
some sort of automatic gain control or edge detection), I am 
willing to try FM reception.  Unfortunately, I know even less
about this, but it does seem that good sensitivity is possible.
FM chips I have seen do not go down to KiloHertz carries frequencies
so I'd have to roll my own.

    It seems that this may be the kind of problem HAMs might
have (though with a twist), so I'm hoping that there are some
suggestions from that sector.  References are good.  Examples, 
discussions, tutorials are even better.  Please copy me by
e-mail on any followups.  I DO read these groups, but news has
been so constipated out here, your responses may get expired
before I receive them.

    Thank you for your patience.  The schematic is included below
for your information (heck, you may want to build your own; it can
test your remote controls if nothing else).


   Vcc
    |
    P1         +----P2--+
    |          |        |     +----P3--+
    +------C1--+->I-\   |     |        |
    |           :OA1 O>-+-R3--+->I-\   |
   PhT         +->I+/          :OA2 O>-+----C2--+
    |          |              +->I+/            |
    |          |              |                 |
    |   Vcc    +--------+-----+                 P4---JAK
    |    |              |                       |
    |    +----------R1--+-R2--+                 |
    |    |                    |                 |
    |    C3                   |                 |
    |    |                    |                 |
   GND  GND                  GND               GND

Legend:
   Vcc - 9V to 12V
   GND - Ground
   PhT - PhotoTransister (IR sensitive)
   C1  - Capacitor (.001 microFarad) (.01 was also tried)
   C2  - Capacitor (.01 microFarad)
   C3  - Capacitor (220 microFarad)
   R1  - Resistor (10KOhm)
   R2  - Resistor (10KOhm)
   R3  - Resistor (1KOhm)
   P1  - Potentiometer (1MOhm, center tap wired to one end for log scale)
   P2  - Potentiometer (1MOhm, center tap wired to one end for log scale)
   P3  - Potentiometer (1MOhm, center tap wired to one end for log scale)
   P4  - Potentiometer (10KOhm, to ground with center tap to jack)
   OA1 - Op Amp 1 (1/2 353) (I- is - input, I+ is + input, O is output)
   OA2 - Op Amp 2 (1/2 353)
-- 
Thomas Roden                                      | tom@syssoft.com
Systems and Software, Inc.                        | Voice: (714) 833-1700 x454 
"If the Beagle had sailed here, Darwin would have | FAX:   (714) 833-1900
come up with a different theory altogether." - me |

yjj@ctr.columbia.edu (Yuan Jiang) (04/07/91)

In article <1991Apr3.211135.21492@syssoft.com> tom@syssoft.com (Rodentia) writes:
>Greetings:
>
>    At this time, I am sending a 25KHz 25% duty cycle square wave
>(at least that's how it looks on the transmitter).  On the reciever, 
>it is very smoothed out, and resembles a sine wave of 50KHz.  I 
>suppose this is due to the harmonics present in the assymetrical
>waveform.
>
Remeber sine wave has negative cycles.  [sin(wt)]**2 is a
2w sine wave plus dc.

>    My second goal is to get the bloody 60Hz and 120Hz out of there.
>This is complicated by the fact that these sources of interference

Will a filter help?


>   PhT - PhotoTransister (IR sensitive)

They are known for being noisy and having low responsivity.
A PIN photodiode costs several dollars.

Good luck

tom@syssoft.com (Rodentia) (04/10/91)

In article <1991Apr6.203416.14689@ctr.columbia.edu> yjj@ctr.columbia.edu (Yuan Jiang) writes:
>In article <1991Apr3.211135.21492@syssoft.com> tom@syssoft.com (Rodentia) writes:
>>Greetings:
>>
>>resembles a sine wave of 50KHz.  I 
>>suppose this is due to the harmonics present in the assymetrical
>>waveform.
>>
>Remeber sine wave has negative cycles.  [sin(wt)]**2 is a
>2w sine wave plus dc.
>
   I understand that there is biasing involved, but the receiver is
AC coupled.  Where does the squaring of the wave come into play?  The
asymmetry to which I was referring was in the duty cycle.

>>    My second goal is to get the bloody 60Hz and 120Hz out of there.
>>This is complicated by the fact that these sources of interference
>
>Will a filter help?
>
   I imagine so, any suggestions as to what stage would be the 
filter?  Is making one of the op amps an active filter best?
>
>>   PhT - PhotoTransister (IR sensitive)
>
>They are known for being noisy and having low responsivity.
>A PIN photodiode costs several dollars.
>
   I wanted to be cheap, but I will look into decent parts.

>Good luck
   Thanks.

   Update:  I changed to a 1458 op amp (from a 353 BIFET).  This
is much more stable at high gains.  It still seems to be sensitive
to the volume control on the Radio Shack audio amp I connect.  Any
suggestions on how to match this to my circuit?  Thanks all.


-- 
Thomas Roden                                      | tom@syssoft.com
Systems and Software, Inc.                        | Voice: (714) 833-1700 x454 
"If the Beagle had sailed here, Darwin would have | FAX:   (714) 833-1900
come up with a different theory altogether." - me |

yjj@ctr.columbia.edu (Yuan Jiang) (04/14/91)

In article <1991Apr10.040631.15699@syssoft.com> tom@ssi.UUCP (Rodentia) writes:
>In article <1991Apr6.203416.14689@ctr.columbia.edu> yjj@ctr.columbia.edu (Yuan Jiang) writes:
>>In article <1991Apr3.211135.21492@syssoft.com> tom@syssoft.com (Rodentia) writes:
>>>Greetings:
>>>
>>>resembles a sine wave of 50KHz.  I 
>>>suppose this is due to the harmonics present in the assymetrical
>>>waveform.
>>>
>>Remeber sine wave has negative cycles.  [sin(wt)]**2 is a
>>2w sine wave plus dc.
>>
>   I understand that there is biasing involved, but the receiver is
>AC coupled.  Where does the squaring of the wave come into play?  The
>asymmetry to which I was referring was in the duty cycle.

I don't remember whether you specified what kind of light source you
used.  The squaring applies to lamps.  If you are using a LED or laser
and operating in the linear region, then I don't have any clue where
the 2w frequency is from.

>
>>>    My second goal is to get the bloody 60Hz and 120Hz out of there.
>>>This is complicated by the fact that these sources of interference
>>
>>Will a filter help?
>>
>   I imagine so, any suggestions as to what stage would be the 
>filter?  Is making one of the op amps an active filter best?

Depending on where this line frequency comes in.  I guess it's from
the light source since 2w(120Hz) is there.  Filter the DC power
supply.  If possible, seal everything is a metal box.
>>