Can anybody work out the frequency of this pulse?

[Andrew]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

Is it 500Hz (period)?

Is it 100Khz (1/10us)?

Is it 50Khz ( 1/ (2xpulse width) )

My best guess is 50KHz because each pulse acts like half a sinusoidal
which, if complete, would have a period of 20us: 1/20us = 50KHz

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why??? It does works though.

I'm confused.

Thanks

Andrew

 

[petrus bitbyter]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

Is it 500Hz (period)?

Is it 100Khz (1/10us)?

Is it 50Khz ( 1/ (2xpulse width) )

My best guess is 50KHz because each pulse acts like half a sinusoidal
which, if complete, would have a period of 20us: 1/20us = 50KHz

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why??? It does works though.

I'm confused.

Thanks

Andrew

Andrew,

If you subtract the DC component, you have a difference of a 500Hz AC signal
left. So your tone decoder has been correctly set to that. If you change the
duty cycle of this signal the base frequency will stay the same although the
shares of the harmonics change.

petrus bitbyter

 

[John Fields]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

---
time on 2000
No. duty cycle = --------- = ------ = 0.995
period 2010

The period equates to a frequency of 500Hz (1/2000us)

1 1
No. frequency = ------- = -------- = 497.5 Hz
period 2010µs

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

---
Since the pattern repeats every 2010µs, 497.5 Hz.

Is it 500Hz (period)? ---
No

---
But each pulse _isn't_ half of a sinusoid, it's part of a signal which
repeats 497.5 times every second.
---

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why???

---
500kHz or 500Hz?

If it's 500kHz, then it's because there's enough harmonic content at
500kHz to trigger the LM567.
---

 

[Jim Backus]

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

I think another poster has answered your question by saying that there
are sufficient harmonics for the detector to work at 500 kHz.

It is well known that a square wave (1:1 duty cycle) only contains odd
harmonics. However if you have a rectangular waveform with a different
duty cycle the pattern of harmonics is different. Essentially there is
a null when the period of the on or off part of the waveform contains
a complete number of cycles. So your waveform wil have a null at 100
kHz, 200kHz etc due to the on period. It will also have nulls at 500
Hz, 1 kHz etc. As the period of the waveform is 2010 microseconds
there will be a peak at 497.5 Hz. I believe that the two dominant
frequencies will be 497.5 Hz and 50 kHz. It might be worth you hooking
the waveform up to a spectrum analyser. I've used a spectrum analyser
to set up exact duty cycles based on this principle.

 

[Andrew]

Petrus, John, Jim

Thank you all for your replies- very helpful.

I'll try setting up my filter to 497.5Hz as recommended. I'll also use
a spectrum analyser as you suggested Jim.

Thanks again

Andrew

 

[Luhan Monat]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

Is it 500Hz (period)?

Is it 100Khz (1/10us)?

Is it 50Khz ( 1/ (2xpulse width) )

My best guess is 50KHz because each pulse acts like half a sinusoidal
which, if complete, would have a period of 20us: 1/20us = 50KHz

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why??? It does works though.

I'm confused.

Thanks

Andrew

Sorry, you can'f filter this signal - its too wide. Maybe just a high
pass filter based on the lowest frequency component.

Luhan

 

[Luhan Monat]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

Is it 500Hz (period)?

Is it 100Khz (1/10us)?

Is it 50Khz ( 1/ (2xpulse width) )

My best guess is 50KHz because each pulse acts like half a sinusoidal
which, if complete, would have a period of 20us: 1/20us = 50KHz

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why??? It does works though.

I'm confused.

Thanks

Andrew

Hi,

More background on what you are trying to do would be helpful. Are
these just 3 data pulses? Is it a continuous set of pulses? Are you
trying to send data or just the presence of a signal?

 

[Pere Pala]

I'm designing an infrared receiver which is to receive the following
transmission:

Light wavelength: 880nm


__ ________ _________ _____
| | | | | |
|____| |____| |____|
|-10us-| |-10us-| |-10us-|

|-2000us-| |-2000us-|

The Duty cycle is 0.005

The period equates to a frequency of 500Hz (1/2000us)

What I need to know is: what centre frequency do I need to set the
filter to on the IR receiver?

Is it 500Hz (period)?

Is it 100Khz (1/10us)?

Is it 50Khz ( 1/ (2xpulse width) )

My best guess is 50KHz because each pulse acts like half a sinusoidal
which, if complete, would have a period of 20us: 1/20us = 50KHz

I have a circuit which detects this pulse using a LM567 Tone Decoder
set to a centre frequency of 500KHz! Why??? It does works though.

I'm confused.

Thanks

Andrew

Could it be that your data stream is actually modulating a 500KHz
subcarrier? This is usually done to avoid interference from other light
sources.