Carrier frequency

[sundar]

why do we choose 38Khz as a carrier frequency for transmission?
i have done a project in infrared remote controlled car , where i have used a t.v. remote and a tsop receiver which also used 38Khz as a carrier frequency.can anyone explain me?

 

[Nigel Goodwin]

Because that's the frequency the receiver IC's use.

It came about because it was derived from the earlier ultrasonic remotes, which worked around that frequency.

 

[sundar]

Because that's the frequency the receiver IC's use.

It came about because it was derived from the earlier ultrasonic remotes, which worked around that frequency.

can i transmit those signals other than 38Khz?

 

[Nigel Goodwin]

can i transmit those signals other than 38Khz?

To some extent, the IR receiver IC's come in various specified frequencies, but they aren't super-critical (a 38KHz IC works perfectly at 36KHz or 40KHz) - but why would you want to use something different?.

 

[MikeMl]

can i transmit those signals other than 38Khz?

Read this data sheet. I think the frequency selectivity of these receivers is much narrower than Nigel thinks.

 

[3v0]

Depends. The last time I ordered TOSP's they had narrow band ones as expected but they also hand one that would cover all of the bands. A sort of one shoe fits all ditty. But then again I could have been dreaming.

EDIT: I feel like I am follow a few people around posting contradictions. It is not my intent to be negative.

 

[Nigel Goodwin]

Read this data sheet. I think the frequency selectivity of these receivers is much narrower than Nigel thinks.

That datasheet makes no such suggestion?, as far as I'm aware the BPF in the chips is fairly wide and flat - the datasheet doesn't even mention it at all.

 

[crutschow]

That datasheet makes no such suggestion?, as far as I'm aware the BPF in the chips is fairly wide and flat - the datasheet doesn't even mention it at all.

The datasheet shows models that differ by 2kHz in carrier center frequency or less, implying that the selectivity is probably better than that.

 

[Nigel Goodwin]

The datasheet shows models that differ by 2kHz in carrier center frequency or less, implying that the selectivity is probably better than that.

Yet no where does it suggest that - you're just making it up

Try them and see!.

 

[crutschow]

Yet no where does it suggest that - you're just making it up

Try them and see!.

If they have a wide bandwidth, then why make models that differ by only 2kHz in center frequency?

 

[Nigel Goodwin]

If they have a wide bandwidth, then why make models that differ by only 2kHz in center frequency?

I've always wondered that myself - like I said, try them and see - in my experience it makes not the slightest bit of difference, even at well over normal remote control ranges. I presume you will lose ultimate range by not accurately following the specified frequency?, but should you be trying to use IR at those distances?.

 

[crutschow]

I've always wondered that myself - like I said, try them and see - in my experience it makes not the slightest bit of difference, even at well over normal remote control ranges. I presume you will lose ultimate range by not accurately following the specified frequency?, but should you be trying to use IR at those distances?.

So it would appear that the units have different BP center frequencies, but the filter rolloff is not very steep, thus they will still operate well at other frequencies. It may be as you say, it only affects the maximum operating range slightly.

I don't have one to try, but it would make an interesting experiment.

 

[Nigel Goodwin]

So it would appear that the units have different BP center frequencies, but the filter rolloff is not very steep, thus they will still operate well at other frequencies. It may be as you say, it only affects the maximum operating range slightly.

I don't have one to try, but it would make an interesting experiment.

I've always only ever stocked one specific type as a replacement, and used it in all manner of different TV's - it's always worked perfectly.

I've also used them with microcontollers, and certainly varied the frequency about considerably with no ill effects - I might give it a try one day, see how far off frequency you have to go .

 

[bill190]

Vishay makes IR recievers with different carrier frequencies. Like these...

30 kHz -TSOP34830
33 kHz -TSOP34833
36 kHz -TSOP34836
38 kHz -TSOP34838
40 kHz -TSOP34840
56 kHz -TSOP34856

Then they make a few with a wide range which could be used as an IR repeater like this one...

30 kHz to 50 kHz -TSOP98238

These are sold at Mouser.com

 

[Nigel Goodwin]

Vishay makes IR recievers with different carrier frequencies. Like these...

30 kHz -TSOP34830
33 kHz -TSOP34833
36 kHz -TSOP34836
38 kHz -TSOP34838
40 kHz -TSOP34840
56 kHz -TSOP34856

Then they make a few with a wide range which could be used as an IR repeater like this one...

30 kHz to 50 kHz -TSOP98238

These are sold at Mouser.com

We know that, but how tight is the bandwidth - in my experience not tight at all.

 

[bill190]

A 38.0 kHz IR receiver I was working with would work if the carrier frequency was in the neighborhood of 37500 Hz to 38500 Hz as I recall.

And would work better from 37800 Hz to 38200 Hz.

But would not work at 37 kHz or 39 kHz.

 

[Nigel Goodwin]

A 38.0 kHz IR receiver I was working with would work if the carrier frequency was in the neighborhood of 37500 Hz to 38500 Hz as I recall.

And would work better from 37800 Hz to 38200 Hz.

But would not work at 37 kHz or 39 kHz.

That's certainly VERY different to what I've found over the years - when I get both eyes back I'll have to do some experimenting