# Missing modulation in IR signal?

I have a cheap IR remote control from Ebay (like this one) and I´m straggling with the modulation of the IR signal. I use an BPV11F photo transistor as receiver and I get this signal with my scope

But this signal looks very identical to the signal which comes from my TSOP38238:

So I would expect to see some kind of modulation (the 38 kHz carrier signal) when I receive this signal over the photo transistor, like in this image.

My guess is that this cheap remote doesn´t use a modulation (is this possible?). Or do I have some misunderstanding?

• do you own a TV with its own remote control? – jsotola Sep 9 '20 at 6:15
• Yeah. I can try it with this. Good hint. Thank you – Kampi Sep 9 '20 at 6:17
• All my cheap remotes work happily. You might like to see their modulation waveforms: (1) "Rpi3 LIRC Library and UART IR Transceiver Setup Problem": raspberrypi.stackexchange.com/questions/103452/…. Cheers. – tlfong01 Sep 9 '20 at 6:53
• Look at the top waveform : you see a very fast turn-on (falling edge) and a very slow turn-off (rising edge). That says your phototransistor is turning off very slowly : you haven't shown its schematic but I'm guessing the pull-up resistor is far too high to respond fast enough to show the 38kHz carrier. Try a 1K to 10K pullup. – user_1818839 Sep 9 '20 at 11:31
• @BrianDrummond thank you for the tip. Changing the collector resistor to 1k "solves" my problem. – Kampi Sep 10 '20 at 6:23

A phototransistor load resistor $$\R_L\$$ should be chosen small enough so that 40kHz is within the passband. A larger-value load resistor limits frequency response so that carrier frequency is not seen. That BPV11F phototransistor data sheet says that you should get 100kHz bandwidth with a 100 ohm $$\R_L\$$, but that low value might give you a rather insensitive detector.
In addition, when probing $$\R_L\$$ with an oscilloscope probe, you should choose X10 attenuator setting, rather than X1 attenuator. The additional capacitance of X1 probe in parallel with $$\R_L\$$ will reduce frequency response even more.