Well, that's not a good basis to start...
These little IR recievers contain a big portion of electronics inside, automatic gain control (AGC), bandpass filtering, comparator, etc. Not knowing at what carrier frequency your reciever modul is tuned, means, that you can totally fail! Bandpass filtering is so sharp, that erroneously working at the wrong carrier frequency can cause a relevant loss of signal, just what you observed. If you combine 38kHz emitter with 40kHz reciever, this will make you loose about 2dB of signal. But when combining 38kHz emitter with 56kHz reciever, you will end up with a totally unsufficient signal level...




Unfortunately performance of this reciever is more sophisticated, then that unwanted noise will simply appear 1:1 at output. You can have noise totally degrading performance of AGC unit, without being able to notice anything by having a look at the output! Nevertheless, the circuit doesn't work properly and you will hardly find the reason.

Don't underestimate the noise that is emitted by microcontroller and its crystal! So, this shielding Russell recommended is extremely helpful, especially if you do not use proper printed circuit board design...




I would also use a power supply filter at reciever, as shown in the link Michael gave: 100Ohm resistor and 10µ/16V Tantal will be very helpful. Don't forget, that inside the reciever modul the signal coming from photo diode is very much amplified! And if there's any unproper shieding or noise on Vcc or ground noise between reciever and micro then the internal high gain amplifier section will not work adequately...

Kai