reciever

[76 warlock]

I received a jr racing 3ch fm receiver through a trade. It just happens to be the same exact freq. as my futaba 3ch fm transmitter, however it didn't seem to work. Should it? If not should I keep the crystals, cause I'll probably toss the receiver if they're not compatible.

[Charlie-III]

AM & FM really don't care the brand, only the freq.

[76 warlock]

That's what I was thinking, however I plugged in 2 servos and a battery directly to the receiver and no response. the freq. is the same between the two. channel 82-75.??? maybe the receiver is bad.

[Scottmisfits]

no, with the FM stuff you can use Hitec (only some) and Futaba together. JR will work with KO, Airtronics, some Hitec, and a couple of others. The receiver is probably just fine.

[Charlie-III]

Originally Posted by Scottmisfits

no, with the FM stuff you can use Hitec (only some) and Futaba together. JR will work with KO, Airtronics, some Hitec, and a couple of others. The receiver is probably just fine.

I've never had an issue, but I haven't tried every combination out there either.

[CreepyCrawl]

The channel number may be the same however the frequency shift is not, hence your incompatibility issue.

Simply explained, the transmitter sends the signal on either the positive or negative side of the frequency.

A more detailed explanation taken from another forum:

R/C equipment transmits the signal from the transmitter as a modulated RF signal. The two basic forms of modulation are called AM and FM. When you say AM and FM, people think of the AM and FM broadcast services. In reality, R/C AM and FM bear little relation to their broadcast cousins.

R/C AM is more like the amateur radio CW (continuous wave) code transmissions where the carrier is turned on and off. The receiver's detector responds to the presence and absence of an RF (radio frequency) signal, creating a high voltage when the RF signal is on and a low voltage when it is off. The servo position information is carried in the 'on' (or mark) portion of the signal, the 'off' portion of the signal is used as seperators between the control signals.

R/C FM is actually FSK (frequency shift keying) where the equivalent of the AM 'mark' is one frequency and its 'space' is another frequency. The transmitter leaves the RF carrier on at all times and simply shifts its frequency between the 'mark' and 'space' frequencies. In negative shift FM, the 'mark' frequency is lower than the 'space', in positive shift, its reversed. Just as the output of the AM receiver's detector is highs and lows corresponding to the RF signal's 'mark' or 'space' condition, the FM detector's output varies from high to low based on its definition of the relationship between the 'mark' and 'space' frequencies.

Remember the servo information is contained in the 'marks' - the spaces are simply there as seperators. If a negative shift transmitter is used with a positive shift receiver, the decoder thinks the 'spaces' are the 'marks' and vis versa, so it doesn't properly decode the signal.

In some receivers with microprocessor decoders, the decoder listens to the detector's output and figures out which condition (high or low) corresponds to 'mark' and 'space' and then adapts. For receivers that are switchable, there is either an inverter that can be inserted between the detector and the decoder or a flag input to the decoder that tells it to flip the polarity of the signal before decoding.

BTW, you can change the shift of a transmitter by adding an inverter between the encoder and the RF modulator.

[76 warlock]

Thanks everybody for clearing that up for me.