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I'm trying to read the ETSI standard regarding my 868MHz transmitter design.

I would like to transmit in one of the channelized bands ex. (868.600 to 868.700), which has a demand on 25KHz channel spacing. This sets a huge demand on the frequency error of the system. See table 5 of EN300 220-1.

A crystal of +/-10ppm would result in frequency error of +/- 8680Hz at 868MHz. So this does not leave much space in the channel for modulation of the carrier, and sets high demands on the modulation not spreading into the adjacent channels.

So, I was looking into high precision crystals, like +/-5ppm but these seems expensive, around 3USD which is more than my transmitter IC.

Does there exist any low cost, but high precision 10-12MHz oscillators that can be used for this purpose ?

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  • \$\begingroup\$ In table 5 it is also stated that the whole frequency band may be used as 1 wideband channel for high speed data, but I cannot find a definition of high speed data. We use 2400bps FSK, is that high speed ? \$\endgroup\$ – JakobJ Mar 7 '17 at 13:58
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These frequency bands are intended for OOK modulated signals, see: https://en.wikipedia.org/wiki/On-off_keying

Basically the RF signal is 100% AM modulated (on or off) in a certain sequence giving a certain code. This code is then used to identify which command is transmitted.

The OOK modulation does not mean that the used frequency space is more than 25 kHz, it is simply on or off at a frequency in that 25 kHz space.

Receivers for OOK signals are generally wideband (much wider than that 25 kHz), the actual separation of information (is this signal for me ?) is done on the transmitted code. So all codes are recived and a microcontroller checks if it matches any code the receiver needs to respond to.

This means that you do not need to worry about that 25 kHz spacing as long as you stay within the assigned frequency band.

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  • \$\begingroup\$ Ok, that seems more easy, so when we use 2-FSK 2400bps, we just need to stay within the defined frequency band. Where did you find this information, just so I know where to look ? \$\endgroup\$ – JakobJ Mar 7 '17 at 14:42
  • \$\begingroup\$ You're talking about FSK, I was not ! But I see no reason why you could not do 2400 bps with OOK as well. This information is just from my "general knowledge". I messed around with 433 and 868 MHz garage door remotes and figured out how they work. And I work on transceivers (at much, much higher frequencies) as a day job ;-) \$\endgroup\$ – Bimpelrekkie Mar 7 '17 at 14:47

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