Transmitters for POCSAG Transmitting datas with POCSAG-protocol via a RF-Transmitter in FSK is not as easy as often thought to be. The following specifications the transmitter must fullfill: 1.) High frequency stability The reason is that all pagers have one feature together: bad receiption antennas! For achieving high sensitivity the receivers need to have narrow filters. This causes high frequency accuracy and stability at the transmitting side. Just keep in mind that 10ppm are 4.4kHz@440MHz 2.) Modulation nearly downto DC Due to the reason that the POCSAG-protocol is not DC-free which means that several '1's or '0's may follow after each other the transmitter must be able to be modulated downto very low frequencies. For a normal XTAL- Transmitter beeing FSKed via a varicap this is no problem at all. For a PLL-Transmitter this is not as trivial as above because if you modulate into the loop (as often proposed in modification instructions for normal FM-Transceivers in PacketRadio) the PLL will recognize that you want to detune the frequency so it will start regulating out this detuning. As you'll probably can imagine the digital one's and zero's will get distorted due to this regulation process. There are several ways for overcoming this problem: One possibility is open-loop-modulation but this will not work at POCSAG because you will not get a VCO so stable during one transmission cycle as it is necessary. At DECT-telephones i.e., this is a common used technique because the timeslots are rather short so the frequencydrift can also be minimized at good circuit designs. Another possibility is closed-loop- modulation and herefore there are several ways to realize that: Imagine, you modulate also the reference frequency in the same phase like the VCO. The PLL will not detect any frequency/phase-changes so the loop cannot regulate out your modulation. That's quite tricky, isn't it? What you have to do is splitting your modulation signal into two paths: The higher frequencies modulate directly your VCO. The lower frequencies modulate your reference-XTAL of the PLL. You have to adjust both modulation levels that the transmitted signal a) has the correct deviation (+/- 4kHz) b) is not regulated out too much. For checking this, one very simple lowcost technique is modulating your transmitter with a rectangular signal (frequency generator, i.e. made with NE555, DutyCycle 50:50) and viewing the outputsignal at another FSK-Receiver with the oscilloscope. Adjust the modulationlevel at low modulationfrequency for VCO and PLL now that the signal at the scope looks like "digital" :-) If the modulationlevel of the PLL is not correct, you will not see your modulation rectangular signal but something strange which is very difficult for digital logic to demodulate. If you own better measuring equipment you can do these adjustment with a SpectrumAnalyzer (ZeroSpan+fast sweeptime) or much better with a modulation-domain-analyzer. Or for best optimization modulate with a PN-Sequence (PseudoRandomNoise) and view the signal with an oscilloscope which is capable to set up a long persistancetime so you can view the eyediagram very comfortable. A very good overview+description how to modulate downto DC is made by RFMD (RF-MicroDevices). Transmitters tested for POCSAG: 1.) The famous XTAL-Transmitter KNET or TEKK is working nice at POCSAG. It is very widely spread over whole Europe for PacketRadio-usage so with this XTAL you will not have any problems if you take the following items into consideration: a) use only good XTALs with low ppm's. It is better to pay 5 Dollars more and having a stable system than spending too much time in readjustings which will cost you in sum more than the XTAL. b) Disable the built-in PA of the TEKK/KNET. The reason is that this little PA with this little cooling is underdimensionated for a continous transmission as it will often occur at Digipeaters and POCSAG-Stations. First, there is the danger that the PA will be blown up by heat but there is another more severe problem. Due to heating up the case the oscillator will also be heat up. This will result in an undesired frequency drift of your transmitter... It is better to disable the internal PA and connect an external PA to the driverstage. Now, you are able to produce much more power than the QRP 2W or 5W which the KNET/TEKK is capable to do by default and you removed the heating-up problem. 2.) Supposed that nearly every pure XTAL-Transmitter will work at POCSAG if it is constructed like 1.) We are just trying to modify a commercial POCSAG-TX which surely will work nice POCSAG. What's interesting to see is that also this commercial TX is only a one-channel highstable XTAL-TX - so no PLL is used. 3.) Commercial HAM-Radio FM mobile transceivers "adverted to be usable for 9k6" often do not work. Very often, only the VCO is modulated directly - so this is a closed-loop-modulation into the loop and the PLL will try to regulate out your modulation. At 9k6-PacketRadio this seems to work - but if you view your signal like mentioned above with an oscilloscope you will be appaled how the signal is looking like. It is working...but not very good and you will have more fun with PacketRadio and will speed up your data-transfer-rates if you bear the explained details above in mind.