[time-nuts] SE880 GPSDO

Ilia Platone info at iliaplatone.com
Thu Apr 28 19:01:49 UTC 2016



Il 28/04/2016 12:28, Attila Kinali ha scritto:
> On Thu, 28 Apr 2016 10:07:56 +0200
> Ilia Platone <info at iliaplatone.com> wrote:
>
>> Please note that not all the frequencies will be utilizable, here only
>> 433MHz is free-for-all and at low power: only under 50mA transmitting power.
> Does this mean you don't have an amateur radio license?
> Then it would be a good time to get one :-)
:) yes, maybe...
> BTW: you also have the 868MHz SRD and the 2.4GHz range available,
> with similar power constraints though.
>
>> There are some very cheap FSK transmitters that can output at a maximum
>> rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO
>> like the 10KHz output of some GPS receivers? The clock being compared to
>> this would be 10MHz downscaled by some decade counters.
>>
>> this would be much simpler to implement.
> I don't get exactly what you mean, but these FSK transmitters for sub-GHz
> radios will not work. Their only purpose is low power radio transmissions
> over a couple of 10m. Yes, you can use them to send data. Yes, you can
> lock to that. But you will not get the control over phase/frequency you
> need to implement a good frequency transfer or time transfer system.
>
> Building your own 70cm is not difficult. There are plenty of amateur
> radio books from the 70s and 80s  around that explain how to build
> one with minimal effort. The QRP community has also quite a few designs
> that are very simple to build (and a bit more modern).
>
> All you actually need for a CW transmitter is some oscillator, ie a VCO,
> that you can either build yourself (L-C tank with a varactor)
> or buy as a chip. Use some simple PLL to lock it to your reference
> (ADF4002 or similar are a decent choice). Add a simple amplifier
> to get the signal to a decent level and a piece of wire as antenna.
>
> The receiver is a bit more involved. There you need to downmix
> the received signal to a frequency that is not higher than your
> OCXO's frequency, feed that to an amplifier and from there to
> a PLL that steers the OCXO. The downmixing LO needs to be derived
> from the OCXO as well.
>
> The PLL of the receiver can be an analog design as with the transmitter
> or a digital design where you digitze the signal and process it in
> an FPGA that produces an output for a DAC that then steers the OCXO.
>
> But all this depends on quite a bit of knowledge on how to design
> analog circuits. If you have never done that, it would be a good
> idea to find a companion where you live that helps you with the
> project.
>
> 			Attila Kinali
Thanks Attila, I know how to build a transmitter and a receiver, and now 
is more clear the system you designed. But as I will propose this system 
to an astro club, and in this astro club there's the possibility that 
not all would have a radio license, I need something "free-to-play", if 
it concern.
I was wondering if it would be more convenient to lock to a signal from 
an AM broadcasting station, if available to a multiple of the OCXO. What 
do you think about?

-- 
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999




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