[time-nuts] SE880 GPSDO

Attila Kinali attila at kinali.ch
Thu Apr 28 10:28:58 UTC 2016


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 :-)

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
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson



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