[Hackrf-dev] short-term frequency stability requirements for the clockIN

[schneider]

Hi,

On 27.03.21 21:43, Kristoff wrote:
> I bought a 10 MHz GPSDO from dxpatrol, but that one seems to create a
> sine-wave instead of a square-wave so I cannot use that one for the hackRF.

I've recently also became interested in GPSDOs and bought one from
dxpatrol. Mine is outputting a square wave though.

If you open up your GPSDO you will find that there is a discrete low
pass filter at the output of the OCXO (assuming that they're always
using a square wave OCXO). If you bridge that filter you will get a
square wave output. This is in fact the state of mine when it was delivered.

That said I'm not super impressed by the build quality or the way it
operates. There is no positive indication of GPS lock and the control
loop is also the most simple one I can imagine. Also: the internal ublox
module is a clone.


> I also have two ublox neo8 receiver-modules that do are able to generate
> a 10 MHz square-wave, .. but the signal seams to be very strange: 2
> longer pulses followed by 3 shorter pulses. Also the time between the
> pulses seems to very, so the jitter is very very bad.

That is expected and why you need a very slow PLL behind it. I doubt the
loop filter of the HackRF clock chip is going to like a signal directly
from the ublox receiver.

Best
schneider