[time-nuts] GPS receiver local oscillator (was: 1PPS for the beginner)

Attila Kinali attila at kinali.ch
Tue Aug 14 16:58:12 EDT 2018

On Tue, 14 Aug 2018 08:34:06 -0500
Dana Whitlow <k8yumdoober at gmail.com> wrote:

> I once read that the LO, at least, in some GPS receivers, was not even
> crystal
> controlled but was rather a ring oscillator based on a string of cascaded
> logic inverters on the chip.  This always sounded improbable to me, and i've
> long wondered it this claim was really true.  Can either of you shed light
> on
> this issue?

This wouldn't work. A simple ring oscillator is not stable enough
and has too much noise to demodulate the GPS signal successfully.
You need at least something of the class of an ceramic resonator
to come close to what you need for successful decoding of the GPS
signal. Most choose a XO simply because they are not much more
expensive and give quite a bit of boost in SNR. Timing GPS receivers
usually go for the TCXO because this allows for longer integration
times and thus higher SNR [1]. If you want to go for even longer
integration times, you have to go for OCXOs. There was a nice
master thesis by Pascal Olivier Gaggero 10 years ago on that topic[2].

What is true, though, is that a lot (most?) GPS receivers use a
ring oscillator as a local oscillator somewhere, either as RF LO source
or for the baseband processing. But this ring oscillator is always
locked to some XO/TCXO to improve its stability.

			Attila Kinali

[1] You can do longer integration times with normal XOs as well,
but then at some point the noise and (in)stability of the XO will
limit the maximum SNR you can get, which won't increase anymore
even if you increase the integration time.

[2] "Effect of oscillator instability on GNSS signal integration time",
by Pascal Olivier Gaggero, 2008
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