Hadn't appreciated that was what it was doing until I read
the description in more detail. I suppose that if soundcards
are going to be used as the interface some sort of locking
will have to be used. G3PLX used his clicklock technique for
phase coherency, where the 1 PPS is bled into the receive
path. You go one further and mix in the NMEA stream audio as
well for time stamping. A neat solution to making use of
what is really unsuitable hardware. Soundcards really aren't
precision measurement tools !
It may be time to revisit the LF receiver in that case.
Something like a direct I/Q downconversion from LF to
baseband, low pass filtering then I/Q sampling into an A/D at
something like 1kHz (or even lower), sending the resulting
samples to a PC using a basic USB (or even a COM ) port. Then
for backwards compatibility adding in the GPS timestamp
information in the same way .
Getting a certain sense of Deja-vue here. Back when LF
was new, and soundcards were rarely used for radio and GPS
receivers rare, I did start to build a custom LF Rx doing just
this technique and managed to receive coherent carriers from
such as MSF and the Swiss one and see how their phase shifted
with the ionosphere. For freqeuncy reference I used my caesium
standard. But it never went anywhere long term on 73 &
137kHz because a) PC technology moved faster and everyone else
was adopting soundcard ideas and b) The PIC processors I was
using were still new than and had pretty basic functionality
making everything hard work and c) no one seemed bothered
since QRSS has just become widely popular and they all used
ow, nearly 20 years later, the direct conversion LF
receiver (Softrock type perhaps) and a new generation of
processor chips make the job a lot easier.
DDS or Fract-N synth LO, Quadrature mixer, simple (just
analogue perhaps) low pass filter, dual A/D , PIC, PC