[time-nuts] Near-perfect chip for Loran-C frequency receiver
Kasper Pedersen
time-nuts at kasperkp.dk
Sat Jul 5 10:28:54 EDT 2008
Bruce Griffiths wrote:
> Your original loop antenna should have a lower phase tempco
> (particularly if a lower TCE coil former than a plastic lid is used)
> than any ferrite antenna, particularly so if the antenna is resonated.
> Quartzlock have abandoned the use of ferrite antennas and substituted a
> balanced shielded loop antenna for this reason for their VLF phase
> tracking receivers.
>
When I built a DCF77-DO (selective I/Q front end, the rest a software
radio in an AVR), I ran into this problem of phase tempco. A loop was
better, but it was unhandy, and I still had phase drift in the
amplifiers, filters and mixer propagation delay.
My fix was to have a tiny transmitter loop near the antenna (just the
feeder cable termination resistor). Once in a while the microcontroller
generates a 77.519kHz carrier, gets a phase measurement using the same
frontend and I/Q mixer, and uses this for compensation.
DCF77 does wiggle quite a bit at dawn, plus there is the occasional 1us
phase adjustments at the transmitter end.
http://n1.taur.dk/dcf/ (~100km NNW of PHK)
I recall having seen better than this with Loran?
/Kasper Pedersen
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