[time-nuts] Cable length calibration

jimlux jimlux at earthlink.net
Fri Jul 1 12:53:43 EDT 2016

On 7/1/16 9:04 AM, Brooke Clarke wrote:
> Hi Mike:
> For quite a while I was heavily into "chirp" transmissions.  These are
> HF ionosphere radio transmissions that sweep from 2 to 30 MHz at 100
> kHz/sec.
> In order to "tune" the radio to a specific station (you can not tune by
> frequency) you need to know the start time schedule for that specific
> station (time nuts content).
> When GPS became popular the transmitters switched to GPS.
> http://www.prc68.com/I/RCS-5A.shtml

I'm building a satellite (actually 2 of them) that is, among other 
things, designed to receive these transmissions.  It turns out that 
accurately measuring the "propagation delay" through the receiver (as in 
from "EM wavefront" to "time stamped samples in the output stream" is 
non trivial.

There's some phase shift/time delay from the physical interaction with 
the antenna and the load impedance presented by the LNA.  Then there's 
the filters and amplifiers in the analog chain. Finally, there's the ADC 
sampling (pipeline delay between voltage at sampling instant to when 
bits appear at the output) delay, and the various delays through the 
digital signal processing (which is fortunately deterministic, but 
non-trivial to actually "measure")

Fortunately, I only claim 10 microsecond timing accuracy so the 100 
kHz/second chirp means that the downconverted stream might have a 
frequency error of 1 Hz.  (that is, if I tell the receiver that the 
chirp starts at 12:34:56.001, and I actually start the ramp at 
12:34:56.00101, I'll see a 1 Hz error in frequency.. if I have a several 
kHz output bandwidth, it will still be in there.

Given that ionospheric delays and propagation delays are substantially 
longer than 10 microseconds, this isn't an issue. 1km is 3 microseconds, 
1000km is 3 milliseconds (or 300 Hz).

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