[time-nuts] Propagation delay within analog radio, (Radio Shack Timecube) and an SDR, at HF
kb8tq at n1k.org
Sat Dec 2 19:30:07 EST 2017
A lot depends on just how this or that was implemented. The bottom line is still that
ionosphere issues will give you a “couple of milliseconds” sort of ill defined variation.
If your RF chain is below 10 MS (which it likely is) is a bigger delay, but not a bigger
> On Dec 2, 2017, at 6:22 PM, jimlux <jimlux at earthlink.net> wrote:
> On 12/2/17 12:57 PM, Patrick Barthelow wrote:
>> Just about to go across town to pick up TWO Radio Shack Timecube radios,
>> that someone will sell cheap. Been 35 years since I have seen one. Used
>> one in College to bring time to astronomy instruments in the field.
>> From a newbie: Has anyone measured or does anyone have an idea of the
>> propagation delay between an audio tick signal on the RF carrier at the
>> antenna port, to an audio waveform on the demodulator/audio amp?
>> Microseconds? more? less?
>> If I hang a scope on the speaker audio out, and RF modulated input signal,
>> how much delay would there be? Same question for an SDR.
>> Best, 73, Pat Barthelow AA6EG
>> apol <apolloeme at gmail.com>loeme at gmail.com
> Milliseconds - mostly from the audio processing chain. For analog systems, the delay is inversely proportional to bandwidth and proportional to number of sections.
> In SDR implementations (obviously not the Timecube) the latency can be quite long, and can be variable. Most SDR implementations don't pay much attention to RF/Baseband delays as long as the pipeline doesn't run dry.
> This was a big, big deal in the early days of the Flexradio, because it used a half duplex processing chain with buffers - Tx/Rx turnaround could only occur on a buffer boundary, and if you cranked the bandwidth down (for CW), the buffer had to be fairly big.
> Gnuradio and Pothos (two popular frameworks for SDR) don't really have any provision for accurate timing - they're basically "signal flow graph" based systems, and the latency through a block is whatever the software gives you.
> The USRP, in the usual "RF up/downconverter from digital samples" mode is also noncoherent between RF and digital side - there's buffering in the USB or Ethernet interfaces.
> The RTL-SDR pod is "coherent within a stream" - that is, once you're streaming, there aren't any dropped samples, but the absolute timing between RF sample and a particular USB data packet is uncertain (due to USB device driver stuff).
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