[time-nuts] Propagation delay within analog radio, (Radio Shack Timecube) and an SDR, at HF
jimlux at earthlink.net
Sat Dec 2 18:22:45 EST 2017
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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