[time-nuts] Propagation delay within analog radio, (Radio Shack Timecube) and an SDR, at HF

jimlux 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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