[time-nuts] timestamps on downconverted data streams

Bill Byrom time at radio.sent.com
Sun Dec 20 22:15:43 EST 2015

This is normally done with a factory or field alignment step. See pages 47-49 of this document for more details:

On the Tektronix MDO4000 series instruments the delay is factory aligned
to within +/- 5 ns, but on page 34 of the document above you can see how
the user can deskew any analog scope channel to the separate RF channel
with much finer resolution. This instrument has a very wideband (> 1
GHz) acquisition system, so filter group delay drift with temperature
and time are only a few nanoseconds. My colleague David Haworth WA9ONY
has made amateur radio astronomy measurements of emissions from our Sun
and Jupiter with this instrument:

The RSA5000/6000 RF Signal Analyzers don't have an oscilloscope
channel, but they have an external trigger inputs (TTL or analog user-
settable threshold). The digital and analog nominal delays between the
external trigger input and I/Q acquisition system are aligned at the
factory, but the user can activate an alignment step which internally
switches in calibration signals which provide automatic adjustment of
amplitude and phase flatness, as well as group delay. These models have
a wideband (25 to 165 MHz depending on option) acquisition system as
well as a narrowband (about 312 kHz) IF system, and these are
automatically aligned.
Bill Byrom N5BB
On Sun, Dec 20, 2015, at 04:42 PM, Jim Lux wrote:
> On 12/20/15 12:17 PM, Bill Byrom wrote:
>> Yes, you can do that. My employer (Tektronix) makes RF Signal Analyzers
>> which sample at a high rate then use a DDC (Digital DownConverter based
>> on decimation and digital filtering) to produce a much smaller output
>> I/Q file at a smaller bandwidth and lower time resolution. The
>> decimation and digital filtering delay acquired signals, so you have to
>> correct for the group delay in these operations.
>> -
> and, how do they do that correction? Or, do they leave up to you to do
> it?
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