Magnus Danielson wrote: > Bruce Griffiths skrev: > >> John Ackermann N8UR wrote: >> >>> Hal Murray said the following on 12/23/2008 01:58 PM: >>> >>> >>>>> 2. Several measurement techniques require a given phase relationship >>>>> (e.g., quadrature) between DUT and reference. For HF frequencies (ie, >>>>> 5 or 10 MHz) is there a *practical* phase shifter design covering >>>>> 180+ degrees that doesn't involve switching various lengths of coax >>>>> in and out of the line? >>>>> >>>>> >>>> PLL up by a factor of N, use that to drive a DDS, then filter. Maybe a >>>> pair >>>> of DDSes will get better tracking. >>>> >>>> For each possible phase offset, you need N slots in the table. (N/4 with >>>> more work) >>>> >>>> >>> If doing phase noise or short term stability measurements, wouldn't the >>> noise of the DDS impact the results? >>> >>> >>> >> John >> >> When measuring the phase noise of amplifiers and other devices with >> relatively low phase shifts at 10MHz (or 5MHz) one can start with a >> quadrature hybrid to produce test and reference signals in approximate >> quadrature. One then only has to compensate for the UUT delay using an >> splitter an, attenuator and another quadrature hybrid to produce the >> small phase shift required. >> One can even use a mixer as a dc controlled attenuator for the fine >> phase adjustment to interpolate between attenuator steps. >> > > Using two mixers programmed by two individual DC signals the quadrature > signal can be setup to arbitrary phase shift. It is trivial to combine > programmed damping and phase-shift in this fashion. > > Cheers, > Magnus > > Hej Magnus
See patent US3803499 for one implementation of this scheme.** **However, where possible, a lower noise more stable system usually results when the phase shift provided by such a system is relatively small.** **Bruce _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.