[time-nuts] Close in phase noise of microwave VCOs

Jim Lux jimlux at earthlink.net
Thu Jun 18 00:14:43 EDT 2015

On 6/17/15 6:15 PM, John Miles wrote:
> Also see http://www.ke5fx.com/gunnpll.html , a quick and dirty but
> successful attempt at locking a Gunnplexer in a relatively low (1
> kHz) loop bandwidth.  The inband noise is likely too high for good
> performance in a radar application, but the basic idea is workable
> enough.  Unfortunately I tried this experiment before I had any way
> to generate log plots. :(
> I can't think of many noise-critical applications where a microwave
> VCO is used without some form of phase locking.  I'd think that a
> homodyne architecture would still need a synthesized source, just
> because the waveform being received is delayed relative to the one
> that was sent.

Yeah, but at short ranges (<100m) that delay is on the order of <1 
microsecond. So low frequency phase/frequency variations cancel 
themselves out.

Police radar speed detectors are a fine example. Gunn oscillators work 
just fine here when operated open loop.

  Without a clean source, I'd imagine that you'd have
> to do autocorrelation between the outgoing and incoming channels
> rather than simple/cheap baseband mixing.  An obvious question would
> be whether it's cheaper to add another digitizer and correlator to
> your pipeline than it would be to clean up your source...

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