[eagle] Phase noise

Grant Hodgson grant at ghengineering.co.uk
Mon Jun 11 03:54:37 PDT 2007


Jim Sanford wrote:

> Do you have a different answer to my original question, "What is a good 
> phase noise number to shoot for in a microwave narrowband (ssb/cw) 
> system?"    If so, I'd like to engage in that conversation, both for 
> Eagle and for my terrestrial microwave station.
> 

Needless to say, there is no right and wrong, it's very subjective. 
However, there are a couple of factors that can be used in calculating 
phase noise requirements.

For analogue systems, i.e. good old CW and SSB, there are two basic 
phase noise requirements for the local oscillators.

The first is that the LO needs to produce a tone that sounds 'clean' 
when listened to on an analogue receiver.  The term T9 is often used, 
but is rarely defined.  The parameter of Residual FM can be used to 
specify the cleanliness of a tone - it's subjective, but a value of 20Hz 
RMS IMHO is more than good enough for narrow-band voice communications. 
  As it's a noise measurement, the bandwidth needs to be specified, and 
so 300Hz - 3kHz is appropriate for SSB.  (Note that FM is different as 
much wider bandwidths are needed).

So, I would suggest a residual FM spec. of 20Hz from 300Hz-3kHz.  This 
can be achieved with a 'flat' phase noise performance of -76dBc/Hz at 
offsets from 300Hz to 3000Hz, for example.

Residual FM can be measured directly, but not easily, or it can be 
determined from an integration of phase noise over the required 
bandwidth.  A Google search will give the maths of converting phase 
noise to FM, I'll post a link if required.  Software such as that from 
KE5FX has the conversion built in.  Note that for the purposes of 
determining residual FM, it doesn't really matter what the shape of the 
phase noise plot looks like,  it is the total area under the curve that 
counts.  Also, the value of 40Hz is based on the response of the human 
ear - it is not based on the actual frequency of the local oscillator. 
Therefore, a 10GHz local oscillator with 40Hz FM will sound just as good 
as one with 40Hz FM at 1.8MHz.  However, maintaining the required level 
of FM at microwave frequencies obviously becomes more challenging as the 
frequency is increased.

The second requirement is that for the phase noise spec. applies to 
reciprocal mixing; the process whereby a weak signal is buried in the 
noise of a strong signal which is at a different frequency.  For the 
purposes of the Eagle analogue receivers, consideration needs to be 
given to the relative strength of any strong signals.  The SAW filters 
will do a good job of reducing out of band signals, so we are left with 
the consideration of strong in-band signals.

For satellite work, the levels of the incoming signals should be much 
closer together than would be the case for terrestrial work.  If we 
assume that the strongest signals will be 30dB higher than the weakest, 
at an offset of 5kHz, and that we don't want the strong signal to 
degrade the S/N of the weak signal by more than 3dB, then we can say 
that the phase noise requirement at 5kHz offset would be approximately 
-30 - log(2.7kHz)  = -64dBc/Hz.

That might seem surprising, but it is based on a relatively narrow range 
of input signals.  It assumes that the uplink signals are perfectly 
clean with no phase noise, and ignores radar interference. If the range 
of input signals is larger, then the phase noise needs to be better than 
this.  Terrestrial stations need much better performance than this as 
they have to deal with a much larger range of input levels.   I've got a 
feeling that this subject has been anlaysed before, but I couldn't find 
a mention of it whilst browsing EagleP.

regards

Grant


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