[eagle] Phase noise
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.
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