[eagle] Re: Phase noise
bill at hsmicrowave.com
Mon Jun 11 08:31:46 PDT 2007
Could you help me relate "long term coherence" with "low phase noise" as
it applies to the LO requirements, say for the U, S2 and C LO's and the
modulation formats we are using?
Also we talk around it, but have we fixed the LO phase noise
requirements starting with the 10 MHz satellite reference?
Regards...Bill - N6GHz
Robert McGwier wrote:
> Thank you all for this great conversation. I want to remind everyone
> that we need long term coherence in the receiver, which can only be
> accomplished by low phase noise, not for the purity of the tone needed
> to be heard by a human ear, but by the need to correlate for long known
> symbols to recover frequency and timing in the SMS text messaging system
> and to do COHERENT detection of the symbols transmitted. We want the
> analog transponder to support this. This is also contributes to the
> requirement for a much larger dynamic range in the system than we have
> had heretofore on our spacecraft.
> This system needed was added to the requirements for the receiver after
> the initial requirements but not before the design was done and
> certainly not before it was built.
> Grant Hodgson wrote:
>> 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.
>> Via the Eagle mailing list courtesy of AMSAT-NA
>> Eagle at amsat.org
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