[eagle] Re: Phase noise

Bill Ress 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.
> 73's
> Bob
> N4HY
> 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.
>> regards
>> Grant
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