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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/7JN5EWRNC5ESXU3RXOGMNHHZYN5SAE3H/?format=api",
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    "message_id": "[email protected]",
    "message_id_hash": "7JN5EWRNC5ESXU3RXOGMNHHZYN5SAE3H",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/RQTXJQ4MSAGDRGTCWPHM4YTJLW4CWPUD/?format=api",
    "sender": {
        "address": "grant (a) ghengineering.co.uk",
        "mailman_id": "74a4d29508634e669020884f51007f55",
        "emails": "https://mailman.amsat.org/hyperkitty/api/sender/74a4d29508634e669020884f51007f55/emails/?format=api"
    },
    "sender_name": "Grant Hodgson",
    "subject": "[eagle] Re: Please check these calculations",
    "date": "2006-11-02T11:50:16Z",
    "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/RQTXJQ4MSAGDRGTCWPHM4YTJLW4CWPUD/?format=api",
    "children": [
        "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/ZCD32TJLNRKGJB52OPXKX7QIOM7Z32QA/?format=api"
    ],
    "votes": {
        "likes": 0,
        "dislikes": 0,
        "status": "neutral"
    },
    "content": "Bob\n\nI think your statement\n\n > and we worry that\n >\n > RN (reciprocally mixed noise) =  S * L * B  will appear and hurt us to\n > the point of signal being covered and for example, SMS not working.\n\nis not quite right, because it doesn't take into account the gain of the \nmixer or the level of the LO signal.\n\nA much easier way to analyse the effect of reciprocal mixing is to just \nconsider the differences, rather than absolute levels.  That way, \nconstants such as mixer noise, gain and LO power get cancelled out.\n\nConsidering the phase noise of the satellite's receiver, and making the \nfollowing assumptions :-\n\nthe phase noise of the LO is flat across the receiver bandwidth at the \noffset of the interferer\nthere is only one interferer\nthe mixer is not being driven into compression\nthe mixer has a flat frequency response\n\nthen, for a given on-channel input level which you called Text, and \nworking in dB, the noise power in the IF channel due to reciprocal \nmixing is :-\n\nX = (I - Text) + [10log(B) + L]    {eqn 1)\n\nwhere I is the interferer level in dBm, Text is in dBm, B is the Rx \nbandwidth in Hz and L is the Rx phase noise in dBc/Hz at the appropriate \noffset.\n\nFor example if the interferer is 45dB higher than the wanted signal, the \nLO's phase noise is -82dBc/Hz at the interferer's offset frequency and \nthe bandwidth is 500Hz, then X = 45 + (27-82) = -10dB.  I.e. in this \ncase the noise contribution to reciprocal mixing would be an extra 10%, \ndegrading the signal/noise ratio by 0.4dB.\n\nHowever, the above model is slightly too simplistic, as one has to also \nconsider the phase noise of the interferer.  This will fall right into \nthe Rx passband, and no amount of filtering will remove it.  We don't \nknow what the phase noise of the interferer will be, but we can guess. \nIf the interferer is another ham transmitter, then a reasonable first \nassumption is to assume that it's phase noise will be similar to the \nreceiver's.  Therefore the equation above needs modifying :-\n\nX = (I - Text) + [10log(B) + L + 3dB]  {eqn 2)\n\nwhich in the above example would give 20% extra noise contribution, \nreducing the S/N ratio by 0.08dB\n\nEquation 2 can be modified to accommodate more than one interfering signal.\n\nWhat are the offset frequencies of the interferers likely to be?\n\nregards\n\nGrant\n\n\nRobert McGwier wrote:\n> We have done all of these calculations for the links and published these \n> spread sheets.  The spread sheets show the typical bias towards taking \n> thermal noise and some \"guesstimate\" as to interference levels but not \n> really taking into account LO noise. \n> \n> \"Everybody\" can understand that noise performance in a mixer that has a \n> 50 ohm input port (which is hooked to a 50 ohm load) with the pretend \n> noise power density of -174 dBm/Hz (k T0).  The noise factor  of the \n> mixer is taken into account  in the usual way\n> \n> Noise Factor = kT0 (F-1)  where k is Boltzman,  T0 is room temperature \n> and F is the noise factor in the mixer.\n> \n> But,  we have these oscillators that we say we want tunable.  I doubt we \n> actually want them to be tunable but let's start to go through this so \n> we can calculate if I am right.\n> \n> Reciprocal mixing is the crap that gets thrown into your receive pass \n> band by a strong signal (strong compared to our weak SMS text signal for \n> example)  and raises the receiver noise floor.  We have these computed \n> signal strengths at the spacecraft and we do not want to raise the noise \n> in our signal bandwidth by more than a few dB (none?)  ;-)\n> \n> Let's make some simplifying assumptions that the bottom end of the \n> passband will be as empty as it has always been and the upper half band \n> will have the usual ten suspects in them and not much more.  The usual \n> 20 people using the linear transponder on the satellite.\n> \n> So we will assume that we are out in the \"flat part\" of the LO noise and \n> that a signal of level  S is out there.\n> \n> Our bandwidth of our desired signal  (the undesigned SMS text message \n> signa for example)  is B and the LO noise floor is L.\n> \n> and we worry that\n> \n> RN (reciprocally mixed noise) =  S * L * B  will appear and hurt us to \n> the point of signal being covered and for example, SMS not working.\n> \n> SNR(in)/SNR(out)  =  F +  RN/(kT0 B)\n> \n> If we wish to see signal level Text at our receiver then  we can \n> estimate the noise floor required of the LO (L) to be (using the \n> previous formulae)\n> \n> L  = Text - S - C/I  - 10log10(B)\n> \n> where C/I is the carrier to interference ratio required at the output of \n> the mixer.   Notice that the reciprocal noise specification depends \n> directly on the input blocking signal strength S.  So this is a bit more \n> complex than one might think at first so some more thought needs to be \n> put into this before we leap to say the 70 cm design is good, bad, or \n> indifferent.\n> \n> We need to resolve this in the next few days so John, Juan, and the rest \n> of our receiver friends can have a design to build to.  Please \n> everyone,  check over my formulae before I go and do a ton of work that \n> is all wasted.  If you don't like the form of them,  suggest something \n> different.  I suggest we build a spread sheet that does these \n> calculations in a \"mutually coupled\" way.  I am off to Lyle's on \n> Thursday and will not be able to do much more on this before I return.  \n> Somebody jump all over it please if you want to.\n> \n> \n> Bob\n> \n> \n> \n\n",
    "attachments": []
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