We have done all of these calculations for the links and published these spread sheets. The spread sheets show the typical bias towards taking thermal noise and some "guesstimate" as to interference levels but not really taking into account LO noise.
"Everybody" can understand that noise performance in a mixer that has a 50 ohm input port (which is hooked to a 50 ohm load) with the pretend noise power density of -174 dBm/Hz (k T0). The noise factor of the mixer is taken into account in the usual way
Noise Factor = kT0 (F-1) where k is Boltzman, T0 is room temperature and F is the noise factor in the mixer.
But, we have these oscillators that we say we want tunable. I doubt we actually want them to be tunable but let's start to go through this so we can calculate if I am right.
Reciprocal mixing is the crap that gets thrown into your receive pass band by a strong signal (strong compared to our weak SMS text signal for example) and raises the receiver noise floor. We have these computed signal strengths at the spacecraft and we do not want to raise the noise in our signal bandwidth by more than a few dB (none?) ;-)
Let's make some simplifying assumptions that the bottom end of the passband will be as empty as it has always been and the upper half band will have the usual ten suspects in them and not much more. The usual 20 people using the linear transponder on the satellite.
So we will assume that we are out in the "flat part" of the LO noise and that a signal of level S is out there.
Our bandwidth of our desired signal (the undesigned SMS text message signa for example) is B and the LO noise floor is L.
and we worry that
RN (reciprocally mixed noise) = S * L * B will appear and hurt us to the point of signal being covered and for example, SMS not working.
SNR(in)/SNR(out) = F + RN/(kT0 B)
If we wish to see signal level Text at our receiver then we can estimate the noise floor required of the LO (L) to be (using the previous formulae)
L = Text - S - C/I - 10log10(B)
where C/I is the carrier to interference ratio required at the output of the mixer. Notice that the reciprocal noise specification depends directly on the input blocking signal strength S. So this is a bit more complex than one might think at first so some more thought needs to be put into this before we leap to say the 70 cm design is good, bad, or indifferent.
We need to resolve this in the next few days so John, Juan, and the rest of our receiver friends can have a design to build to. Please everyone, check over my formulae before I go and do a ton of work that is all wasted. If you don't like the form of them, suggest something different. I suggest we build a spread sheet that does these calculations in a "mutually coupled" way. I am off to Lyle's on Thursday and will not be able to do much more on this before I return. Somebody jump all over it please if you want to.
Bob