I have a question if someone could help me with that… I am from I.T. background so am weak at r.f… till now I used to look at link budget in terms of output power, sensitivity and fspl… but now I am looking at terms like noise figure, noise temperature etc. Now when I was going through IARU form just for the sake of knowing what all info is required in it and whether I have it or not, it asks about noise temperatures of receivers onboard satellite… As an example, if we consider si4464 is being used onboard, how do I find its noise temperature… there is nothing in the datasheet which says about it… or if there is a way to derive it from any given value please guide me on how to
According to the datasheet of Si4464x:
Receive sensitivity = –126 dBm
This implies that the power at the transceiver’s input must be greater than –126 dBm. This can be derived from the link budget (
P_Rx = P_Tx + G_Tx – L_Tx – L_FS – L_M + G_Rx – L_Rx), taking the gains (EIRPs, LNA gains etc.) and losses (FSPL, filter insertion losses, impedance mismatch losses etc.) into account. For the noise temperature, I expect you to be using a LNA in front of the transceiver. According to Friis’ equation for noise, this will be the primary factor affecting the total noise figure of your system, so I suggest calculating the noise temperature for that component (given its noise figure at the center frequency of operation obtained from the corresponding LNA datasheet).
The noise temperature (K) is equal to
290 × (10^(NF/10) – 1), where
NF is the noise figure of the LNA (in dB). You should plug the result of that formula into the IARU calculator.
Thank you @0xCoto for help. I do know about the sensitivity and how to calculate link budget. Rather I have it ready with me. The problem is, we are not using LNA as link budget is satisfied even without it. And module’s datasheet doesn’t mention noise-figure or noise factor… the only relevant numbers available are reciever sensitivity and phase noise, so I wonder if there is any way noise factor, noise temperature and noise figure could be calculated fro these values.
According to this source:
The noise figure of the Si446x family of chips is typically 7.5dB in the 868M/915M band, and about 6.5 dB in the 315M/434M bands.
The noise factor is easily computed using the formula
F = 10^(NF/10) where
F is the noise factor and
NF is the noise figure in dB. For
NF = 7 dB,
F ≈ 5.
I would suggest adding a first-stage pre-amplifier with a low noise figure right after the on-board antenna. According to Friis’ equation for noise, this will significantly reduce the total noise factor of your system, which would otherwise be ~7 dB with the transceiver’s seemingly-poor LNA. Even if your link budget appears to work out without it (e.g. due to a high-gain (but noisy) LNA built into the transceiver module, P_Rx can be greater than –126 dBm), that does not guarantee your system link margin (i.e. your “signal-to-noise ratio” in simple terms) will be very high, or that you will obtain a high data rate equivalently. This is because you are not taking noise temperatures into account, but only signal power.
This helps… Thanks. Most probably, most of the off-the-shelf transcievers would have similar performance… thus I guess I should look at designing a proper one with external PAs and LNAs. I wonder if PQ9ish transcievers have external lna and what is their noise figure.
Sensitivity and noise figure (or factor) is essentially the same:
MDS = kB × T0 × F × B × M
where MDS is the minimum detectable signal (aka “sensitivity”), kB is the Boltzmann constant in units of J/K, T0 is the temperature of the device, F is the noise factor, B is the bandwidth of measurement, and M is power ratio required for the MDS. In other words, you need to know the bandwidth and SNR required for the given sensitivity in order to calculate the noise figure from it.
I think 10 dB noise figure is sufficient for an uplink receiver and I’m not convinced that an LNA in front of your uplink receiver will do you much good. The complete system temperature is the sum of the receiver temperature and the antenna temperature, which is around 290 K for an Earth facing antenna. Furthermore, your uplink data rate is presumably very low and you can also compensate for reduced sensitivity by increasing the uplink EIRP within reasonable limits. Even if an LNA improves your link budget you have to consider the added mass, power, and extra potential point of failure it brings into your satellite.
Depends on a variety of factors (including EIRP limits imposed by the ground station as you very well mentioned), but by introducing an LNA with a low noise factor, you help bring down the total noise factor (or figure) of your system, according to Friis’ equation for noise. Whether pre-amplification is necessary also depends on various in-line losses (which for practical reasons are often ignored) and the sensitivity of the receiver module, although that already seems to work out for @rnbokade (assuming his calculations are correct).