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[SOLVED] Balanced LNA Noise Figure

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youngguns21

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I read in a rfic.co.uk tutorial that using a balanced amp design for a LNA will automatically add ~0.5dB NF. Is this true? Why is this?

If this is deemed a problem, then either a balanced amplifier be used or an isolator added (NOTE in both cases this would add ~ 0.5dB to the noise figure of the amplifier).
**broken link removed**
 

Isolator/Lange Coupler insertion loss will directly be added into NF as dB if they are used to improve the Return Loss ( in the doc. S11 is very poor as seen )
 

The problem is only in that the noise sources and signal sources are available only as unsymmetrical, 50 or 75 Ohms. If you use such source to measure a symmetrical- input device, a balun or similar device is assumed to be necessary which has an insertion loss, to be added to the device parameters.
To measure a low-noise amplifier noise figure if its input is symmetrical, you can avoid this problem if you can connect a symmetrical antenna to its input, and a high-gain amplifier after the amplifier, and a RF power meter.
Then this "radiometer" will generate a RF output power P = kTB G, where k is the Boltzmann constant (k = 1.38 E(-23)), T is the noise temperature in Kelvins the antenna senses from the surrounding space (usually ambient, but...), B is the bandwidth in Hertz, and G is the overall gain (times).
In dB scale, the same equation reads P = -174 + NF + G, where -174 comes from kT, T ~ 290 Kelvins, and NF and G are in dB.

At mm waves we use this easy and accurate method to measure receiver noise figure, by "Y-factor" method. The antenna is a small horn, once pointed to a wall; it senses the ambient temperature, ~290 K, and the corresponding output power reading is for the "high" temperature. The second step requires a coffee cup with a microwave absorber with liquid nitrogen added. If this cup is held before the horn , the output power drops to that of "low" temperature. The two known temperatures and two measured output power levels allow to calculate the Y-factor, and then receiver noise figure in dB.

For longer wavelengths, you can use a directional antenna with a well-defined radiation pattern, to introduce the ambient temperature for "high" and e.g. sky-temperature for "low". I used the method at 12 GHz with a standard-gain horn antenna; the clear-sky antenna temperature was ~ 70 K if the horn pointed to zenith.

Or, you can use a 300-Ohm resistor connected to LNA symmetrical input (or another value to which LNA is matched). It will generate the ambient-temperature level for the "high" reading. Then you can cool the resistor in liquid nitrogen (or water ice or in dry ice), to get the "low" output reading. Again use the Y-factor method to get the noise figure.

In case of symmetrical LNA input, make sure the receiver does not amplify interfering signals from around. You can locate the input resistor in a lossy enclosure for screening. Do not use a metal box as its walls will not radiate well the ambient temperature; the walls must be lossy to do so.
 

BigBoss: I understand that any losses before the LNA will add to the NF, but I wasn't sure if the author was stating some kind of Rule-of-Thumb for using a balanced LNA. I suppose the author was thinking of a coupler that has 3.5dB loss in the through and 2.5dB loss in the coupled paths? Anyway, thank you for the quick reply.

jiripolivka: WHAAAAAATTT?!??!!!!
 

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