If we use the specification for a low noise amplifier, invariably the noise performance is a Noise Figure. However, in a particular system design we calculated the input referred voltage that could be a limiting factor for the very first stage LNA. The issue was how to convert from the noise figure of a selected LNA ( from Analog Devices no less) to the input referred noise voltage to make sure the amplifier was being chosen correctly. Well here is the conversion at least in one form.

Note: The noise factor is simply 1 + NA/Ni. Ni is the noise power coming in from a 50 Ohm matched source and is equal to -174 dBm/Hz. ( Pretty standard usage).

The noise voltage being generated by the 50 Ohm source is vni=4.46E-8 Vrms/Hz. This can then be used to compare whether the amplifer will work with a particular noise figure ( from the expression 1 + NA/Ni).

Check and see if the number NA, the noise input referred power generated by the amplifier itself, converted from a voltage to power is acceptable or not. Must remember to use the impedance level of 50 Ohm. Simple?

Example: If the NF is = 0.8, then 1+ NA/Ni = 10**0.08 = 1.2 ( approx). We can calculate vna as above for vni.

Here is a note on input noise. It has been found that the -174 dBm/Hz should be modified to -162 dBm/Hz for the rural environment in the US and to -98 dBm/Hz for the urban environment. The -174 dBm/Hz is therefore a theoretical figure used to specify and calculate noise figures and noise factors!

Yes, another thought; we need to make sure that the derivation for the noise factor is elaborated: Here it is:

Noise factor F = SNRi/SNRo where i stands for input and o stands for output.

So = Si X G ( G = Gain)

No = [Ni noise power from the 50 Ohm source + NA, noise power generated by the amp].

F = [Si/Ni] / [GSi/G(Ni+NA)] = 1 + NA/Ni.

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**The input interface.**

The frequency divider has a differential analog interface. The following parameters apply:

The minimum frequency that can be input is 500 Mhz and the maximum frequency is 6.0 Ghz.

The RF input level is 5 dBm to – 5 dBm. For lower frequencies make sure that the slew rate is

greater than 560 V/us. The input is biased by two 500 Ohm resistors connected to a 1.6V DC bias.

Therefore AC coupling is used at the input. These are two 100pF capacitors.

**The output interface.**

The output is single ended. The output driver is capable of sourcing and sinking 24 mA. The

equivalent output impedance is 50 Ohm. To avoid reflections it is recommended that the divider

work into a 50 Ohm load.

**General operation.**

The inputs are applied to the input SMA I/O. The product will work with both a differential input as

well as a single ended input. However, a differential input works best. The division ratio is applied

to the N1 and N2 control inputs as follows:

N2 N1 Division ratio

0 0 8

0 1 16

1 0 32

1 1 64

The logic levels are:

Logic level Voltage

1 1.4V minimum

0 0.6V maximum

The supply voltage interface.

The operating supply voltage is 3.3V typical. The quiescent (DC) operating supply current is 2 mA.

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**RF Switch typical features**

Supply voltage = Vcc = 0/+5 Vdc

Operatng temperature = TA = -50° C to 125 Deg C

Operating impedance = 50 Ohm

Input power for 1 dB

compression ( 5.0V system) = 37 dBm ( f = 0.5 to 3 Ghz)

Input third order Intercept = 64 dBm ( 0 to 5.0V system, f = 0.5 to 3 Ghz)

Operating frequency range = DC to 3 Ghz.

Insertion loss DC to 3 Ghz = 0.8 dB

Isolation DC to 3 Ghz = 14 dB minimum

Return loss DC to 3 Ghz = 20 dB

50% contl to 10/90 %

( ON/OFF) = 120 ns

Summary Specifications

Gain, Operating: 19.5 dB

Operating frequency range: 1.0 – 2700 Mhz

OIP3: (Pout = 19.0 dBm), -8.5 dBm

P1dB: 4.6 dBm

N.F: 4.2 dB

Supply voltage Operating: 3.3 – 5.5 Volts

Price: single unit $7.50, 100 units : $5:50.

Free delivery, shipping lead time 2 days.

30 day return policy, buyer ships.

Supply current Supply = 5.0V, 23.0 mA