Active Mixer RF Input Power and Input IP3i, P1dB Question

Question

I am new and learning in this field.

The received signal is first amplified with SKY65404 LNA and secondly with HMC313 Amplifier. Lastly it is connected to Active Mixer ADL5801's RFIN pins.

In ADL5801's mixer datasheet, i could not find what is the maximum rf input power accepted in decibels. LO level is given in datasheet as -10 to 10 dBm. The datasheet gives information about Input IP3 and Input P1db values but i do not know how to read them.

My questions are:

1. How can i learn what is the max. allowed rf input power for mixer so i can select amplifier.

2. Does Input IP3 and Input P1db give any information related to the first question or how should i interpret them.

3. How should i calculate the received signal's power from antenna. I mean it is said the rf signal gets so weak in the air to nano or micro volts. What value is the start point as decibel to start adding gains of amplifiers so i calculate mixer input level correctly.

Thank you.

ADL5801 Datasheet

Answer 1

Assume your IP3 is +20dBm. The 3rd order distortion drops 30dB, per 10dB input power reduction. Thus -10dBm input RF power drops the distortion by 3*(+20dBm - (-10dB)) or by 90dB below the IP3, or to +20- (90) to -70dBm;

[ note my Initial Error in computing the distortion level: had gone 90dB below +10dBm and that is wrong; should be 90dB below the IP3, which is -70dBm distortion products power; another (correct) way to compute would be to take 2/3 of the drop in distortion (or 2/3of 90dB) and refer that reduced level to the RF input; that would give 60dB below RF input of -10dBm, or the -70dBm power level for distortion products.]

Your input RF is -10dBm; the distortion is -70dBm, your Harmonic Distortion ratio is 60dB.

Is that satisfactory for your signal chain?

However, the two input amplifiers may overdrive your mixer.

You need to sketch out the gain chain and the IP3, and compute the resultant 3rd order.

What input power level to expect from the antenna? Suppose you need a +10dB SNR, for 10MHz bandwidth. Suppose your front end antenna matching, coax, and LNA produce a 4dB degradation of SNR. Just add these up, and reference to the Boltzmann Thermal Noise in 1Hertz bandwidth at +17 degree Centigrade:

-174dBm/rtHz

+4dB match/cable/firstLNA

+70dB for the increased noise power of a 10,000,000Hz bandwidth

+10dB SNR for acceptable bit errors, packet errors and re-transmission requests, and trivial Bit_Error_Correction protocols

-174 + 4 + 70 +10 = -174 + 84 = -90dBm.

Since 0dBm across 50 ohm is 0.223 volts RMS and 0.632 vols PeakPeak, and -120 dBm is 0.223 microVolts RMS and 0.632 microVolts PeakPeak, your power level of -90 dB is 30dB more power (1,000X) and is sqrt(1,000) or 31.6 higher voltage; thus your input RMS power (needed for LNA thinking) is -90dBm and 0.223 * 31.6 or about 7 microVolts RMS.

Do you understand this diagram?

^{simulate this circuit – Schematic created using CircuitLab}