Help some basic concepts about RF amplifier

[Osawa_Odessa]

Hi, I want to ask some basic question about RF amplifier GNSS architecture.
Here is the overall block diagram of the architecture.

And here is the RF specification of its in the IEEE paper attached:

Now I want to ask two basic questions. Please note that I am only a beginer and not angry!
1. In the paper, the voltage gain of RF amp is 13.3 dB. But in my case, I need to increase the voltage gain to at least 50dB. Do you think this is doable?
Is it so hard to get the voltage gain 50dB?
2. When we said that the RF amp has input power is -110dB, then how can we know about input voltage? I think we need to know about input impedance of RF amp and then use the formula:
V = sqrt (P*Zin)

 

[jiripolivka]

In frequency ranges above say 10 MHz it is not common to talk about "voltage gain" as the quantity we can measure is power. So the gain in dB is the same but we care for power, not voltage.
At your GPS frequency band, designing an amplifier with 50 dB or more gain is possible but in your block diagram the receiver gain is concentrated in the IF section where it is easier and cheaper to make high-gain amplifiers.
If you put a high-gain RF amplifier just after the antenna, you must consider its linearity, and due to a high gain, also system vulnerability by external interference.
Designing such system needs some experience. The best way to learn it is to see how it is done by others, and why. Then you may improve the state of art, and present a better design.

- - - Updated - - -

To your point 2: We refer the impedance to the common 50 Ohms, used by the most test instruments. If you specify the RF input power level as -110 dBm, you should also know receiver NF and bandwidth. The noise floor of the receiver should be ~20 dB lower. You can use the equation or input noise power, Pn :

Pn = -174 + NF +10 log B where Pn is in dBm, NF in dB and B in Hertz.

 

[FvM]

Due to limited selectivity of the RF filters, RF gain can't be arbitrarily high, otherwise off-band components could cause amplifier and mixer overload. So if you plan higher RF gain, you have to check that the signal is keeping the linear range in every part of the receiver.

When we said that the RF amp has input power is -110dB

It's -110 dBm, not dB. RF measurements are usually referred to 50 ohms.

 

[Osawa_Odessa]

Thanks, jiripolivka and FvM, for help!
Jiripolivka,

In frequency ranges above say 10 MHz it is not common to talk about "voltage gain" as the quantity we can measure is power. So the gain in dB is the same but we care for power, not voltage.

I am happy to know that. I want to ask more about this. Do you mean that in the frequency range we usually can only measure power not voltage? Why we don't measure voltage in this frequency range?

At your GPS frequency band, designing an amplifier with 50 dB or more gain is possible but in your block diagram the receiver gain is concentrated in the IF section where it is easier and cheaper to make high-gain amplifiers.

I has some intuitive about that but don't really know why. If possible, can you give me a link explanning this.
I have been said that the input signal going into antenna is about -140dBm and they said that RF amplifier has to have the gain at least 50dB to make sure that the signal going to ADC is about 0.3V.

If you put a high-gain RF amplifier just after the antenna, you must consider its linearity, and due to a high gain, also system vulnerability by external interference.

Thanks, this is good one. I will search more about this.

Designing such system needs some experience. The best way to learn it is to see how it is done by others, and why. Then you may improve the state of art, and present a better design.

Thanks for the advice. I thought about this but you know I have almost no experience about this and therefore the searching also get more difficult.
If you know any materials about this, please let me know.

To your point 2: We refer the impedance to the common 50 Ohms, used by the most test instruments

I don't get this point. Can you give me a link?

If you specify the RF input power level as -110 dBm, you should also know receiver NF and bandwidth. The noise floor of the receiver should be ~20 dB lower. You can use the equation or input noise power, Pn :

Pn = -174 + NF +10 log B where Pn is in dBm, NF in dB and B in Hertz.

Can you tell me how you get the formula?

FvM,

Due to limited selectivity of the RF filters, RF gain can't be arbitrarily high, otherwise off-band components could cause amplifier and mixer overload. So if you plan higher RF gain, you have to check that the signal is keeping the linear range in every part of the receiver.

Thanks I will check that now.

It's -110 dBm, not dB.

How can you know it is -110dBm not dB?

RF measurements are usually referred to 50 ohms.

I don't get this. If you know the link explanning this, please let me know.

 

[chuckey]

-110 dBm means its 110 db down on 1 mW (into 50 ohms), though for telephony its 1 mW into 600 ohms. So V^2 /50 = .001, so V^2 = .05, so V ~.2236 V.
Frank

 

[jiripolivka]

We can talk for a long time about the basics. I think it is time you read a basic RF electronics textbook, like ARRL Radio Amateurs' Handbook where all the basics are explained.

First, above ~10 MHz RF voltmeters do exist but their probes introduce stray capacitances, so voltage readings have large errors. Measuring power with a 50-Ohm load is easier and with much less error.

With a signal level of -140 dBm at input it takes a lot of gain to get 0.3V for the ADC, but look on your schematic again. Making high-gain amplifiers is easier at a lower IF than at RF. Also, down conversion must be done to get a lower frequency to the ADC.

Concerning why 50 Ohms are standard, it is history and big RF makers who unified the line impedance. We use available equipment for testing, so we must use their standard to refer to a common line impedance.

The Pn formula is the common P=kTB taken as a logarithm. Here the units are dB and dBm if referred to one mW. If you do not understand, you need a high-school electronics course first, Forget about designing system without knowing the basics.

 

[Osawa_Odessa]

The Pn formula is the common P=kTB taken as a logarithm. Here the units are dB and dBm if referred to one mW. If you do not understand, you need a high-school electronics course first, Forget about designing system without knowing the basics.

I don't need to go back to high-school. I know clearly the difference between dB and dBm but I ask because I my first post I wrote is dB but he said that it should be dBm not dB.

 

[Osawa_Odessa]

I want to ask one more question. In the picture above, can I consider RF amp as a power amplifier?

 

[RCinFLA]

For your selectivity/dynamic range analysis, be aware of 1800/1900 MHz cellular band that although low power is everywhere and often close by. On the higher side, weather doppler radars in the 2700 MHz range running mega-watts of ERP can also destroy GPS reception.

Close in noise performance on L.O. is very important for GPS receivers.

 

[Osawa_Odessa]

Can anyone answer my question? Is "RF amp" in the picture "RF power amplifier"?

 

[FvM]

Is "RF amp" in the picture "RF power amplifier"?

It is not.

 

[Osawa_Odessa]

It is not.

Hi, can you tell me where can I find some tutorial about RF amplifier in MOS? It seems that all the tutorials and books that I have found are RF power amplifier.