RFIC and 50 Ohm matching

[amnon]

RFIC and 50 Ohm

Hello,

In RFIC, there is no need for 50 Ohm matching since the distance between the source and the load is small compared to the wavelength.
this implies that there is no need for matching inside the RFIC.
So now the output impedances and input impedances are not 50 Ohm (for example, input impedance of Mixer or output impedance of LNA).
so how can we calculate the NF,OIP3 and the rest of the information?
since P=V^2/R and R isn't 50 Ohm, how do we handle with a system with different load resistances? how can we deal with the noise figure which is references to 50 Ohm source.

Thanks in advanced,

 

[vfone]

Re: RFIC and 50 Ohm

Impedance matching doesn’t have anything with the distance between stages.
Impedance matching is necessarily to get the maximum power transfer between stages, doesn’t matter how far or close they are.

Noise Figure matching doesn’t have anything with 50ohms.
A Low Noise Amplifier is to design an input matching circuit that terminates the transistor with conjugate of Gamma optimum (Γopt), which represents the terminating impedance of the transistor for the best noise match. This is not necessarily 50 ohms.

 

[amnon]

Re: RFIC and 50 Ohm

The impedance matching stems from the fact that since the source and the load may be separated higher then ~wavelength/10 -> there are reflection in the trace due to power mismatch which means, the rest of the power that doesn't go to the load return to the source.
for that, matching is important since it minimize the reflected waves and thus maximize the power transfer.

but in the case were there is no possibility for a reflected waves since the source and load are closely (as in RFIC) power matching is not neccessary and voltage matching becomes important.

how do we deal in RFIC with voltages and not powers? suppose you design an LNA with IIP3 =10dBm and Gain=20dB.

how do you translate these parameters to voltages?

I know there is an definition of IIP3 [dBV] (which is derivated from IIP3[dBm]) and concerend to the RFIC.

Does anyone familiar with this? or know any good source of information for that?

Thanks.

 

[BigBoss]

RFIC and 50 Ohm

You'll modelize input/output interface circuits like bondings,package parasitics,ESD components etc.

For mixers, characteristic impedance is not a serious issue. IP3 can be analyzed as dBV independent from termination impedance.And then it is converted to power and dBm..

For LNA, optimum NF impedance is found in a simulation and then this impedance is macthed by aid of parasitic components mentioned before..

For PA's, the same method is valid but you'll check optimum load impedance in this case..

 

[amnon]

Re: RFIC and 50 Ohm

Can you give me some learning material about this?

I've search at Razavi's book and many other sources and didn't find any.

Thanks

 

[noiseless]

RFIC and 50 Ohm

amnon,

I believe, for the circuit on the chip, matching is not nessary. If the specifications are addressed in dBm, you can convert it to voltage by default 50 Ohm. then use voltage to design your circuit.

 

[stober]

RFIC and 50 Ohm

if ur output impedance of LNA is 800 ohm for example, then the power gain is calculated by 800 ohm, which means you soppose ur load is 800 ohm. but this is not a surious issue because there's no need to do impedance match between LNA and the follows like mixer.

 

[rautio]

Re: RFIC and 50 Ohm

Wow! This thread has some aspects that are a little bizzare. Think a little bit, guys. It is incorrect to say that because the source and load are close to each other in terms of wavelengths allows you to ignore impedance matching. For example....

Take a 1.5 volt DC battery (wavelength = infinity). If it's internal resistance is, say 10 Ohms, what resistor do you connect to it to get maximum power out of the battery and into your resistor? That's right, 10 Ohms!

If you have an FET power amplifier and its input impedance is 1 Ohm, you better match to 1 Ohm on the driver amp output if you want to get maximum power output. Watch those wirebonds! Really close distance between source and load does not matter. You still have to impedance match.

If optimum power transfer is not so important, then you can worry less about impedance matching. But keep in mind, if you do not have a good match, voltages (i.e., VSWR) can get quite high, even when << wavelength between source and load.

 

[vfone]

Re: RFIC and 50 Ohm

I found that IC designers with only Analog experience (no RF experience) they apply this concept, to don’t use impedance matching for power transfer.

Impedance matching between stages it will give a lot of benefits not only for maximum power transfer, but also for Noise, Conversion Gain, IPn, Spurious Emissions, etc.