ADS question

[Leiser]

Hi,

does anyone know how in S-parameter simulation set the power level of the input? I want to get the S21 for an amplifier for diferent input power levels.

Thanks,

 

[IRstuff]

I think you're misunderstanding S-parameters:

http://en.wikipedia.org/wiki/S-parameter#2-Port_S-Parameters

TTFN

FAQ731-376

 

[VEBill]

"...S21 for an amplifier for diferent input power levels..."

That seems like a question about linearity, compression, distortion and third order intercept points, isn't it?

http://en.wikipedia.org/wiki/Third-order_intercept_point

 

[Leiser]

Hi,

thanks all for your comments. I will review the principle behind S-parameters.

My question arose, because I was taking the S-parameters with the network analyzer for which you have to set the power level. For exemple, my design has an AGC in a FM amplifier, so I measure S21 for diferent input power.

So, why in the network analyzer you have to set the power level? then S-parameters will be different for diferent power leves?

 

[IRstuff]

Did you read the responses? Perhaps you need to review what AGC means, as well:

http://en.wikipedia.org/wiki/Automatic_gain_control

At the very minimum, you woulnd't want to damage your network with excessive power, would you?

TTFN

FAQ731-376

 

[VEBill]

"...my design has an AGC in a FM amplifier..."

I assume that you mean AGC = Automatic Gain Control (variable gain for output amplitude control under various input conditions).

Most wideband RF amplifiers (preampliers, for receive) do not use AGC for a very simple reason...

If someone is parked next to 104.3 MHz and that nearby signal is huge, then the AGC circuit (not having any idea what station he's actually listening to), will turn down the gain, and the already-weak 88.1 MHz station that he is listening to will fade away for ** no apparent reason **.

Perhaps having just a little bit of AGC would do more good than harm, but much more than a few dB of variable gain is perhaps going to cause more problems than it solves.

The better approach is to make the preamplifier so that it has an incredibly high dynamic range and stays linear even when parked next to 104.3 MHz.

Now if the automotive radio could somehow communicate the frequency information (station being listened to) back to the antenna, and the antenna contained narrow active filters, then AGC would be perfect and the resultant system would be the best car radio antenna in the world.

All the theory and industry standard plots typically assume constant gain (not AGC). To describe your preamplifer_with_an_AGC, you're going to have to run multiple plots (perhaps printed with different colors) to describe what happens at different amplitudes.

And you should plot the results for "two tone" (widely spaced, in band) to see the potential disadvantages (side effects) of preamplifier AGC.

 

[Leiser]

You are right. However, OEMs ask for AGCs that trigger at
a fixed power. Some car receivers send the IF to the antenna block to adjust antenna impedance and amplifier gain, but it is not usual. I guess at the end of the day all comes to price, specially in Automotive market.