Quote:I have a narrowband signal (approximately 5k bandwidth).
It appears to me that I get different results when two tones f1, f2 are applied at the input of PA.
I considered the following cases.
I am running QPSS analysis and consider multiple intermodulation products.
1) f1, f2 are 5kHz apart (two tones are edge of channel)
2) f1, f2 are 500 Hz(two tones quite close)
Any ideas which is more realistic case of making the two tone test ?
A detailed explanation is much helpful
Center frequency of your TX system is just one ?
Usually TX system have some channel frequencies, e.g. cellular, WLAN, Bluetooth, etc.
I don't think bandwidth of TX system is just same as modulation bandwidth.
Even though modulation bandwidth is relative narrow such as 5kHz,
TX system, especially PA, must have far wider bandwidth than 5kHz.
If your PA truely have very narrowband characteristics such as 5kHz
and you have interest in IM products up to 5th,
it is desirable that 5*(f2-f1) must be enough inside bandwidth of PA.
So I can't judge whether f2-f1=500Hz is appropriate or not from informations you described.
BTW what modulation is used ? FSK ?
What characteristics is important ?
Generally two tone test is alternative test.
What characteristcs of DUT do you intend to evaluate actually ?
Depending on modulation scheme and chacteristics you intend to evaluate,
sometimes a simple one tone test could be enough.
Two tone drive is equivalent to double sided AM modulation with carrier suppression.
This means an input amplitude to DUT changes dynamically.
So you can evaluate AM/AM and AM/PM characteristics under dynamic amplitude change.
This is a merit of two tone test.
You can also evaluate AM/AM and AM/PM characteristics by one tone test.
But this is under static amplitude change.