Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

question about distortion

Status
Not open for further replies.

danda821

Full Member level 2
Joined
Jun 18, 2002
Messages
139
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,296
Activity points
1,083
HI,

I have a question about distortion in LNA. There are two tones (f1 and f2) fed into the LNA. At the output of the LNA, there are f1, f2, 2*f1-f2 and 2*f2-f1 terms. Ideally the amplitude of 2*f1-f2 and 2*f2-f1 should be equal. But in really normally they are not equal, can someone help me explain that? Thank you.
 

Boowei

Member level 1
Joined
Dec 20, 2006
Messages
37
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Location
China
Activity points
1,522
the Gp of LNA is unequal at 2*f1-f2 and 2*f2-f1
 

abcyin

Full Member level 4
Joined
Apr 15, 2005
Messages
237
Helped
12
Reputation
24
Reaction score
3
Trophy points
1,298
Activity points
2,855
the reason is that the passband gain of your LNA is not equal at different frequency.
 

A.Anand Srinivasan

Advanced Member level 5
Joined
Oct 15, 2005
Messages
1,804
Helped
257
Reputation
514
Reaction score
39
Trophy points
1,328
Location
India
Activity points
10,680
the coefficient of both r different... one would have the square of amplitude of one freq and the other would have that of the other...
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,203
Helped
1,532
Reputation
3,065
Reaction score
1,147
Trophy points
1,393
Activity points
32,949
In general they should be equal in amplitude if the spacing between tones f1 and f2 is small.

There is only one reason that can make the 2f1-f2 and 2f2-f1 to don’t be equal (if the spacing is small), and this is related to the Memory Effect that could appear into the amplifier.

Are two kinds of Memory Effect: Electrical (due to the envelope termination on the bias filtering) and Thermal (due to the junction temperature which is modulated by the applied signal).
 

danda821

Full Member level 2
Joined
Jun 18, 2002
Messages
139
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,296
Activity points
1,083
Thanks. Can you explain the memory effect in detail? esp. the electrical one. Thank you.


vfone said:
In general they should be equal in amplitude if the spacing between tones f1 and f2 is small.

There is only one reason that can make the 2f1-f2 and 2f2-f1 to don’t be equal (if the spacing is small), and this is related to the Memory Effect that could appear into the amplifier.

Are two kinds of Memory Effect: Electrical (due to the envelope termination on the bias filtering) and Thermal (due to the junction temperature which is modulated by the applied signal).
Added after 1 minutes:

Suppose the amplitude of f1 and f2 are the same (A). Thank you.

A.Anand Srinivasan said:
the coefficient of both r different... one would have the square of amplitude of one freq and the other would have that of the other...
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,203
Helped
1,532
Reputation
3,065
Reaction score
1,147
Trophy points
1,393
Activity points
32,949
Memory Effect could be explained as a time lag between AM-PM and AM-PM response of the amplifier. The Electrical Memory Effect is introduced by poor gate/base and drain/collector decoupling at low frequencies causing a distortion of the envelope currents which results in IMD asymmetry.
Low frequencies mean baseband/video frequencies or the frequency spacing between two tones.

The most significant Memory Effect appears in Class AB amplifiers, with reduced conduction angle where drain/collector varies with output power. In Class A amplifiers the Memory Effect reduces.

For best characterization for Memory Effect do a two-tone IMD vs Output Power test (plotting both 2f1-f2 and 2f2-f1) for different spacing between tones (e.g. from 100kHz to 10MHz). See the difference, and tune the bias filtering for best equivalent level between tones at the spacing you are interested.
 

itzikhaim

Advanced Member level 4
Joined
Jul 24, 2006
Messages
100
Helped
12
Reputation
24
Reaction score
1
Trophy points
1,298
Activity points
2,007
Hi danda821

As far as I know, the equal level of the intermodulatin is caused by an equal level of the input signals (f1, f2)
If you'll change one of the input level you'll see the output intermodulation level will change.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top