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Amplifier oscillation possible solution?

D

Dummy

Jan 1, 1970
0
Half carrier spur was seen in one of the PA design.
Spur amplitude is quite high, at about -45dBc.
Would it be caused by insufficient phase margin or some other factors?
Between gate and drain, there's a series RC feedback.
I've tried tweaking the RC values, but to no avail.
Frequency of interest is 30MHz - 42MHz.
Any good articles out there regarding amplifier oscillation, so that I
could put some possible fixes on use. Please help.
 
T

Tim Wescott

Jan 1, 1970
0
Dummy said:
Half carrier spur was seen in one of the PA design.
Spur amplitude is quite high, at about -45dBc.
Would it be caused by insufficient phase margin or some other factors?
Between gate and drain, there's a series RC feedback.
I've tried tweaking the RC values, but to no avail.
Frequency of interest is 30MHz - 42MHz.
Any good articles out there regarding amplifier oscillation, so that I
could put some possible fixes on use. Please help.

It's called the "varactor effect" or "parametric oscillation". I've
never had to deal with it myself, so everything I know about it comes
from books.

The one reference I can find on short notice is "Radio Frequency
Transistors, Principals and Practical Applications" by Norm Dye and
Helge Granberg, published by Butterworth-Heinemann, (c) 1993. They say
that there's nothing to be done (I think Wes Heyward had some solutions,
but I can't remember), and suggest filtering the output. They also say
that it only happens on class C or B amplifiers, that it's a result of
the wide bias swings on the collector-base diodes, and that it doesn't
happen on class AB or A amplifiers.
 
J

John

Jan 1, 1970
0
Dummy said:
Half carrier spur was seen in one of the PA design.
Spur amplitude is quite high, at about -45dBc.
Would it be caused by insufficient phase margin or some other factors?
Between gate and drain, there's a series RC feedback.
I've tried tweaking the RC values, but to no avail.
Frequency of interest is 30MHz - 42MHz.
Any good articles out there regarding amplifier oscillation, so that I
could put some possible fixes on use. Please help.


Destroy subharmonic resonance's in transistor PA circuits
Sprat magazine

Part 1 issue 91 page 6
Part 2 issue 107 page 9

Available on CD containing all back issues 1 to 109
Sprat http://www.gqrp.com click on Club Sales
 
R

Robert Baer

Jan 1, 1970
0
Tim said:
It's called the "varactor effect" or "parametric oscillation". I've
never had to deal with it myself, so everything I know about it comes
from books.

The one reference I can find on short notice is "Radio Frequency
Transistors, Principals and Practical Applications" by Norm Dye and
Helge Granberg, published by Butterworth-Heinemann, (c) 1993. They say
that there's nothing to be done (I think Wes Heyward had some solutions,
but I can't remember), and suggest filtering the output. They also say
that it only happens on class C or B amplifiers, that it's a result of
the wide bias swings on the collector-base diodes, and that it doesn't
happen on class AB or A amplifiers.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Well, if it *is* due to Vcb variations, then that might be fixed with
a cascode configuration.
 
D

ddwyer

Jan 1, 1970
0
Robert Baer said:
Well, if it *is* due to Vcb variations, then that might be fixed with
a cascode configuration.
Before TTL dividers used it to induce 5MHz xtal oscillator to give 1 MHz
o/p
 
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