Amplifier oscillation possible solution?

D

Dummy

Guest
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.
 
Dummy wrote:
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.
Click to expand...
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
 
Tim Wescott wrote:
Dummy wrote:
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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Click to expand...
Well, if it *is* due to Vcb variations, then that might be fixed with
a cascode configuration.
 
"Dummy" <ahkit1021@yahoo.com> wrote in message
news:74bb84c0.0405232306.3fc4e94e@posting.google.com...
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.
Click to expand...

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
 
In article <40B2ECE3.15289BF6@earthlink.net>, Robert Baer
<robertbaer@earthlink.net> writes
Tim Wescott wrote:

Dummy wrote:
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.

--

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.
Before TTL dividers used it to induce 5MHz xtal oscillator to give 1 MHz
Click to expand...
o/p
--
ddwyer
 
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