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FET oscillator design - HELP! - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: FET oscillator design - HELP! Posted: Fri
Oct 28, 2005 12:32 am
i am trying to build
a balanced FET oscillator (2 FETs resonate each other through a transmission
line) and i have a strange problem with the bias. I bias the FET independently
at gate and at the drain. At some point, i cannot change my gate bias
anymore (to a more positive value, say at around -0.6V i cannot go to
-0.5 anymore) unless i decreasy my drain bias. What can this mean?
In addition, my oscillator oscillates at the wrong frequency
but this frequency does not come from the circuit i built but apparently
from the instability in the transistor even if i did not measure any
instability when i identified the S-parameters. What can the problem
Thank you very much! I am not very experienced with active
circuits, so i appreciate every every answer....
I really don't
know how to go on
subject: FET OscillatorPosted: Tue Nov 01, 2005 2:29 pm
bias condition means is that the feedback is strong enough to cause
the gate-to-channel diode to conduct (assuming you're using a pair of
JFETs or MESFETs - you didn't specify...)
The more details you
provide the easier it is to answer questions. Frequencies, voltages,
device part numbers, etc. all help us to answer your questions.
In general, oscillation occurs at the frequency or frequency where
the loop gain is 1 and the phase shift around the loop is n x 360 degrees,
where n is a non-negative integer (0,1,2,3, and so on). So just because
the FET is stable in and of itself doesn't mean much about a circuit
- and it's easy to get unintended feedback! Layout often makes the difference
between an excellent design and a poor one - and an abstract schematic
doesn't always capture the important details at RF.
Post subject: Posted: Wed Nov 02, 2005
Joined: Fri Sep 02, 2005 7:25 pm
Location: Hampshire UK
Theres a hundred ways this can
happen to you.
First thing you / we need to know is the frquency
range over which the FET devices have gain. Next is the (apparently
fixed) frequency of the oscillator.
For an (extreme!) example, if
you are using something like a pair of PHEMTs with gain up to 7GHz and
you are trying to make a 180MHz oscillator, then it will take off by
itself unless you get a good grip on the layout!
I don't know
the circuit, though I suspect its maybe of the balanced type known as
"kaliatron", but whatever the mechanism, you need to understand completely
where your oscillator gets the gain energy to amplify some noise that
will break into the desired oscillation mode, and more, you need to
know what gain non-linearity is going to set the level it tops out at.
This might, for example, be done in a controlled way using dual-gate
A thing like "I cannot change my gate bias from -0.6 to
-0.5" usually means the poor thing is revving away at some frequency
you didn't think of, possibly even a low frequency (squegging - some
folk call it). Your transmission line with a FET switching away at each
end is going to present to that line some impedances that may well reflect
energy way outside of the frequency you intend, and have a journey time
that allows this (unwanted) oscillation to arrive with a phase that
Post subject: VoicePosted:
Thu Nov 03, 2005 5:32 pm
Theres a hundred ways this can
happen to you.
You sound rather stressed. )