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oscillation - RF Cafe Forums
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karthik
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Post subject: oscillation
Posted: Fri Jan 29, 2010 12:22 am
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
I have been testing an L-band front-end with a 2
port RF switch followed by a lna, phase shifter
and attenuator. I get the gain that I expect out
of the board, but when I hook it up to a spectrum
analyzer (with the input of the board terminated
in a load), I see some low level oscillations (<-75dBm)
starting at a few KHz, all the way up to 2-ish GHz
and then goes quiet after that. I've been trying
to find the source of the oscillation and havent
yet found it. When I switch ports on the RF switch,
the amplitude of the oscillation (and some of the
peaks) change. The only time the oscillation goes
away is when I remove the dc blocking cap between
the switch and LNA, leaving the input of the LNA
open. I've played around with the bias & blocking
caps and inductor on the LNA and even bypassed the
on-board voltage regulators/power conditioning to
rule noise on the power lines out. I am using 2
pins on a PIC microcontroller to directly toggle
the switch control lines.
I would appreciate
any guidance on what to do next.
Karthik
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fred47 |
Post subject: Re: oscillation
Posted: Fri Jan 29, 2010 4:32 pm
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 104 |
Low-level and covering a large range of frequencies:
1. Doesn't sound like LNA instability - that would
almost certainly be bigger in amplitude, unless
your LNA has an unbelievably bad IP3. 2. Disappears
when RF switch is disconnected - can't be bias related,
as the capacitor blocks DC. 3. RF switch is driven
from a PIC - which continues to run.
Sounds
like you're getting a signal coupled into the RF
path from the PIC via the switch.
First
thing to do is to put a scope probe on the control
leads of the switch, AC coupled, and set to the
highest possible sensitivity. Any noise, and you've
found your culprit.
To test this hypothesis
in-circuit, disconnect the two leads from the PIC
to the switch, drive them to the desired voltages <i>
with a clean source </i> (not the PIC supply
voltage!), but leave the capacitor between the switch
and the LNA.
Good luck!
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karthik |
Post subject: Re: oscillation
Posted: Fri Jan 29, 2010 4:44 pm
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
Hi fred,
just did that...cut the traces from
the pic and soldered jumpers hooked to a power supply...did
not change a thing. Im beginning to wonder if the
ground pad on the lna under the chip isnt soldered
properly. But then, i have a hundred boards, and
all seem to be behaving similarly. Is there any
way to check for a floating ground without pulling
everything apart?
Karthik
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fred47 |
Post subject: Re: oscillation
Posted: Fri Jan 29, 2010 4:51 pm
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 104 |
Is there any chance that noise could be coupled
in via the power pin of the RF switch? (I'm assuming
you have a solid ground plane under the whole thing.)
Another thing to try would be lifting the power
to the PIC on your test board. (That's a subset
of "try to find where the source of the noise is"...)
Its on-chip oscillator may be affecting things.
As far as the ground is concerned, you haven't
given many details, so I can't help much there.
good luck - and keep us informed, this sounds
like it might get interesting!
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karthik |
Post subject: Re: oscillation
Posted: Fri Jan 29, 2010 5:00 pm
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
The only power going into the switch is through
the control lines. As for the ground plane, the
LNA is a qfn package with a ground paddle on the
lower surface...I dont know how to verify if that
ground connection has been made. The board layout
does have that pad connected to gnd, but.....
So far, the only time the oscillation/noise
died away was with the dc blocking cap disconnected.
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fred47 |
Post subject: Re: oscillation
Posted: Fri Jan 29, 2010 5:08 pm
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 104 |
Where I was going with removing the power
from the PIC
is that the current leaving the
ground pin
of the PIC might be coupling into the switch via
a common impedance in the ground return.
Paddles on the bottom of a chip are sometimes
a problem - but if you have multiple vias on the
PCB under the paddle, I think you don't need to
worry about the ground connection. Still, if your
LNA chip has a pin or pad which isn't the paddle,
you should check there. (Not that I think that's
likely - you would already have checked that, I
think).
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karthik |
Post subject: Re: oscillation
Posted: Sat Jan 30, 2010 9:08 pm
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
fred,
As you suggested, I hooked the oscilloscope
on the switch control lines, and there was noise
on the lines. Here is the part that I dont have
an answer for. When I disconnected the switch control
lines from the PIC and left the lines floating,
there was noise on the output dc blocking cap of
the LNA! When I connected the control lines to a
'clean' source, the noise went away - both on the
output of the LNA and the 'oscillation'. Now, this
worked on one board. I need to repeat this on another
board/boards to make sure this is in-fact the solution..
Thanks for the help..Truly appreciate it
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fred47 |
Post subject: Re: oscillation
Posted: Mon Feb 01, 2010 12:59 pm
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 104 |
Hi Karthik,
When you say "noise on the lines"
going to the RF switch, is that
random noise
or a periodic
signal?
Just curious!
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karthik |
Post subject: Re: oscillation
Posted: Mon Feb 01, 2010 1:32 pm
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
It looks like a 1KHz sawtooth riding on top of the
control pulse. And it is 180 deg out of phase between
the 2 control lines (though they are on different
outputs of the PIC). Other output lines on the PIC
are clean. And this noise is visible on the PIC
even when the switch is not connected to it....might
need either a pullup on the PIC or a cap to filter
it out...atleast that is the thinking right now...till
im proved wrong
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karthik |
Post subject: Re: oscillation - update
Posted: Fri Feb 05, 2010 10:21 pm
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Joined: Tue Apr 11, 2006
8:13 pm Posts: 34 |
I've spent the last week running more tests. I find
that cleaning up the noise on the switch control
lines removed most of the 'oscillation' on the RF
side. Most of it, not all. I eliminated the noise
using a series resistor and a cap to ground on each
control line. I have 2 switches hanging on the line
with the control lines Tee'd off from the PIC to
each switch. The cap to ground is on each control
line of each switch, while the resistor is common
to both switches on each line. The control lines
on the 2 switches are interchanged. By that, I mean,
the control lines on switch 1 is connected as A-B
and on switch 2 it is B-A. That way, when I send
A=1, B=0, switch 1 looks at port A and switch 2
looks at port B and vice versa. Now, after cleaning
up the control line (with a series resistor and
a cap to ground), A=1,B=0 (switch 1 looking at port
A, switch 2 looking at port B) is clean, but A=0,B=1
(switch 1 looking at port B, switch 2 looking at
port A) is still noisy on both switches. Both switches
are looking at identical loads. When I remove the
input cap (on the noisy port) on the switch and
place a 50ohm resistor to ground, the noise goes
away. When I put the cap in, the noise returns.
Is this an impedance mismatch issue? If it is, why
is it oscillating on one port and not another? The
microstrips are as identical as they can be on both
switch ports. Why would this happen? The noise on
both switch line look identical (in the sense that
it does not appear as though there is periodicity
on either line) and it is fairly small <5mVp-p.
Any suggestions will be greatly appreciated
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Posted 11/12/2012
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