help needed: reducing 2nd harmonic in 1W transmitter

J

John

Guest
I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!
 
On 24 May 2004 23:09:41 -0700, John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!
Click to expand...
Assuming the LPFs are working, the obvious (to me) problem is that
the signal is making an end run around them. PCB layout and
shielding should be suspect.

Sound possible?
--
Best Regards,
Mike
 
John wrote:
I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Click to expand...
It is unlikely that an LPF consisting of a few L & C does not
work as expected. You are aware that an LPF doesn't absorb the
2nd harmonic but reflects it ? You therefore cannot cascade filters.
Their transmission behaviour is based on 50 Ohms at the input and 50
Ohms at the output. And a filter doesn't have 50 Ohms at the
frequency where it reflects.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
Activ8 <reply2group@ndbbm.net> wrote in message news:<159m33dmybdf4$.dlg@news.individual.net>...
On 24 May 2004 23:09:41 -0700, John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!

Assuming the LPFs are working, the obvious (to me) problem is that
the signal is making an end run around them. PCB layout and
shielding should be suspect.

Sound possible?
Thanks, I think that's the most logical explanation.
Click to expand...

I'm thinking of putting inductor beads in series with all the ground
connections of my power amp. Is this a good idea? If I do this will
there be a potential difference between system ground and my power
amp's ground? I put beads on all Vcc connections of the power amp and
it lessened harmonics and spurious signals.
 
On 25 May 2004 03:26:50 -0700, John wrote:

Activ8 <reply2group@ndbbm.net> wrote in message news:<159m33dmybdf4$.dlg@news.individual.net>...
On 24 May 2004 23:09:41 -0700, John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!

Assuming the LPFs are working, the obvious (to me) problem is that
the signal is making an end run around them. PCB layout and
shielding should be suspect.

Sound possible?
Thanks, I think that's the most logical explanation.

I'm thinking of putting inductor beads in series with all the ground
connections of my power amp.

Is this a good idea?
Click to expand...
Hell no. You want low inductance ground paths. In fact, your
grounding might be the main problem. If you can post a picture of
your layout to abse, it'd help.

If I do this will
there be a potential difference between system ground and my power
amp's ground?
Click to expand...
Yeah, at RF. What is "system"?

I put beads on all Vcc connections of the power amp and
it lessened harmonics and spurious signals.
Click to expand...
Chokes on Vcc are typical.
--
Best Regards,
Mike
 
On Tue, 25 May 2004 12:09:27 +0200, Rene Tschaggelar wrote:

John wrote:
I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?

It is unlikely that an LPF consisting of a few L & C does not
work as expected. You are aware that an LPF doesn't absorb the
2nd harmonic but reflects it ? You therefore cannot cascade filters.
Their transmission behaviour is based on 50 Ohms at the input and 50
Ohms at the output. And a filter doesn't have 50 Ohms at the
frequency where it reflects.

Rene
Click to expand...
IIRC, you could put an LC shunt between the two LPFs to deal with
the reflections, right? I've seen it called a diplexer, but I've
always used the term "diplexer" for a frequency selective splitter
like passing one band upstream and another downstream.
--
Best Regards,
Mike
 
John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!
Click to expand...

How about a shorted 1/4 wave stub? At that sort of frequency you may be able
to fit it right on the board?

Regards, Dan.
--
And on the evening of the first day, the lord said.... LX1, Go!
And there was light.
The email address *IS* valid, do not remove the spamblock.
 
"Dan Mills" <dmills@spamblock.demon.co.uk> wrote in message
news:c90det$708$1$8300dec7@news.demon.co.uk...
John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level.
Click to expand...
Alarm.

If you insert a known working filter, and the signal you're trying to filter
didn't go down (much) then you have a problem. Possibly a bad ground path
between the filter and the system.
Or, the signal is getting to the output by other means.

PCB layout is crucial up here.

The other thing to look at, is why do you have so much to get rid of in the
first place?
The easiest way to control noise is not to make it.
Change the bias on an oscillator/amp stage?


I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.
Click to expand...
Unless the filter isn't rated for the application, you should see the same
40dB loss when in circuit.


How about a shorted 1/4 wave stub? At that sort of frequency you may be
able
to fit it right on the board?
Click to expand...
Not a bad idea, but it may be rather large.
 
On Tue, 25 May 2004 16:35:00 -0500, Dave VanHorn wrote:

"Dan Mills" <dmills@spamblock.demon.co.uk> wrote in message
news:c90det$708$1$8300dec7@news.demon.co.uk...
John wrote:

I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic). I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level.

Alarm.

If you insert a known working filter, and the signal you're trying to filter
didn't go down (much) then you have a problem. Possibly a bad ground path
between the filter and the system.
Or, the signal is getting to the output by other means.

PCB layout is crucial up here.

The other thing to look at, is why do you have so much to get rid of in the
first place?
The easiest way to control noise is not to make it.
Change the bias on an oscillator/amp stage?

I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.

Unless the filter isn't rated for the application, you should see the same
40dB loss when in circuit.

How about a shorted 1/4 wave stub? At that sort of frequency you may be
able
to fit it right on the board?

Not a bad idea, but it may be rather large.
Click to expand...
Around 19 mm on FR-4 - .75 in - might be able to make it into a
hairpin or just put one 90 in it to save space.

what about a radial stub at the second harmonic? Can't say that'd be
smaller off the top of my head, but it would save needing a j0 cap -
well, er, that's assuming the stubs location has a DC path that
would affect the bias - a real short to ground would screw that up.
--
Best Regards,
Mike
 
I guess it's safe to conclude that the harmonics are bypassing my LPF.
The next step for me would be to redo the layout. Thanks for all those
who replied!
 
"John" <juan.alcasabas@eazix.com> wrote in message
news:b2ed7347.0405251825.22a4cc8d@posting.google.com...
I guess it's safe to conclude that the harmonics are bypassing my LPF.
The next step for me would be to redo the layout. Thanks for all those
who replied!
Click to expand...
Before you redo the layout, you need to find out why it's happening.
Hard to help more, without more information.
(I am looking for a job at the moment.. :)
 
I just finished experimenting on my transmitter and I found something
strange. I put a coax on my power amp's output and fed the signal to
an LPF assembled on a different PCB. The 2nd harmonic (1830MHz) was
not attenuated at all in fact it was a bit larger. I tried an agilent
wide-band 6dB attenuator (DC-12.4GHz) but the harmonic was not
attenuated unlike the fundamental. I have no idea why the harmonic is
not affected but I think it can't be caused by my layout since the
filter and attenuator I tested were not on the same PCB as my
transmitter's power amp.

Do filters or the passive components in them have an input power
threshold that degrades their frequency selectivity? I'm inputting a
915MHz signal @ +30dBm into my filters and their selectivity is not as
good as when I input +14dBm from my signal generator.
 
On 26 May 2004 01:38:24 -0700, John wrote:

I just finished experimenting on my transmitter and I found something
strange. I put a coax on my power amp's output and fed the signal to
an LPF assembled on a different PCB. The 2nd harmonic (1830MHz) was
not attenuated at all in fact it was a bit larger. I tried an agilent
wide-band 6dB attenuator (DC-12.4GHz) but the harmonic was not
attenuated unlike the fundamental. I have no idea why the harmonic is
not affected but I think it can't be caused by my layout since the
filter and attenuator I tested were not on the same PCB as my
transmitter's power amp.

Do filters or the passive components in them have an input power
threshold that degrades their frequency selectivity? I'm inputting a
915MHz signal @ +30dBm into my filters and their selectivity is not as
good as when I input +14dBm from my signal generator.
Click to expand...
That's also possible. You should check the specs, an app note, FAQ,
or call the mfg. Could be that the rf is making an end run around
the internal circuitry.

You may end up using a stripline filter or stub or something.
--
Best Regards,
Mike
 
On 26 May 2004 01:38:24 -0700, juan.alcasabas@eazix.com (John) wrote:

I just finished experimenting on my transmitter and I found something
strange. I put a coax on my power amp's output and fed the signal to
an LPF assembled on a different PCB. The 2nd harmonic (1830MHz) was
not attenuated at all in fact it was a bit larger. I tried an agilent
wide-band 6dB attenuator (DC-12.4GHz) but the harmonic was not
attenuated unlike the fundamental. I have no idea why the harmonic is
not affected but I think it can't be caused by my layout since the
filter and attenuator I tested were not on the same PCB as my
transmitter's power amp.

Do filters or the passive components in them have an input power
threshold that degrades their frequency selectivity? I'm inputting a
915MHz signal @ +30dBm into my filters and their selectivity is not as
good as when I input +14dBm from my signal generator.
Click to expand...
Test gear certainly does. You may need to verify that your problem is not in
the level measuring device. The quickest way is to feed in via coax and note
the level of fundamental and harmonic of interest. Then insert an attenuator
and note the reduction in both these signal components. If the harmonic drops
by say double the attenuator value, there is a high probability that your signal
detector (specan or whatever) is being overdriven.
 
"John" <juan.alcasabas@eazix.com> wrote in message
news:b2ed7347.0405260038.3cd913d8@posting.google.com...
I just finished experimenting on my transmitter and I found something
strange. I put a coax on my power amp's output and fed the signal to
an LPF assembled on a different PCB. The 2nd harmonic (1830MHz) was
not attenuated at all in fact it was a bit larger. I tried an agilent
wide-band 6dB attenuator (DC-12.4GHz) but the harmonic was not
attenuated unlike the fundamental. I have no idea why the harmonic is
not affected but I think it can't be caused by my layout since the
filter and attenuator I tested were not on the same PCB as my
transmitter's power amp.
Click to expand...
Could you be overdriving the input to your analyzer?
Drop in a 10 or 30dB pad, and see if you get the same results.
I hope you didn't smoke the input! These things do have limits.

Do filters or the passive components in them have an input power
threshold that degrades their frequency selectivity? I'm inputting a
915MHz signal @ +30dBm into my filters and their selectivity is not as
good as when I input +14dBm from my signal generator.
Click to expand...
Generally not, but everything has an upper limit.
I've never really tried to do what you're describing, but common sense says
that any filter has an upper limit on power.
 
John wrote:
I have a +30dBm, 915Mhz transmitter. My power amp stage consists of an
RFMD IC (RF2173). The 2nd harmonic is at -17dBm which is consistent
with the power amp's specs (-38dbc, 2nd harmonic).
Click to expand...
-17dBm at +30dBm put means +77dBm OIP2 which sounds about right.

I have tried
reducing this by putting 2 chip LPFs (900MHz cut-off, 3W rating) but
the 2nd harmonic is roughly at the same level. I tested the LPF up to
+14dBm (the max power I can get from our signal generator) and the
attenuation at 1830MHz is around 40dBm.
Click to expand...
40dBm is NOT a measure of attenuation- and the 3W rating of the LPF is
too vague- usually means a maximum CW input without permanent damage and
says nothing about its filtering characteristics at that power level-
but the switch from dBm to Watts is a giveaway that the rating is a
damage limit and not a performance spec.

I am new to RF microwave design. I have no idea why the LPFs are not
working. Does anybody have an explanation/solution to my problem?
Thanks!
Click to expand...
Assuming the LPF is in fact working, the 2nd harmonic at its output
would be -57dBm and this is -87dBc below carrier- NOT a measurement that
can be made or characteristic preserved without extreme care. The most
likely cause is a combination of cable radiation and overdriving the
instrumentation. Think shielding- get rid of that cable run between the
PA and LPF- and use a directional coupler for the instrument tap.
 
juan.alcasabas@eazix.com (John) wrote in message news:<b2ed7347.0405260038.3cd913d8@posting.google.com>...
I just finished experimenting on my transmitter and I found something
strange. I put a coax on my power amp's output and fed the signal to
an LPF assembled on a different PCB. The 2nd harmonic (1830MHz) was
not attenuated at all in fact it was a bit larger. I tried an agilent
wide-band 6dB attenuator (DC-12.4GHz) but the harmonic was not
attenuated unlike the fundamental. I have no idea why the harmonic is
not affected but I think it can't be caused by my layout since the
filter and attenuator I tested were not on the same PCB as my
transmitter's power amp.

Do filters or the passive components in them have an input power
threshold that degrades their frequency selectivity? I'm inputting a
915MHz signal @ +30dBm into my filters and their selectivity is not as
good as when I input +14dBm from my signal generator.
This can also be caused by the output impedance of your amp's output
Click to expand...
not being 50-Ohm (unlike signal generator's), so the filter doesn't
work properly.
At these frequencies RF leakage and bypass effects can also be caused
by poor quality cables and connectors - just like bad PCB layout.
Regards,
Eugene
 
In article <c90det$708$1$8300dec7@news.demon.co.uk>,
dated Tue, 25 May 2004 22:22:37 +0100,
Dan Mills, <dmills@spamblock.demon.co.uk> says...




How about a shorted 1/4 wave stub? At that sort of frequency you may be able
to fit it right on the board?
Click to expand...
How about an open 1/4 wave stub for the second harmonic ?

--
"You cannot move mountains if you believe them to be mountains.
You must think of them as collections of small stones,
Which can be moved one at a time, and then reassembled."
-- The Tao of Meow
 
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