Shielding data lines to RF circuits

[Richard Hosking]

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Thanks

Richard

 

[Fred Bartoli]

"Richard Hosking" <richardh@iinet.net.au> a écrit dans le message news:
406ad05b$0$16580$5a62ac22@freenews.iinet.net.au...

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Richard,

I don't have direct experience about this but I'm developping a VNA with
pretty tight specs so I had to think about things like this.

Here are some of my reflexions, and also some ideas provided by other people
:

Low output level is not the same thing as low leakages : it depends on where
leakages couples.
What you don't want is leakages coupled to the output port, but you may have
some leakages that will be stopped by your main enclosure (OK, not mW
though) and still maintain low level at the output port. Whether this can be
tolerated or not depends on the rest of your equipment.

Besides the DC/command lines, to achieve accurate low level output will
require good internal shielding between the DDS section and the attenuator :
1dB accuracy at -100dBm means leakages coupled to the output has to be
under -120dBm and 0.1dB lower this to -140dBm.
At this level, ground plane management can be "difficult". Depending on your
frequency range it might also help to provide separation between the DDS and
the attenuator with a small xformer.
You'll have to think to current paths for both cases, high frequency and low
frequency, if your frequency span doesn't allow an xformer, because currents
spreads differently, depending on freq.
I've used fasthenry (a freeware prog) with great success to model this kind
of things in another application.

If you really need very, very good shielding (radiated power inside the
enclosure) it might be easier/better to "double shield" the beast and to add
a lowpass section between the 2 shields.
Again, think to current paths and that common mode is your enemy ("ground"
potential that's shifted by return currents).

For the DDS command lines, optos might help, but maybe not as much as
expected if you want a high speed link.
If you really want high speed and isolation I'd go for an optic fiber. Not
very expensive and coupled with a double shielded box, you'll have
vanishingly low leakages.

Just think to neglect nothing and you're OK.


Thanks,
Fred.

 

[Tom Bruhns]

Fred gave you a very nice bunch of ideas.

For data lines, you might consider using differential signalling. You
can apply common-mode choking fairly easily, without disturbing the
data. But if you put capacitance to ground, you'd have to be very
careful to keep it balanced, and to not have so much it causes signal
degradation.

We fairly regularly deal with high isolation out to 100MHz and a bit
beyond, and it hasn't proven to be terribly difficult. We have things
like switching power supplies, whose noise would cause lots of
problems if it got into the wrong places. One of the key tricks is to
keep loop areas very tight. Another is to be very sure that there is
a return path for everything, and that the return path is close to the
outgoing path. That's really the same as keeping loop areas low.
There is a lot of advantage to using multilayer boards and SMT parts.

Not long ago, I revamped a board that had a heavy cast shield "clam
shell" bolted around a critical area of the board. (It digitizes
wideband analog inputs, and has lots of digital communications going
on not far from the analog stuff.) My new design uses a simple
snap-on folded metal shield, and I'm not even convinced it needs
that...and the spuriouses are at least 20dB lower than the old board,
down in the -120dBfs region as I recall. In laying out the board, we
simply paid a lot of attention to keeping things very tight, taking
advantage of SMT parts and differential signalling. Think of
everything as an antenna that can radiate and/or pick up signals, and
minimize their ability to do that. And realize that current in any
copper conductor causes a voltage drop that you must avoid sharing
with some other circuit you don't want the signal getting into.

I gather you want a stepped attenuator on the output of your DDS. You
may well want to have the attenuator itself be a separate board with
some decent shielding between sections, and essentially DC controls
(to relays??) to control the steps. Common mode choking on the RF
signal cable should be helpful.

Cheers,
Tom

Richard Hosking <richardh@iinet.net.au> wrote in message news:<406ad05b$0$16580$5a62ac22@freenews.iinet.net.au>...

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Thanks

Richard

 

[Rene Tschaggelar]

Richard Hosking wrote:

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Have a look at the ADuM1400 series of magnetic isolators from AD.
Yes, double shielding might be necessary and probably differential
signals. Put the whole into a solid block of Alu.
What attenuator are you going to use ? For what frequency range ?

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[John Jardine]

Richard Hosking <richardh@iinet.net.au> wrote in message
news:406ad05b$0$16580$5a62ac22@freenews.iinet.net.au...

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Thanks

Richard

When I was very poor, I built a spectrum analyser. It took an eternity,
albeit much was learnt. By far the costliest and most time consuming lesson
was finding I hadn't a hope in hell of meeting a decent performance without
enclosing every circuit block in it's own screened casing, each with
extensive 'cleaning' of the incoming power and control lines. Even the power
supply transformer had a mu-metal screen to reduce magnetic field effects.
regards
john

 

[Richard Hosking]

Many thanks for the replies
That has given me some ideas for a start

Richard


Richard Hosking wrote:

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Thanks

Richard

 

[Rene Tschaggelar]

What attenuator are you using ?
-100dBm is a lot of attenuation.

Rene

Richard Hosking wrote:

Many thanks for the replies
That has given me some ideas for a start

Richard


Richard Hosking wrote:

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I
note there are at least two stages of bypassing/low pass filtering
with an intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in
a shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

 

[John Woodgate]

I read in sci.electronics.design that Rene Tschaggelar <none@none.net>
wrote (in <406c4230$0$709$5402220f@news.sunrise.ch&gt about 'Shielding
data lines to RF circuits', on Thu, 1 Apr 2004:

What attenuator are you using ?
-100dBm is a lot of attenuation.

No, it's only 1 dB of attenuation from -99 dBm. 100 dB is a lot of

attenuation (or gain).
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Rene Tschaggelar]

John Woodgate wrote:

I read in sci.electronics.design that Rene Tschaggelar <none@none.net
wrote (in <406c4230$0$709$5402220f@news.sunrise.ch&gt about 'Shielding
data lines to RF circuits', on Thu, 1 Apr 2004:

What attenuator are you using ?
-100dBm is a lot of attenuation.


No, it's only 1 dB of attenuation from -99 dBm. 100 dB is a lot of
attenuation (or gain).

Got me John,
So how to get -99dBm, considering the DDS outputs in the
order of 0dBm.
Are there any decent digital step attenuators around ?
The better switches available at minicircuits have an isolation
of 80dB when off, their analog Attenuator has a range of 55dB.
In 2 stages then ?

Rene

 

[John Woodgate]

I read in sci.electronics.design that Rene Tschaggelar <none@none.net>
wrote (in <406c8cc9$0$709$5402220f@news.sunrise.ch&gt about 'Shielding
data lines to RF circuits', on Thu, 1 Apr 2004:

John Woodgate wrote:
I read in sci.electronics.design that Rene Tschaggelar <none@none.net
wrote (in <406c4230$0$709$5402220f@news.sunrise.ch&gt about 'Shielding
data lines to RF circuits', on Thu, 1 Apr 2004:

What attenuator are you using ?
-100dBm is a lot of attenuation.


No, it's only 1 dB of attenuation from -99 dBm. 100 dB is a lot of
attenuation (or gain).

Got me John,
So how to get -99dBm, considering the DDS outputs in the
order of 0dBm.
Are there any decent digital step attenuators around ?
The better switches available at minicircuits have an isolation
of 80dB when off, their analog Attenuator has a range of 55dB.
In 2 stages then ?

Yes, it's often the simplest solution.

--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Richard Hosking]

I thought I would use switched sections with various sections switched
by relays - each section shielded if necessary.
The DDS will give an output down to -50dBm or so under software control,
though spurious outputs are not necessarily attenuated by the same
amount, so it is probably not a good idea to use the full attenuation range
My board will be double sided unless I *need* to use a more expensive board

Richard

Rene Tschaggelar wrote:

What attenuator are you using ?
-100dBm is a lot of attenuation.

Rene

Richard Hosking wrote:

Many thanks for the replies
That has given me some ideas for a start

Richard


Richard Hosking wrote:

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I
note there are at least two stages of bypassing/low pass filtering
with an intermediate shielded section

My question is: how do I get data and power lines into my DDS chip
(in a shielded enclosure) and prevent RF leakage out which will limit
the useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean
data rates in the MHz region.

 

[Rene Tschaggelar]

I'm on a similar project and had a look at these issues.
Depending on the frequency and the relay, the isolation of an
off contact can be as low as 25dB. I assume you'd plan on changers
(SPDT) with the idle on 50 Ohms. The relay attenuator sure uses
a lot of board state.
The minicircuits variable attenuator unfortunately doesn't go to DC,
and while spec'ed to 10MHz, I didn't measure the lower border myself
yet. The minicircuits step attenuator have no steps greater than 20dB
and are rather pricey. They also have a lower border of 10MHz.

I'm also not yet sure how to get rid of the digital noise, especially
when using high update rates. I'm considering an FPGA to have the
lowest possible digital clock frequency for the amount of operations.

Rene



Richard Hosking wrote:

I thought I would use switched sections with various sections switched
by relays - each section shielded if necessary.
The DDS will give an output down to -50dBm or so under software control,
though spurious outputs are not necessarily attenuated by the same
amount, so it is probably not a good idea to use the full attenuation range
My board will be double sided unless I *need* to use a more expensive board

Richard

Rene Tschaggelar wrote:

What attenuator are you using ?
-100dBm is a lot of attenuation.

Rene

Richard Hosking wrote:

Many thanks for the replies
That has given me some ideas for a start

Richard


Richard Hosking wrote:

Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (<-100dBm)
I have a venerable HP8640B which can give a calibrated output to
-137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I
note there are at least two stages of bypassing/low pass filtering
with an intermediate shielded section

My question is: how do I get data and power lines into my DDS chip
(in a shielded enclosure) and prevent RF leakage out which will
limit the useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data
lines and extensive bypassing on the power lines. I want reasoanbly
quick update speeds for my DDS, which is a serial port, which will
mean data rates in the MHz region.