Offbeat microwave question

[Phil Hobbs]

So one day soon I'm going to have these two SMA-mounted photodiodes that
work up to about 10 GHz. Conveniently, they have built-in bias tees but
don't have built-in load resistors, so they need a decent-quality
termination.

They're part of a differential Doppler interferometer producing tone
bursts of very roughly 200 cycles, immersed in wideband white noise, to
the point of being completely invisible on a scope.

I need to build/buy/cobble together a splitter/combiner that works from
DC to 8 GHz. I can buy resistive ones, but they have a 6- or 7-dB
insertion loss, which means that by the time I'm done attaching an amp
with a 3-dB noise figure, I'll wind up with about a 3000K noise
temperature. Those photons are expensive, so this is clearly a non-starter.

I'd also like the two paths to be phase-matched to within 5 degrees or
so, and the gains to be equal to within, say, 0.3 dB. That way I have
some hope of getting 15 or 20 dB of laser noise suppression. On the plus
side, because of the signal characteristics, I don't care much about the
time domain response, and a few amplitude whoopdedoos aren't worrisome
as long as they aren't too deep or too broad.

So, I'm thinking of just siamesing the two detectors together via
matched pieces of RG-402 semirigid coax (0.141 inch) and either a T
connector or some sort of artistically sculpted microstrip thing, e.g. a
vanilla Wilkinson coupler tuned to some higher frequency.

I'm sort of leaning towards the tee connector to start with, because the
actual input resistance of the average connectorized amplifier is around
25 ohms, iirc, so the match should be reasonable, and the degradation in
the noise figure due to going a bit off the noise match shouldn't be too
bad.

True? False? Smart? Stupid? Other suggestions?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Joerg]

So one day soon I'm going to have these two SMA-mounted photodiodes that
work up to about 10 GHz. Conveniently, they have built-in bias tees but
don't have built-in load resistors, so they need a decent-quality
termination.

They're part of a differential Doppler interferometer producing tone
bursts of very roughly 200 cycles, immersed in wideband white noise, to
the point of being completely invisible on a scope.

I need to build/buy/cobble together a splitter/combiner that works from
DC to 8 GHz. I can buy resistive ones, but they have a 6- or 7-dB
insertion loss, which means that by the time I'm done attaching an amp
with a 3-dB noise figure, I'll wind up with about a 3000K noise
temperature. Those photons are expensive, so this is clearly a
non-starter.

I'd also like the two paths to be phase-matched to within 5 degrees or
so, and the gains to be equal to within, say, 0.3 dB. That way I have
some hope of getting 15 or 20 dB of laser noise suppression. On the plus
side, because of the signal characteristics, I don't care much about the
time domain response, and a few amplitude whoopdedoos aren't worrisome
as long as they aren't too deep or too broad.

So, I'm thinking of just siamesing the two detectors together via
matched pieces of RG-402 semirigid coax (0.141 inch) and either a T
connector or some sort of artistically sculpted microstrip thing, e.g. a
vanilla Wilkinson coupler tuned to some higher frequency.

I'm sort of leaning towards the tee connector to start with, because the
actual input resistance of the average connectorized amplifier is around
25 ohms, iirc, so the match should be reasonable, and the degradation in
the noise figure due to going a bit off the noise match shouldn't be too
bad.

True? False? Smart? Stupid? Other suggestions?

A Wilkinson is resonant when it comes to isolation but I guess you don't
need isolation here.

If you use coax make sure both end right at the amplifier input. If you
can't do that go from the point where the two cables joint to the amp
input with two parallel coaxes. So that the impedance is half. Otherwise
you'd see a big reflection from that point.

Regarding input impedance, are you sure the noise-optimal input
impedance is 25ohms? If not, maybe it can be tweaked to be around there.

 

[Robert Baer]

So one day soon I'm going to have these two SMA-mounted photodiodes that
work up to about 10 GHz. Conveniently, they have built-in bias tees but
don't have built-in load resistors, so they need a decent-quality
termination.

* DIY termination? Perhaps use two 100 ohm 0603 in a physical "Y"to
ground plane (am presuming 50 ohm system)?

** snipped to keep aioe happy **

I need to build/buy/cobble together a splitter/combiner that works from
DC to 8 GHz. I can buy resistive ones, but they have a 6- or 7-dB
insertion loss,

* Am assuming a balun transformer is not much better..

** snipped to keep aioe happy **

So, I'm thinking of just siamesing the two detectors together via
matched pieces of RG-402 semirigid coax (0.141 inch) and either a T
connector or some sort of artistically sculpted microstrip thing, e.g. a
vanilla Wilkinson coupler tuned to some higher frequency.

* Did not know about the Wilkinson coupler and was going to suggest
tapered transmission line "Y".
But hoo nose? maybe the darn resistor can be tossed (when properly
matched) for better noise and maybe lower loss.

** snipped to keep aioe happy **

 

[Robert Baer]

Joerg wrote:
** snipped to keep aioe happy **

If you use coax make sure both end right at the amplifier input. If you
can't do that go from the point where the two cables joint to the amp
input with two parallel coaxes. So that the impedance is half. Otherwise
you'd see a big reflection from that point.

Regarding input impedance, are you sure the noise-optimal input
impedance is 25ohms? If not, maybe it can be tweaked to be around there.

Tapered transmission lines are one conventional way for impedance
"conversion".

 

[Phil Hobbs]

Joerg wrote:
** snipped to keep aioe happy **
Tapered transmission lines are one conventional way for impedance
"conversion".

Hard to do over multi-decade bandwidths, though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

Why not dump each pd into a 20+ GHz MMIC first? That would get your
s/n up before you start mixing and such.

If you're already SMA, the MMICS could be bought in cans, with
connectors. Tweak the MMIC power supplies to trim differential gain,
and stick an SMA trombone in one or both legs of the outputs to tweak
phase.

The input impedances of MMICS does tend to be low, but Vcc trims that,
too. I've seen some that cross right through 50 ohms. It also depends
on the output load impedance, because of the feedback resistor in the
usual Darlington gain stage. I have low-level-TDRd MMICS to measure
their wideband input impedance.

I'll have to get a few and do that. The difficulty with separate paths
is that it'll be really hard to get the gain and phase to track well
enough over frequency to have any decent subtraction performance.

Ideally I'd like to use a split cell, but they're not that easy to get.
Discovery Semi supposedly makes them, but they give you the third
degree before they'll even give you a datasheet, and that's pretty
off-putting.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

* DIY termination? Perhaps use two 100 ohm 0603 in a physical "Y"to
ground plane (am presuming 50 ohm system)?

** snipped to keep aioe happy **
* Am assuming a balun transformer is not much better..

I thought about wiring them in series instead, with some Type 61 ferrite
sleeves over RG-405 coax for broadbanding, and a pHEMT or something as
the front end. I'd do that at 1 GHz in a heartbeat, and I've done the
copper tape and XActo thing up to about 3 GHz, but 8 GHz is a little
more difficult.


Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Joerg]

Hard to do over multi-decade bandwidths, though.

Yes, and you optical guys often need "DC to visible light" bandwidth.

 

[Tim Williams]

I've been trying to get ballpark quotes on deep-drawn aluminum cans,
and some of the makers do everything possible to avoid selling them.
One told me that they will have to go out and get pricing on aluminum
before they can quote.

Needing something bigger than cookie tins? ;-)

Tim

 

[Neon John]

Why do so many companies defend themselves against customers? It's not
as if their competitors have any real trouble getting data sheets.

I've been trying to get ballpark quotes on deep-drawn aluminum cans,
and some of the makers do everything possible to avoid selling them.
One told me that they will have to go out and get pricing on aluminum
before they can quote.

John, that kind of crap is why we went to China for our custom
extrusions. The prices were less than one tenth that of anything we
got quoted in the US and the vendors fell all over themselves to work
with us.

John
fluxeon.com
John DeArmond
http://www.neon-john.com
http://www.fluxeon.com
Tellico Plains, Occupied TN
See website for email address

 

[amdx]

I've used coax Christmas trees to make pulse sequences from an SD-24.
Working out where to place the stubs and how long they should be is fun.

In the photodiode case it isn't as bad, because I can float the cases.
Thus I'd only need the balun to work down to a megahertz or so, which is
probably doable with some combination of 43 and 61 ferrite beads on a
reasonable length of RG 402 or 405. I ordered a few dozen from Amidon
yesterday, so they should be in by the time I get back from the wilds of
Orange County. (How do people stand living in Irvine? Inquiring minds,
and all that.)

BTW when did RG-402 get so stinking expensive? I used to pay more like
50 cents a foot, and now I see it's more like six bucks a foot. Pretty
steep for just tinning the shield of some RG188!

Cheers

Phil Hobbs

Do think this RG-402 made in China has the quality desired?
$1.50 per foot.
http://www.ebay.com/itm/40-1M-3FT-s...888?pt=LH_DefaultDomain_0&hash=item4ab437ae50

Mikek

 

[[email protected]]

Well, I think I'm the local record holder, anyway--I've built 200 THz
crystal radios.

Cheers
Phil Hobbs

Can't help but join the who-does-the-brodaest-band bragging
contest by mentioning the Athena+ detectors, which are square-law
from dc to two-and-half million terahertz. Well, input match is
less than phenomenal in some sections of the band. Joel Ullom does
even broader a band, but he does not write to s.e.d....

Regards,
Mikko

 

[[email protected]]

Yeah, but we were talking 200 THz _circuitry_, here.

Quasi-optical, I suppose? What's wrong with Wolter optics?

What constitutes 'circuitry' is an interesting question, actually.
I wonder how much microwave gadgetry could be reproduced with
grazing-angle techniques? Phase relations would be tough to maintain.
Still, I recall vaguely some bold plans to construct an interferometric
X-ray telescope. It was supposedly going to be able to resolve surface
patterns on Sirius.

(What's Finnish for "Yo' momma"?)

" Äitis oli ", I guess. Fits well

Regards,
Mikko

 

[[email protected]]

Nope. Real circuitry--waveguide-coupled metal antennas and travelling
wave metal-insulator-metal tunnel junctions.

Wow, that looks pretty neat. That's way above the superconductive
gap frequencies, so there's no way for me to even try to counter. You're
the champ. I feel too tired now to figure out how you manage low-loss
propagation at such high frequencies. Reminds me about the suggestion in
the Kraus' old book on antennas about harvesting visible light with
antennas and rectifiers.

With how many years' integration time? The shot noise in a measurement
like that would _not_ be pretty.

Right. I haven't been following the subject, but a bit of digging
revealed this...

http://bhi.gsfc.nasa.gov/docs/vision.html

....which sounds remotely familiar.

Regards,
Mikko

 

[Charlie E.]

In the photodiode case it isn't as bad, because I can float the cases.
Thus I'd only need the balun to work down to a megahertz or so, which is
probably doable with some combination of 43 and 61 ferrite beads on a
reasonable length of RG 402 or 405. I ordered a few dozen from Amidon
yesterday, so they should be in by the time I get back from the wilds of
Orange County. (How do people stand living in Irvine? Inquiring minds,
and all that.)

Phil Hobbs

Ah, the People's Republic of Irvine. Very nice place, as long as you
like lots and lots of other people, and are not in a hurry to get
anywhere!

Lived there for ten years before moving out to the desert. In Irvine,
it took me about 20 minutes to get to Costco (about 2.5 miles.) Today,
it take me about 20 minutes to get to Costco (25 miles!)

But, if you like High Suburban life, Irvine is a nice place. It has
just about anything you might want close by...

 

[Phil Hobbs]

Ah, the People's Republic of Irvine. Very nice place, as long as you
like lots and lots of other people, and are not in a hurry to get
anywhere!

Lived there for ten years before moving out to the desert. In Irvine,
it took me about 20 minutes to get to Costco (about 2.5 miles.) Today,
it take me about 20 minutes to get to Costco (25 miles!)

But, if you like High Suburban life, Irvine is a nice place. It has
just about anything you might want close by...

Except restaurants that aren't part of a chain, or a used bookstore, or
walking trails with trees, or anyplace that isn't covered in six inches
of glitzy roofie plastic from China. (Or so it seems.)

Cheers

Phil Hobbs

(Whose aversion to Orange County grows every time he visits the place.)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[[email protected]]

Ah, the People's Republic of Irvine. Very nice place, as long as you
like lots and lots of other people, and are not in a hurry to get
anywhere!

Lived there for ten years before moving out to the desert. In Irvine,
it took me about 20 minutes to get to Costco (about 2.5 miles.) Today,
it take me about 20 minutes to get to Costco (25 miles!)

But, if you like High Suburban life, Irvine is a nice place. It has
just about anything you might want close by...

OK, but it seems that even though it is close by, you can't get there
from here. ...and I suppose if it isn't close by, you don't want it
(or better not)? No thanks. You can have the whole state.

 

[Charlie E.]

Except restaurants that aren't part of a chain, or a used bookstore, or
walking trails with trees, or anyplace that isn't covered in six inches
of glitzy roofie plastic from China. (Or so it seems.)

Cheers

Phil Hobbs

(Whose aversion to Orange County grows every time he visits the place.)

Oh, there are non-chain restaurants, as well as some small, local
chain places. My favorite mexican place is Jalapeno's, a small family
owned chain there. I am not sure about a used book store, been a
while since I saw one anywhere! There are several walking trails,
such as the one along the creek, that have trees, but they are also
right outside someone's windows!

Have they ever opened the park yet? They took over the old El Toro
airbase, and were supposed to build a park there...

 

[Charlie E.]

OK, but it seems that even though it is close by, you can't get there
from here. ...and I suppose if it isn't close by, you don't want it
(or better not)? No thanks. You can have the whole state.

Well, nothing wrong with Orange county that a few million people
leaving wouldn't fix! ;-)

 

[[email protected]]

Well, nothing wrong with Orange county that a few million people
leaving wouldn't fix! ;-)

No, that's OK. They can stay there. You all can. Quarantine, as it
were. ;-)