On Sat, 07 Jan 2006 16:49:52 +0100, Paul Burridge
k wrote:

On Fri, 06 Jan 2006 19:40:27 -0700, Wes Stewart
wrote:


http://cgi.ebay.com/HP-909A-50-Ohm-C...QQcmdZViewItem

Thanks, Wes. I'll keep an eye on it.

Secondly, a 50 ohm power splitter (one feed-in; three outputs) N-type
connections, again flat up to 1.3Ghz. No switching needed, thankfully.

I must ask, what is the purpose of this?

The (thumping great) service manual that came with this VNA gives
various levels of tests that can be performed oneself prior sending
the thing away for calibration. The power splitter together with a few
other basic items enables the analyzer to 'check itself' for
fundamental operating fitness.

Okay, I couldn't figure out what operational use it would have.

On Sat, 07 Jan 2006 10:25:18 -0700, Wes Stewart
wrote:


Okay, I couldn't figure out what operational use it would have.

Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?
--

"What is now proved was once only imagin'd" - William Blake

On Sun, 08 Jan 2006 01:07:20 +0100, Paul Burridge
k wrote:

On Sat, 07 Jan 2006 10:25:18 -0700, Wes Stewart
wrote:


Okay, I couldn't figure out what operational use it would have.

Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?

It's probably a lumped element Wilkinson power divider design and they work at a
specific frequency range the optimum Z match is where the 1/4 wavelength is set.

Paul Burridge wrote:
On Sat, 07 Jan 2006 10:25:18 -0700, Wes Stewart
wrote:



Okay, I couldn't figure out what operational use it would have.


Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?

If all you need is a resistive power divider, the frequency limits
should be generous. If you want efficient power dividers (with little
loss in the splitter itself) you use wideband transformers to get the
power split.

On Sun, 08 Jan 2006 01:07:20 +0100, Paul Burridge
k wrote:

On Sat, 07 Jan 2006 10:25:18 -0700, Wes Stewart
wrote:


Okay, I couldn't figure out what operational use it would have.

Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?

Without knowing which devices you're referring to, I would still guess
that they are based on broadband transformers. "Broadband" does not
mean DC to daylight. And yes, there might be some compensation caps
involved too.

If you want better bandwidth, including DC, and loss is not a concern,
then fully resistive dividers are what you want. An example of a
two-port is he

http://www.k6mhe.com/n7ws/HP-11549.pdf

Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?

These are ferrite transformer hybrid splitters with fractional dB
insertion loss beyond the 3dB split and typically 30dB isolation between
output ports. If you want a DC splitter then they have those too- but
you're not swift enough to see them. They are simply this. What do you
think the loss and isolation is now?:
View in a
fixed-width font
such as Courier.

(Seems like this should be r.r.a.homebrew instead of the antennas
group!)

Beware of the difference between a "power divider" and a "power
splitter." (The terminology different folk use can be confusing,
because there's not good consistency in the useage...) I gather,
without reading every word of this thread, that you want to make
network measurements, and the splitter is to provide levelling: either
a reference channel or actual amplitude control of the exciting signal.

So for a two-way, you want simply an input connector and two output
connectors, with a 50 ohm resistor from the input to each output. The
trick is that you want as near perfect symmetry between the two
channels as you can get. Ideally, each 50 ohm R will be in a coaxial
environment where the output end is 50 ohms and the input end is 100
ohms, but you don't have to control that very accurately to get it to
work well to 1.3GHz. With that arrangement, the source "sees" 50 ohms
if each output is loaded with 50 ohms, and if you use one output
channel to control the level, the level stays constant at the input
connector (since that's what's being monitored) and that is an
EFFECTIVE zero impedance point, much as the inverting input to an op
amp is a virtual ground. The the second output port then shows an
effective 50 ohms source impedance. You can show that monitoring the
reference channel instead of actually using it to control the level is
functionally the same for circuits which are not significantly level
dependent.

For a three-way, you want 50 ohms to each output port as with the
two-way, but now since the three outputs in parallel (each 50 ohms load
plus 50 ohms series resistor) result in 33.3 ohms at the junction where
they come together, you need 16.7 ohms in series from there to the
input port, to provide a 50 ohm load for the source (assuming 50 ohm
loads on each output). Note that 16.7=50/3. So you can make that
splitter with 6 50 ohm resistors.

Being just resistive devices, the only thing that limits the frequency
response is parasitic inductances and capacitances--not being able to
make the environment "perfect". But that's only a problem on the high
end, not the low end. DC is not a problem for these devices. However,
they are much more lossy than a power divider, which ideally has no
loss in the divider itself, at least with proper loads.

We had a need for some DC-6GHz splitters. The commercial ones we had
were rated to 3GHz, and indeed they weren't all that wonderful much
beyond that. A search for commercial ones that were rated to 6GHz came
up empty. So I designed a 6GHz one, using 50 ohm 0805 SMT resistors,
with some help from a mechanical engineer. The three output ports are
in one plane, radial spokes from the central node where all four
resistances come together. The input port is perpendicular to that at
the center. The resistors are all soldered together at one point, zero
lead length except for their terminations and the tiny ball of solder.
To get good performance to 6GHz, there's a screw that adjusts the
capacitance from ground to the central node. It's all housed in a
hexagonal aluminum block, with SMAs radiating out. I was able to build
a prototype that worked acceptably, using only PC mount SMAs soldered
together. I'd have a lot of confidence, based on that, that I could
make one that would work quite well to 1.3GHz even without much in the
way of test equipment to check it.

And you should have no trouble at all making some very decent loads
using the techniques others have mentioned. FWIW, I've had better luck
using two 100 ohm SMTs radially opposed than using four 200 ohm SMTs
with 90 degree spacing. I didn't investigate just why, but assumed it
had to do with the parasitics inherent in the parts.

Cheers,
Tom

(I'm a bit surprised you need a three-way splitter. Are you measuring
two DUT paths simultaneously??)