Hi chaps,

Is there a way of determining what basically amounts to S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

"Paul Burridge" a écrit dans le
message de news
Hi chaps,

Is there a way of determining what basically amounts to
S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of
doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)

p.
--

"What is now proved was once only imagin'd." - William Blake,
1793.
You can use a Vector Voltmeter like the HP8405A (up to 1300 MHz),
and a signal generator. The test will be done on a single
frequency but the 8405 support some low manually sweep of the
frequency to cover the bandwith you test.
A 8405 can be found at cheap prices and is cheapest than a VNA.

"Paul Burridge" wrote in message
news
Hi chaps,

Is there a way of determining what basically amounts to S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)
p.
--
"What is now proved was once only imagin'd." - William Blake, 1793.

I'd consider one of the common antenna impedance analyzers, but I don' know
what the output level is. Scope & sig gen is asking a lot unless you think
you can measure RF voltage and current magnitudes and phase while driving
the device with its "normal" signal level.
If you are talking about RF power transistors, the pros use matching devices
like shorted stubs and stretch lines to get power out, then remove the
device and measure the matching networks with the available equipment. Good
luck.
--
Steve N, K,9;d, c. i My email has no u's.

On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci" wrote:

Here's an idea that's just whacky enough that it might work...has anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be able use
Lissajous patterns. The idea would be to drive the complex load through a
directional coupler with fairly good directivity. A sample of the incident
signal would go into the scope X input and a sample of the reflected signal
would go into the Y input. With a pure reactance as a calibration load,
adjust the X and Y scope gain for a perfect circle on the display. Replace
the test load with a complex load, and the magnitude and inclination of the
line/oval display can tell you amplitude and phase of the reflected signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near 75
ohms, that directivity won't be a big issue. What about the phase response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...
--
Steve N, K,9;d, c. i My email has no u's.


"Joe Rocci" wrote in message
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci" wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the two
waveforms. The scope horiz gain can be adjusted for some nice number of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

On Wed, 11 Aug 2004 13:30:01 -0500, "Steve Nosko"
wrote:

Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near 75
ohms, that directivity won't be a big issue. What about the phase response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...

Well it was a neat idea in principle, certainly. Let's see if there's
a workaround for the issues you raise...

--

"What is now proved was once only imagin'd." - William Blake, 1793.

Most people think of a directional coupler as a directional coupler, and a
splitter as a splitter. In reality, the splitter is simply the extreme case
of a directioanl coupler where the coupling factor is 50%.

The fact that it was designed for 75 ohms is somewhat incidental. The
impedance on any of the ports is a function of the impedance on the other
ports. As to phase, it has to be well-behaved or you wouldn't have good
directivity.

Joe
W3JDR


Steve Nosko wrote in message
...
Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near
75
ohms, that directivity won't be a big issue. What about the phase
response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...
--
Steve N, K,9;d, c. i My email has no u's.


"Joe Rocci" wrote in message
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci"
wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the
reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and
inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional
coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference
in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the
two
waveforms. The scope horiz gain can be adjusted for some nice number
of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do
also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the
bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

"Joe Rocci" wrote in message
news:EUwSc.9258$dG.759@trndny02...
Most people think of a directional coupler as a directional coupler, and a
splitter as a splitter. In reality, the splitter is simply the extreme
case
of a directioanl coupler where the coupling factor is 50%.

The fact that it was designed for 75 ohms is somewhat incidental. The
impedance on any of the ports is a function of the impedance on the other
ports. As to phase, it has to be well-behaved or you wouldn't have good
directivity.

Joe
W3JDR

Sure Joe,

you are right. I just never thought of it on that light.

Lets think about this phase thing, however. As long as there is the proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow and
the reverse power with one of these and you need to have them both constant
with respect to each other--otherwise you cant measure phase.. If the fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made, so I
have know first hand knowledge.
--
Steve N, K,9;d, c. i My email has no u's.

On Thu, 12 Aug 2004 15:32:00 -0500, "Steve Nosko"
wrote:


Lets think about this phase thing, however. As long as there is the proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow and
the reverse power with one of these and you need to have them both constant
with respect to each other--otherwise you cant measure phase.. If the fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made, so I
have know first hand knowledge.

Let's assume for one moment the splitter idea would function
acceptably as a directional coupler for this purpose. What's next in
the block diagram? Phase comparitor? DC amp?

--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul & Steve,

Steve,
I'm pretty confident that the phase shift will be constant and can be
calibrated out. If not, it can be made irrelevant by using two splitters,
one for the forward path sample and one for the return path sample.

Paul,
The idea is to use the splitter 'backwards"; drive the RF into one of the
splitter legs and feed the load through the common port. Assuming good
directivity, any RF coming out of the other splitter leg must be reflected
energy. If you put a sample of the forward energy into a scope's X input
(horizontal) and a sample of the reflected energy into the Y input
(vertical), you will get an elliptical display called a Lissajous pattern.
If X and Y are equal in magnitude and exactly 90 deg out of phase, this
will be a perfect circle. Any other phase angle will result in a elliptical
pattern whose inclination angle relative to the X axis (or Y axis) is a
function of the phase angle. The length of the ellipse is a function of the
magnitude. This is classic stuff...look it up if you're not familiar with
it.

Joe
W3JDR


Paul Burridge wrote in message
...
On Thu, 12 Aug 2004 15:32:00 -0500, "Steve Nosko"
wrote:


Lets think about this phase thing, however. As long as there is the
proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow
and
the reverse power with one of these and you need to have them both
constant
with respect to each other--otherwise you cant measure phase.. If the
fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made,
so I
have know first hand knowledge.

Let's assume for one moment the splitter idea would function
acceptably as a directional coupler for this purpose. What's next in
the block diagram? Phase comparitor? DC amp?

--

"What is now proved was once only imagin'd." - William Blake, 1793.