开云体育

That's a good point -- if you want a voltage level at your meter terminals
that's proportional to power at the input of a square-law detector, you
don't need to worry about the "root" part of "rms." And the integration
behavior of your metering circuit takes care of the "mean" part, whether you
want it to or not...

-- john, KE5FX

--- John Miles <jmiles@...> wrote:

A thermal sensor will also give true-RMS power
readings for all waveform
shapes, for obvious reasons, while readings taken
with a simple diode
detector will be most accurate for clean sinewaves.

The typical domain for the diode detector is below
-20dBm, where the diode is operating in the
"square-law" region. I believe that the response to
complex waveforms is identical to the thermal
converter in this case. So, for example, a pulse train
with peaks of -20dBm could be accurately measured with
the diode detector. However, a pulse train with peaks
of -10 or 0 dbm and an average power of -20dBm would
not be, unless you attenuated the signal accordingly.

AW




____________________________________________________________________________________
The fish are biting.
Get more visitors on your site using Yahoo! Search Marketing.

Hi Mike:

The characteristic impedance of free space is 120 * PI. So an antenna is just an impedance matching device from whatever impedance you start with (say 50 Ohms) to 120*PI Ohms. There are no mechanisims for reasonance in free space but there is one for the earth called the Shuman resonance. You can Google it.

Something I just learned in the past couple of years is that the impedance of transmission lines is not constant for audio frequencies. For more on that see:

Have Fun,

Brooke Clarke, N6GCE

--
w/Java
w/o Java

Hello group--

I have two pieces of surplus equipment -- an Anritsu MW98A OTDR with 1300 nm source, and a Photodyne Optical Attenuator, 1975XQ. If you are interested in purchasing either of these, please email me off list.

Thanks!

--

Thomas B. Fowler, Sc.D.
Senior Principal Engineer
Mitretek Systems
3150 Fairview Park Drive
Falls Church, VA 22042
703-610-2944 Fax 703-610-2399
tfowler@...

--- In hp_agilent_equipment@..., "antoon_on6ea"
<antoon.debosschere@...> wrote:

Hi all, has anybody some service information, schematic of a HP117A

VLF

comparator ?
Any help appreciated.

Regards
Antoon

Hi
I have the manual .
May be I can scan and mail to you the
pages you are interested in
Regards
Gianfranco

Greetings,

I spent 46 years as an electronic technician without knowing free space had an
impedance of 376.7 ohms. How did that happen?

Jerry





Maybe you needed a space ohmmeter!





It probably was something you didn't need to know.



Stuart K6YAZ

Los Angeles, CA



________________________________________________________________________
Check out the new AOL. Most comprehensive set of free safety and security tools, free access to millions of high-quality videos from across the web, free AOL Mail and more.

Chris,
good that you solved the problem.
One more article for my archive, may come in handy if my 141T acts up.

I also have a 181A storage scope here.

Jos
PA0AMX

Hi Chris....collective egg-on-face :-))
I think I for one assumed that because the display worked in the
"non-storge" mode the EHT was probably OK. Thanks for sharing your success
with us....that is another message I will file in with the manual.
Alan G3NYK

Hi all,

The problem has been solved. After re-checking and re-checking voltages, waveforms and magnitudes, as well as checking the integrety of crimped cables, fitting a new CRT socket, there was still no joy, with my suspicion focussing more and more on the "junction box". Well I was close........ When I could not fix the problem, I put in yet another CRT, this one also almost new. It came out of a mainframe with powersupply issues in which I had just replaced the tube. Again no joy.
Back to the junction box. I decided that maybe I should find some way to check the 6.6KV post accelerator voltage from the tripler. This is not easy, as the tripler is a sealed unit and the output HT wire is moulded into the junction box, from where it is routed to the CRT. I opened the junction box and poked the HV voltage probe at the HV input and behold only about 1.1KV. I replaced the tripler, which is a real pain and time consuming, as it requires almost removing the HV power supply. I switched the gear back on and you guessed it - the variable persistence worked.

When I first posted my question, I mentioned that I had not checked the 6.6KV post accelerator voltage, because that should be OK, since the unit worked fine in conventional mode. Nobody came back to me to tell me that I was wrong, wrong, wrong. This remark is not meant to be a smart a*** snipe, because I am grateful for all the suggestions and ideas. The point I am trying to make is that to engineers familar with storage CRTs, I imagine it would have been so obvious that the post accelerator HV is part of the variable persistence system, that my remark was overlooked. Not being an electronics engineer but a mere amateur (dabbler if you like), I was firmly convinced that the post accelerator HV was only involved in the conventional operating mode. I barely understand how conventional CRTs work and the variable persistence version is a complete mistery to me.

Anoher lesson learned, another problem solved. Once again my thanks to all who bothered to respond.

Cheers,

Chris

On Fri, 19 Jan 2007, John Miles wrote:

There are a couple of differences. Thermally-driven sensors can be somewhat
harder to burn out accidentally, because they use a resistive element
instead of a tiny diode chip. You won't tend to fry them with ESD on the
workbench, for example. For the same reason, they can theoretically look
like a better 50-ohm load than a diode sensor, although I don't believe the
difference matters much in the real world.

I would take issue with you on this, there is a greater 'safe
area' on the 8484A diode sensor (measures -70 to -20, max safe
input +20dBm) than there is on an 8481A thermocouple (measures -30
to +20, max safe inut +25dBm). Of course, you have to put a 40dB
pad in front of the 8484A, but hey, that's not a bad idea!

The biggest drawback to thermocouple-based sensors that I've personally
noticed is that they don't work well at low power levels, where a zero-bias
Schottky diode excels. With a resistor-thermocouple sensor, it can take
several seconds for a power reading near the bottom end of the range to
settle down. They really don't do well below -20 dBm.

This is true, diode sensors are good to below -60dBm.

A thermal sensor will also give true-RMS power readings for all waveform
shapes, for obvious reasons, while readings taken with a simple diode
detector will be most accurate for clean sinewaves.

Thermistor sensors, the 8478A etc. have less dynamic range and are
less stable, but have the unique advantage of being transfer
standards, i.e. can be calibrated at DC (given the right power
meter).

I *believe* the 435/436 wattmeters can use both types of elements, but don't
quote me on that...

Yep, all those I have seen can, as can the 437 and 438. None of
them work with thermistor sensors though, for them you need a 431
or 432.

BTW, you may want to add GMC and Narda to your list.

Geoff

--
Geoff Blake G8GNZ located near Chelmsford, Essex, U.K.
Please reply to: geoff (at) palaemon (dot) co (dot) uk
Using Linux on Intel & Linux or NetBSD on Sun Sparc platforms

Please avoid sending me Word or PowerPoint attachments.
See <>
------------------------------------------------------------------------------

This E-mail and any attachment(s) are strictly confidential
and is intended solely for the addressee(s). If you are not the
intended recipient please notify <postmaster(at)palaemon.co.uk>
and the sender by return and permanently delete the message.

You may not disclose, forward or copy this E-mail or any of its
attachments to any third party without the prior consent of the
sender.

------------------------------------------------------------------------------

John Miles wrote:

There are a couple of differences. Thermally-driven sensors can be somewhat
harder to burn out accidentally, because they use a resistive element
instead of a tiny diode chip. You won't tend to fry them with ESD on the
workbench, for example. For the same reason, they can theoretically look
like a better 50-ohm load than a diode sensor, although I don't believe the
difference matters much in the real world.

The biggest drawback to thermocouple-based sensors that I've personally
noticed is that they don't work well at low power levels, where a zero-bias
Schottky diode excels. With a resistor-thermocouple sensor, it can take
several seconds for a power reading near the bottom end of the range to
settle down. They really don't do well below -20 dBm.

A thermal sensor will also give true-RMS power readings for all waveform
shapes, for obvious reasons, while readings taken with a simple diode
detector will be most accurate for clean sinewaves.

I *believe* the 435/436 wattmeters can use both types of elements, but don't
quote me on that...

-- john, KE5FX

Yes, the 435 and 436 power meters accept both thermocouple and detector based sensors.

I second John with regard to the response time of thermocouple based sensors at low power levels. Each time you go down a decade below 0 dBm, the response time takes a hit. Below -20 dBm, the response time is approximately forever.

While some of the newer detector based sensors have a range going up to +20 dBm, some of the older detector models will blow up (or be damaged) with much less than that.

Even though thermocouple based sensors will accept high peak power (at low duty cycle), they are still deceptively easy to blow up. Witness the number of sensors for sale on eBay "as-is" and how much vendors want for "as-is" versus "warranty" types. The repair cost for a sensor is about the same as a guaranteed used sensor because the cost of the housing is negligible compared to the cost of the thermocouple element and the labor to replace it, so unless you know exactly what you are doing, or you are curious to see what's inside, do not buy one that does not or may not work.

Some companies, such as Narda, make thermocouple and detector based standalone sensors (they look weird but output 0-10VDC) that do not require a bench meter. Simply provide well regulated +/- 12V, a pot to adjust zero and a digital voltmeter, and voila, you have a fairly good thermal low power (<10 mW) meter going from 10 or 50 MHz all the way to 12.4 or 26.5 GHz, depending on the model.



In my experience, these sensors are quite reliable and very useful in a ham's VHF to Microwave lab.

Didier KO4BB

There are a couple of differences. Thermally-driven sensors can be somewhat
harder to burn out accidentally, because they use a resistive element
instead of a tiny diode chip. You won't tend to fry them with ESD on the
workbench, for example. For the same reason, they can theoretically look
like a better 50-ohm load than a diode sensor, although I don't believe the
difference matters much in the real world.

The biggest drawback to thermocouple-based sensors that I've personally
noticed is that they don't work well at low power levels, where a zero-bias
Schottky diode excels. With a resistor-thermocouple sensor, it can take
several seconds for a power reading near the bottom end of the range to
settle down. They really don't do well below -20 dBm.

A thermal sensor will also give true-RMS power readings for all waveform
shapes, for obvious reasons, while readings taken with a simple diode
detector will be most accurate for clean sinewaves.

I *believe* the 435/436 wattmeters can use both types of elements, but don't
quote me on that...

-- john, KE5FX

David

John,

Isn't what you've described called "entropy"? I see no resonance or storage in this, simply transmission of energy from A to B.

Pete

John,

The capacitor you're talking of must have electrodes of some sort to
define
it. I agree that the energy is stored in an E-field, but it must still
have
a pysical definition, no?

Pete


No. The sun emits EM radiation. That energy is stored in the E & M
fields and eventually reaches us. Where are the plates or coils ??

It may be necessary to have something like plates or coils to INSERT or
EXTRACT energy from the fields, but the energy is stored in the fields

-John

"Pete" <peterawson@...> wrote:

Chuck,

It sure would, but so would aligning a perfect pool ball to
continually
bounce from one perfectly elastic collision to another & back.
But, is it useful, or even interesting?

Pete



Pete,

You miss the point. In between the mirrors (or collisions in your
example) the energy is stored in the E and M fields (or in your
example... the kinetic energy of the pool ball). ...... NOT in the
'device' or the plates of the capacitor for xample.

-John

John,

The capacitor you're talking of must have electrodes of some sort to define it. I agree that the energy is stored in an E-field, but it must still have a pysical definition, no?

Pete

Chuck,

It sure would, but so would aligning a perfect pool ball to continually bounce from one perfectly elastic collision to another & back.
But, is it useful, or even interesting?

Pete

You mention resonance, which implies stored energy. What physical
device are
you thinking of? I can't think of
any way to store energy without defining a physical device/mechanism
for the
storage medium.


The energy is stored in the FIELDS and a 'device' is not needed. Think
of a vacuum capacitor.
-John


Free space is
certainly large enough, but, at or near 0 deg K & very low density, it
seems
a poor candidate for energy storage.

Pete Rawson

Hi Guys,

Apologies for the hugely off topic question but as you guys are so into
RF (and its
been
a little quiet in last few hours ;) I was hoping someone might be able
to answer a
simple
question, well fairly simple, with caveats no doubt...

As free space has a characteristic impedance of some 376.7 Ohms (or
close to)...


Roughly:

Impedance depends on Mu (u) and Epsilon (e)[ and geometry. In free
space, geometry is constant. Since there is nothing in free space, u and
e are constant and independent of frequency, so impedance is constant
and independent of frequency.

Real space has gas molecules so there may be dispersion (variation of
index of refraction with wavelength) i.e.: impedance variation with
frequency.

-John