¿ªÔÆÌåÓý

  1. HP-Agilent-Keysight-Equipment
  2. Messages
Expanded Topics Messages
Date
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

On Sat, 17 May 2025, jmr via groups.io wrote:

There are separate manuals for all those Hxx 478A sensors. There was a whole
bunch of those, all seem to be nearly identical according to their manuals.

H55 -- 00478-90029
H63 -- 00478-90030
H72 -- 00478-90031
H73 -- 00478-90032
H75 -- 00478-90033
H13 -- 00478-90036

They all use 0.1uF capacitors.

There was also a 8478B option H01 with the same specs. No manual for that,
just a product note telling it has a VSVR of 1.05 or less at 50MHz.

And here is kinda addendum from HP (before Agilent/Keysight) for H75 that
they suggest using along with original 478A 00478-90021 manual (attached,
hopefully it will get through).

Just to clarify, the latest service manual for the standard 478A sensor is available on the Keysight website and this lists the different internal circuits for the early and later variants of the standard sensor (as I have posted on this thread) and it provides a serial number checker in the appendix. This is how I know my sealed and unused sensor is one of the later types with a ferrite bead. It has a serial number greater than 65601 so the B changes in the manual apply to this sensor.

My early 478A sensor was probably made in the mid 1960s so it has the RC damper in it and no ferrite bead.

Note that this service manual doesn't cater for the H7x variants that use 0.1uF caps. My aim in this thread is to see how well I can improve the input VSWR of an early (or late) standard 478A sensor so it has a similar or better VSWR at 50 MHz compared to the H7x variants.
--
Regards
Jeremy
---
*
* KSI@home KOI8 Net < > The impossible we do immediately. *
* Las Vegas NV, USA < > Miracles require 24-hour notice. *
*

HP 478A-H75 Operating & Service.pdf
HP 478A-H75 Operating & Service.pdf

Re: HP 478A Thermistor Sensor. Input return loss and matching

 

Here's a screenshot from my old Agilent VNA showing the input VSWR of my 478A sensor with the matching section added at the input. You can see that it now achieves a really low VSWR at 50 MHz.
?
Because I've used low loss parts, there is no through loss penalty when the matcher is inline. This is because the approx 0.004dB mismatch loss caused by the capacitive input of the standard sensor is made up for by having the series matching inductor inline. However, due to the finite Q of the matching inductor the model predicts that there will be about a 0.004dB loss caused by the matching inductor. So the two cancel out. So when I next check my 1mW source from my power meter, I can do it with a 478A sensor that has very low input VSWR. Probably as low as the VSWR of the H7x variants at 50 MHz.?
?
I don't need to worry about harmonics because the harmonic distortion levels from the 1mW source are very low. Better than -60dBc.
?
--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

On Sat, 17 May 2025, jmr via groups.io wrote:

H72 is SPECIFICALLY designed for low frequency measurements, especially that
50MHz one. It is not high frequency device going into gigahertz range so it
didn't require tiny special microwave capacitors with low inductance.

All other 478A (except similar H75/H76, maybe some other Hxx) on the other
hand are totally unsuitable for that 50MHz (and general low frequency)
measurements.

Those Hxx are special calibration devices, not general purpose power
sensors.

OK thanks. I think the standard 478A sensor uses a tiny (and very thin) SMD disc cap of about 1350pF for the AC ground end of the thermistor. The later 478A sensor uses 1200pF here.? I don't know what package type is used for the series blocking cap at the input.
Because the sensor is designed for use to 10 GHz I think this capacitor will have relatively low package inductance compared to a regular MLCC in a 1206 package.

I'm not sure what package the 0.1uF caps use in the H72 but if they are allowed to be as large as a 1206 MLCC then maybe a MLCC with a class 1 COG dielectric can be used here. This might help explain why the upper frequency range is much lower for the H72 version.

--
Regards
Jeremy
---
*
* KSI@home KOI8 Net < > The impossible we do immediately. *
* Las Vegas NV, USA < > Miracles require 24-hour notice. *
*
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

Just to clarify, the latest service manual for the standard 478A sensor is available on the Keysight website and this lists the different internal circuits for the early and later variants of the standard sensor (as I have posted on this thread) and it provides a serial number checker in the appendix. This is how I know my sealed and unused sensor is one of the later types with a ferrite bead. It has a serial number greater than 65601 so the B changes in the manual apply to this sensor.
?
My early 478A sensor was probably made in the mid 1960s so it has the RC damper in it and no ferrite bead.?
?
Note that this service manual doesn't cater for the H7x variants that use 0.1uF caps. My aim in this thread is to see how well I can improve the input VSWR of an early (or late) standard 478A sensor so it has a similar or better VSWR at 50 MHz compared to the H7x variants.
--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

OK thanks. I think the standard 478A sensor uses a tiny (and very thin) SMD disc cap of about 1350pF for the AC ground end of the thermistor. The later 478A sensor uses 1200pF here.? I don't know what package type is used for the series blocking cap at the input.
Because the sensor is designed for use to 10 GHz I think this capacitor will have relatively low package inductance compared to a regular MLCC in a 1206 package.
?
I'm not sure what package the 0.1uF caps use in the H72 but if they are allowed to be as large as a 1206 MLCC then maybe a MLCC with a class 1 COG dielectric can be used here. This might help explain why the upper frequency range is much lower for the H72 version.
?
--
Regards
Jeremy
Re: Recap 6205B

 

Hi Joe,
?
If you just change some parts without knowing why you only make it worse. You give exact 0 information what was your error-picture, what you did or what voltages did you measure. My advice:
print out the schematc, mark the GND reference against which there is the masurement. If you measure, take the DC and AC Voltages and write them in the schematic. Check all voltages from left to right. Only C10, C12 or C14 may have up to 0,1V AC without load.
There may be a defect capacitor. But defect diodes or transistors are more common.
?
before you open the device: unplug it. If open: Isolate ALL life points, no exception. Use an isolation transformer.
?
Are the bridges on the rear according to the schematic?
?
Roland
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

On Sat, 17 May 2025, jmr via groups.io wrote:

There are 0.1uF C0G capacitors in 1206, nothing special. I have a reel of
those and use them often.

Dunno what capacitors are inside H72 probe although I have one here. Judging
from the head external dimensions I don't see any problems fitting a couple
of 1206 capacitors in there.

I'd expect the 0.1uF caps in the H72 sensor to be class 2 ceramic types as it's hard to imagine COG/NP0 being used here. This would presumably make the reactance at 10kHz susceptible to temperature changes and this might also worsen the drift vs temperature if you tried to use it with a 431 meter. So there's probably more than one reason the H72 can't be used with the 431 meter.

I'm not sure how stable the ESR of the 0.1uF caps will be over temperature up at 50 MHz but presumably these caps were chosen carefully by HP. The ESR of these caps will contribute to the efficiency factor stamped on the sensor as the ESR in these caps will rob a tiny amount of the incident power to the thermistor. However, the ESR will probably be tiny at 50 MHz. The ESR will increase up towards 1 GHz and this will cause some loss in efficiency as the ESR of class 2 ceramic caps does tend to climb rapidly up in the GHz range.

The standard sensor will use thermally stable caps here with low ESR and I would expect the loss resistance of these caps to be dominated by metal losses as the frequency is increased although the dielectric material will also contribute some loss. Normally the metal losses go up by sqrt(2) per octave and this is one way to try and model the efficiency vs frequency of the standard sensor. I plan to add this info at some point in this thread as the efficiency does degrade at just worse than sqrt(2) per octave.

--
Regards
Jeremy
---
*
* KSI@home KOI8 Net < > The impossible we do immediately. *
* Las Vegas NV, USA < > Miracles require 24-hour notice. *
*
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

Here's an image showing the completed matching adaptor that contains the series 11nH inductor.
?
This is constructed using two precision N bulkhead connectors, one male and one female. I salvaged them from an old prototype RF filter but they are available from Mouser for about ?27 each. This is a very good price for an extremely good connector. These have 8 slots in the N connector and are designed to work well to many GHz.
?
Whilst this connector choice may seem like overkill at 50 MHz, these high quality N connectors are similar quality to those used in a VNA in terms of the mating parts and this is what you would want for something mating to a power meter where you want repeatable connections.
?
You can see that I've soldered them together with a soldered metal outer ring and there is a series inductor inside. This construction method looks ugly but it will deliver very low additional losses.
?
The N connectors are stripline launcher types which give a clean interface to the inductor. I think the Suhner part number for the male version is 13 N-50-0-33/133 NE
?
When I measure with this inline I get the same VSWR response as the model with about 1.003:1 on the VNA. This is exploring the measurement limits of my VNA.?
?
When I put it inline with the 1mW 50 MHz output of my power meter, there is no discernible change in level (compared to not having it inline) when I use the 432A with a DVM on the recorder output. I get 0.996x mW in both cases where the fourth digit drifts the same with and without the matcher inline. It requires careful operation of the zero to achieve repeatable results here.
The difference between 0.995 mW and 0.996 mW is? 0.004dB and the difference appears to be much less than this.
?
So it looks like this is working just as well as the model predicted it would. I think I managed to get a Qu of much better than 65 for the 11nH inductor but I think the Q diminishes a bit once inside the metal enclosure. But it still performed as expected so I think the Q was still good enough once inside the matching enclosure.
?
?
?
?
?
?
--
Regards
Jeremy
Re: Recap 6205B

 

For C14 & C20 the can has several tabs that secure them to the board - these are common with the "-" of the capacitor and the can itself is often used to complete the circuit.? Make sure that all of the tabs connected to the case are also connected to the "-" terminal if required.? The voltage rating of any replacement should be the same or higher than the originals and the capacitance value itself should be 20-50% higher than the original - the actual capacitance of the originals tended to be close to or above the indicated value while modern capacitors tend to be below the rated value (as close to the lowest limit of the tolerance band as the manufacturer can get away with.) No need to worry about low ESR on these however and, while not required, 105 deg C parts are a good choice IMO as they really don't cost much more than 85 deg parts and they will probably outlast us both in that application.
?
Hal
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

I'd expect the 0.1uF caps in the H72 sensor to be class 2 ceramic types as it's hard to imagine COG/NP0 being used here. This would presumably make the reactance at 10kHz susceptible to temperature changes and this might also worsen the drift vs temperature if you tried to use it with a 431 meter. So there's probably more than one reason the H72 can't be used with the 431 meter.
?
I'm not sure how stable the ESR of the 0.1uF caps will be over temperature up at 50 MHz but presumably these caps were chosen carefully by HP. The ESR of these caps will contribute to the efficiency factor stamped on the sensor as the ESR in these caps will rob a tiny amount of the incident power to the thermistor. However, the ESR will probably be tiny at 50 MHz. The ESR will increase up towards 1 GHz and this will cause some loss in efficiency as the ESR of class 2 ceramic caps does tend to climb rapidly up in the GHz range.
?
The standard sensor will use thermally stable caps here with low ESR and I would expect the loss resistance of these caps to be dominated by metal losses as the frequency is increased although the dielectric material will also contribute some loss. Normally the metal losses go up by sqrt(2) per octave and this is one way to try and model the efficiency vs frequency of the standard sensor. I plan to add this info at some point in this thread as the ESR does degrade at just worse than sqrt(2) per octave.
--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

I can't be certain, but I think the reason the H72 can't be used with the 431 meter type is because the H72 sensor uses a 0.1uF decoupling cap on the output of the 100R + 100R thermistors inside the 478A sensor. This would have a reactance of about? -160 ohms at 10kHz and this may be low enough to upset the 431 bridge operation as it operates at 10kHz with 200 ohm elements in the bridge.
?
By contrast, there's about 100 times higher capacitive reactance in the regular (non H7x) 478A sensors, either early or old versions. So these can be used with both the 431 and 432 meters with no issues.
?
--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

¿ªÔÆÌåÓý

The HP 478A Thermistor Sensor ?Option H72 and others state? only to be used with HP 432

?

So it sounds like the new HP 478A Thermistor Sensor. Without the option might also not be useable / compatible ?with the 431

?

So has anyone a handle to the serial number of the HP 478A Thermistor Sensor. that go with the HP 431?? and Those for the HP 432

Paul

?

From: [email protected] [mailto:[email protected]] On Behalf Of ebrucehunter via groups.io
Sent: 17 May 2025 21:29
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 478A Thermistor Sensor. Input return loss and matching

?

I was under the impression (although possibly incorrect) that resistance and capacitance elements found in early 478A sensors were needed corrections for 431 power meters that utilized a 10 kHz bridge circuit.

?

Bruce, KG6OJI
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

I was under the impression (although possibly incorrect) that resistance and capacitance elements found in early 478A sensors were needed corrections for 431 power meters that utilized a 10 kHz bridge circuit.
?
Bruce, KG6OJI
5 files uploaded #file-notice

Group Notification
 

The following items have been added to the Files area of the [email protected] group.

By: Philip Freidin <philip@...>

Description:
Service notes for E3610
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

Hmmmm?? messages passing in the night (or day)
It looks like you were typing message 151895 while I was typing 151896.
Hopefully my info will be helpful.
?
Philip
?
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

I think the 8757A uses an HP 1345A display module, which is used in multiple
different instruments. You can isolate the display subsystem by unplugging the
26 signal flat flex cable, at either end. This will cause the 1345A to go into test
pattern mode, that should look like this:
?
?
The service manual for the 1345A is available online from the usual sources.
?
I recently repaired one of these displays that apparently had a functional Y axis, but
the X axis was dead, thus just a vertical line with varying intensity. As part of my debug
process, I took scope measurements of all the nodes in the Y axis starting at the
pre-amp chip at U28 and continuing to the final transistors that drive the deflection
plates. All the scope images, test pattern, and faulty display can be accesses at:
?
?
Many of these signals have a significant DC offset (10V to 100V) with an AC
component (the actual display data) of 20mV to 40mV P2P. My scope traces
either show that in two separate pictures, or on the same picture with two
channels, one showing average DC level, and the other trace shows the AC
coupled version of the signal with 20 mV/Div .
?
From your description, it sounds like your issues are earlier in the signal path,
but this test pattern is an easy starting point to localize the fault to be either in
the display section, or in the 8757A supplying display commands over the
flat flex cable.
?
None of the analog signals representing frequency or amplitude are directly
connected to the display. They are processed in the rest of the analyzer
and a list of display commands (a little like HPGL) are sent over the flat flex.
Depending on the options for the 1345A display, it ether has a display memory
that contains the display command list, and repeatably plays it over and over,
or it doesn't have this memory, and it is the responsibility of the host instrument
(8757A) to continuously send the data to refresh the screen.
?
With display memory, the host instrument only has to send the display commands
when the display has to change. This is best for generally static displays. Without
the memory, the host instrument processor has to continuously send display
commands, so a heavier load, but more appropriate for a continuously changing
display.
?
Philip
?
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

This afternoon I read more of the manual. I then checked the "X-Y-Z Analog" board that drives the video - gently "banging" (tapping with a plastic rod!) the rear corner of that board suddenly but intermittently changed the vertical position of the display. I dismounted this board and I saw that someone in the past had re-soldered 4 of the preset adjustable pots - that's an indication that they also maybe had this problem in the past, so I checked around them and properly cleaned those joints and re-soldered them together with the pins for the wites that connect to the backside of the board.
?
This solved the problem of the display being off-centre - it now appears in the correct place (also with the test pattern selected), although the problem of the sequencing/cycling of the 4 different 'video' channels being far too slow is still there.
?
I checked the CPU board and the battery - the battery is still good, and I can store the setup and recall it perfectly okay.
?
So I think the problem that remains is the speed of the 'frame sequencing' if that's the correct terminology, and maybe that is on the CPU board since that sends the outputs to the display board. I have no further ideas.
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

When I fitted a ferrite bead to my early 478A I found that the VSWR curve became smoother but it degraded from about 1.068:1 at 50 MHz to about 1.075:1 with the ferrite added. This is unavoidable as the resonance was slightly improving the VSWR at 50 MHz.
?
A VSWR of 1.075:1 is still OK for measuring the output of a typical 1mW reference at 50 MHz but I had a go at improving this by fitting an external matching network at 50 MHz. Obviously, the matching network should provide minimal insertion loss.
?
The complex input impedance of the early sensor at 50 MHz is about 1000pF in series with 50 ohms. So the simplest matching network would be a series inductor of about 11nH.
?
See the plot below that shows the input VSWR of the early 478A sensor (complete with an added ferrite) in the unmatched state
against what happens when matched with an external series 11nH inductor with an unloaded Q of about 55 at 50 MHz.
?
You can see that the response is about the same if I set port 2 of the simulator to have the same port impedance as my early 478A sensor with the ferrite added. Without the matching network, the insertion loss is about 0.005dB.
?
The resistive loss of the matching inductor means that matching it makes no improvement in the insertion loss but there should be an improvement in mismatch uncertainty.
?
By changing port 1 to 52R (VSWR of 1.04:1) and adding a long lossless 50R transmission line I can graphically proved an indication of the mismatch uncertainty caused by a 1mW source that might have a VSWR of 1.04:1. See the second plot.
?
You can now see there is a tiny advantage in having the matching network as there is much less uncertainty ripple (at 50 MHz) in the red trace compared to the green trace. However, in reality, I think a typical 1mW source from an HP power meter will have a fairly accurate 50R resistive part of its source impedance. So I'm not convinced that there will be much advantage in having this matching network inline. This is because of the way the 1mW 50 MHz reference circuit is designed in a typical HP power meter. It's likely to look like (maybe just over?) 50R resistive in series with a tiny reactance.
?
However, I have gone ahead and built a matching network and I can describe and show it it a later post. I hope this stuff is interesting and not too nerdy :)
?
?
?
?
?
?
?
?
--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

On Sat, May 17, 2025 at 03:14 PM, Ed Marciniak wrote:
It would be reasonable to try 61 mix NiZn ferrite with a 300C curie temperature, and mu of 125
Thanks. I tried lots of ferrite types on a VNA and quite a few were OK to use but I chose one that gave at least 70R ESR across 50 MHz to several GHz that was fairly stable on a VNA when heated. However, I would much rather find out what HP used and fit this instead. Maybe someone here may know what ferrite was used. I think the ESR at 50 MHz has to be at least 50 ohms as this is 'big' compared to the 2.7R of R1.
--
Regards
Jeremy
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

It sounds like it could be in the analog display area, the video and sweep drive to the CRT. Check the horizontal and vertical drive to the CRT during the analog portion of the sweep. With no signal input the vertical should be a negative voltage (check the manual for values), the horizontal voltage should be equal positive and negative values (check the manual). Yours should not be horizontally balanced, if it is offset. Trace that signal back through drive circuitry until it changes to a balanced signal.

Don Bitters
121 - 140 of 151493