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Re: HP 478A Thermistor Sensor. Input return loss and matching
Here's a comparison showing the VSWR of an early sensor (VNA measurement of my HP 478A) against a model based on the schematic of the later sensor. You can see that the VSWR at 50 MHz should improve to about 1.05:1 with the later sensor type and there is no resonant blip at 53 MHz.
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I do have the later type of sensor here but it is still factory sealed (see the image below to see it has never been used) and it seems a shame to disturb it just to measure the VSWR. You can see in the image that the red internal washer for the N connector is supplied external to the sensor. I'm not sure why this is. I doubt it has fallen out on its own but it is lying flat just below the black outer of the N connector. I think this sensor is actually over 30 years old but never been unpacked and used.?
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I also have a brand new unused (also made over 30 years ago) 432A meter with sealed cable and paperwork from HP and I keep this carefully stored for the day my other 432A or 478A finally fail BER.
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You can also see that the efficiency chart on the side of the sensor shows the efficiency of the later sensor type is very close to 100% at 0.01GHz and the efficiency drops to about 98% by 2 GHz.
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My early sensor also has a minor VSWR wiggle and (in)efficiency bump at about 850 MHz but I've not investigated this as it is almost certainly caused by something inside the metal body containing the two thermistors and I don't want to disturb this.
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Regards
Jeremy |
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Re: HP 478A Thermistor Sensor. Input return loss and matching
¿ªÔÆÌåÓýIt would be reasonable to try 61 mix NiZn ferrite with a 300C curie temperature, and mu of 125.
From: [email protected] <[email protected]> on behalf of jmr via groups.io <jmrhzu@...>
Sent: Saturday, May 17, 2025 8:34:17 AM To: [email protected] <[email protected]> Subject: Re: [HP-Agilent-Keysight-equipment] HP 478A Thermistor Sensor. Input return loss and matching ?
The reason the efficiency also suffers with a blip at about 40-60 MHz is that some of the incident RF from a 1mW reference source will end up in the 2.7R damper resistor via the resonant path to R1. This means the DC substitution (used by the HP 432A meter)
can't correctly indicate the incident power in this frequency range unless the efficiency change is known across this tight frequency range. It's the equivalent of the sensor losing efficiency and in my case this was a loss of over 1% efficiency at the peak
of the bump. This was always really annoying for me because it meant I had to have an efficiency chart showing how the efficiency sharply changed by over 1% across 40-60 MHz and this required lots of cal points.
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Adding the ferrite achieves two things. It removes the bump in the input VSWR and it also flattens the efficiency. At first, it might appear that adding the ferrite would suck even more energy from the thermistors as a ferrite bead looks resistive up at
VHF.? This would make the efficiency even worse. However, as long as the ferrite introduces 'lots' of series resistance (ESR), there will be a big mismatch loss at 50 MHz so only a tiny amount of incident RF will get burned up in the high resistance of the
ferrite. The efficiency won't be perfectly flat but there should be a big improvement in efficiency across 40-60 MHz and I was able to achieve this.
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Obviously, the choice of ferrite is fairly critical here as it has to provide a high ESR at 50 MHz and it also has to be fairly immune to temperature changes. This means that the initial permeability needs to be reasonably stable wrt temperature changes
across about 10-50degC.
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The latest manual for the 478A sensor lists this ferrite as E1 but it doesn't give a part number for it. It would be really nice to know what the official ferrite material is.
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Regards
Jeremy
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Re: HP 478A Thermistor Sensor. Input return loss and matching
The reason the efficiency also suffers with a blip at about 40-60 MHz is that some of the incident RF from a 1mW reference source will end up in the 2.7R damper resistor via the resonant path to R1. This means the DC substitution (used by the HP 432A meter) can't correctly indicate the incident power in this frequency range unless the efficiency change is known across this tight frequency range. It's the equivalent of the sensor losing efficiency and in my case this was a loss of over 1% efficiency at the peak of the bump. This was always really annoying for me because it meant I had to have an efficiency chart showing how the efficiency sharply changed by over 1% across 40-60 MHz and this required lots of cal points.
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Adding the ferrite achieves two things. It removes the bump in the input VSWR and it also flattens the efficiency. At first, it might appear that adding the ferrite would suck even more energy from the thermistors as a ferrite bead looks resistive up at VHF.? This would make the efficiency even worse. However, as long as the ferrite introduces 'lots' of series resistance (ESR), there will be a big mismatch loss at 50 MHz so only a tiny amount of incident RF will get burned up in the high resistance of the ferrite. The efficiency won't be perfectly flat but there should be a big improvement in efficiency across 40-60 MHz and I was able to achieve this.
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Obviously, the choice of ferrite is fairly critical here as it has to provide a high ESR at 50 MHz and it also has to be fairly immune to temperature changes. This means that the initial permeability needs to be reasonably stable wrt temperature changes across about 10-50degC.
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The latest manual for the 478A sensor lists this ferrite as E1 but it doesn't give a part number for it. It would be really nice to know what the official ferrite material is.
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Regards
Jeremy |
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Re: HP 478A Thermistor Sensor. Input return loss and matching
Here's the schematic of the early 478A sensor. You can see there is a 460pF capacitor C3? in series with a 2.7R damping resistor R1.
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I've added some extra detail to show that there is a short red wire that connects between C1 (inside the metal sensor block) and C3 on the PCB on the back of the block. This wire is probably an inch or so long.
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Sadly, this causes a resonance near 50 MHz and this is the cause of the blip in the VSWR. I believe that R1 is there to damp out this resonance. To demonstrate this, I've put together a very crude model of the sensor in Genesys and modelled the red wire as a 22nH inductor. I modelled with and without the 2.7R damping resistor R1 and the results are shown below. I also included the real VNA measurement of a real (early) 478A sensor.
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You can see that the VSWR has a huge resonant wiggle at about 53 MHz when R1 is removed (see the green trace). So I think R1 was added to try and damp out this resonance.
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Later sensor variants appear to deal with this issue by fitting a ferrite bead over the red wire and the damper network is no longer needed. I've tried upgrading my sensor with a ferrite bead and can confirm that it is well worth doing as it completely damps out the resonance, leaving a nice smooth VSWR curve.
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I hope the simulation plot is readable. I've enlarged the fonts to try and improve things.
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Regards
Jeremy |
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HP 478A Thermistor Sensor. Input return loss and matching
In the last few weeks, I've had my old HP 432A power meter on the bench along with its 478A thermistor sensor. I normally only use it about once a year to check the 1mW (50 MHz) reference on another power meter.
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However, I've spent a bit more time looking at the 478A sensor and wondered if it was worth sharing some of the knowledge gained and also some of the options for reducing the measurement uncertainty compared to using a standard sensor at 50 MHz.
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I've looked at the input match, the efficiency and also at how the 478A has had some changes internally over the years. One obvious option to improve the input match at 50 MHz is to buy a special version with option H75 or option H76. These are probably quite rare and expensive. Another option is to improve the input match of a standard sensor by adding a low loss (external) matching section inline with the standard sensor. I have done this and have achieved good results. I'll show the design process and test results in this thread.
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The standard sensor has a fairly grim spec for input VSWR at 50 MHz of about 1.3:1. However, it shouldn't be this poor at 50 MHz if you look at what is inside the sensor and model the VSWR. In theory, an older version of the 478A (like mine) should manage a VSWR of about 1.07:1 at 50 MHz. The later versions might be closer to 1.05:1 at 50 MHz. So in this sense, there probably isn't much point trying to improve the match if the aim is to measure a 1mW reference from a typical HP power meter. This is because the source VSWR of the 1mW reference is likely to be much lower than its spec of about 1.06:1. So the mismatch uncertainty should be negligible. Only a calibration house should really be concerned here, but I thought I would have a go at improving my sensor anyway.
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The following posts show the thought process and the modelling and the test results.
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I've attached a VSWR plot of the input VSWR of an early HP 478A sensor. This was taken with a lab VNA set to a really slow sweep time of about 20 seconds, although slowing the sweep time from less than 1 second to 20 seconds didn't seem to make much difference.
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You can see there is a kink in the VSWR at 50 MHz and this has always bugged me because it causes a bump in the efficiency across about 40-60 MHz. I've lived with this 'bump' issue ever since I first got this sensor back in the 1990s. But now I've managed to get rid of it and I'll show what causes it and how to upgrade an older sensor like mine to get rid of it.
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Regards
Jeremy |
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Re: 1 mw - 0dbm lab calibration
¿ªÔÆÌåÓýHi Jeremy, Nice thread on old stuf I hardly ever use. But now I got encouraged to do some tests as well. You certainly discovered that Appendix II of the O/S Manual of the HP432A gives instructions to install long cable options. So HP is very much aware of the total resistance values. Mine has Option 009 (10 feet cable) but I choose to disregard for the moment, I have the standard cable. The calibration procedure 2 is quite convenient making use of my HP34401 DVM. You need a long stabilization time.? I was wondering: how about thermocouple effects of the different connections in the chain from the thermistors to the internal bridges. For fun I measured the return loss of one of my HP8478B
(1833A13571) sensors. Quite good actually and certainly within
spec. See below. Thanks for the nice work and observations. Best 73 de Harke
On 8-5-2025 22:14, jmr via groups.io
wrote:
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HP8757A Display Problem (Scalar Network Analyzer)
Hi all,
I have a HP8757A scalar network analyzer with a display problem which has appeared after some 6 months of no use. It was previously working just fine, with a HP8350B sweep generator and the 10MHz - 20GHz plug-in.
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What happens is that on switch-on, it goes thru the self-test and then reverts to a display which seems to contain some correct information such as frequency span, key button names (at the right), and a 'noisy' scan waveform (as expected) - BUT the display refresh is very slow and is well off-centre - it is clearly cycling through the 4 'display channels sources' but it is nearly unreadable (certainly unusable).
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I have another 8757A which I use with another sweeper which works well, and by using this as a 'test reference' I can determine which buttons to the right of the screen to use to perform some tests. In this way, I have called up the test pattern display (SYSTEM -> MORE -> SERVICE -> DISPLAY -> TEST PATTERN) and this generates the wanted test pattern BUT this is also well off-centre and refreshed slowly.
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I have checked the error-code LEDs (removed the top cover of course) and they are all off (zero) after switch-on or after "PRESET" which indicates no error found, and also checked the +15V, -15V and +5V on the small power board with the fuses. The +105V rail there is also correct.
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This problem is independent of whether the 8350B sweeper is on or off, and if it is on the PRESET button initializes both the 8757 and the 8350 as it should. I have compared the problem unit with the working unit and the basic functionality would appear to be present. I am wondering if the battery on the CPU board (backing up some constants?) could be a reason, but I doubt it - I can replace this tomorrow if I think it might be.
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I know that the refresh frequency of the display will be lower with the more channels that are being scanned (A, B, C, R) but this does not appear to change the problem on the unit that I am dealing with.
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For what it's worth, I have a comprehensive workshop and lab at home and I am used to working with both microprocessor designs and (more so) with microwave / ATV designs. I have read the service manual and operating manual but with so many pages and different references I am not sure where I should be looking.
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Would anyone here have a good idea of where the problem could be?
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My sincere thanks in advance.
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Re: Looking for service manual/schematic for early E3611A DC Power Supply
¿ªÔÆÌåÓýI have started a new folder and uploaded all I have??? New folder?? E 3610,1,2 A ? Paul ? From: [email protected] [mailto:[email protected]] On Behalf Of Philip Freidin via groups.io ? You didn't specify which version of the schematic you had. I have 5959-5304? Edition 6 , October 2007.? Is this earlier than what you have? ? The document does not have a detailed component list, and many transistors, diodes, and op-amps on the schematic have no part numbers. The schematic does not include the meter section. ? Cheers, ? ? |
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Re: Looking for service manual/schematic for early E3611A DC Power Supply
You didn't specify which version of the schematic you had.
I have 5959-5304? Edition 6 , October 2007.? Is this earlier than what you have?
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The document does not have a detailed component list, and many transistors,
diodes, and op-amps on the schematic have no part numbers. The schematic
does not include the meter section.
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Cheers,
Philip ?
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Re: HP 8566B repair
I have checked the A3A8 board as indicated in the service manual and the results seem ok. The first photo shows the result with the "A" band selected. The second photo with the high band (Instrument preset) selected. Unfortunately I do not get the noise intervals on the high band, but only a continuous screen, as in the low band. Also I can no longer see any signal connected to the input, not even in the low band! |
Trevor Gale
Paul Bicknell
Harold Foster
Philip Freidin