|
I'm at the stage now where I'm going to need to improve the test setup. This means using a sig gen with lower harmonic distortion and also using external levelling using a thermocouple power meter. The external levelling also improves the source match because I use an 11667A splitter in the external levelling system.
I will also have to factor in the response of my homebrew 'tee'. This replaces the official HP 11536A tee. I designed it many years ago using SMA connectors and it includes compensation for the 2.5pF input capacitance of the 8405A probe. I'm fairly certain there will be something similar inside the 11536A although I've never take one apart. The compensation improves the input VSWR when the probe is attached. I get similar VSWR vs frequency as the 11536A up to about 700MHz. With the 8405A probe attached, my homebrew tee manages a VSWR < 1.03:1 below 500MHz but it degrades to about 1.1:1 by 700MHz. Sadly, I can't match the VSWR or amplitude performance of the 11536A above 700MHz. There will be some extra compensation somewhere that I haven't got in my homebrew version. All I can hope to do here is try and get the same frequency response as the healthy A probe above 700MHz.
The simulation of my homebrew tee does show an upwards slope of about 0.1dB by 500MHz when a 1 port VNA model of the 8405A probe? is connected to the tee connection. So some of the slope I'm seeing will be in the tee. I suspect that the 11536A will be similar or maybe slightly better.
These upgrades in the test setup are needed because I'm hoping for yet more improvement when I try the BAT-62 diodes. There's a chance they will be here tomorrow, but they will probably arrive later in the week. I need to be able to measure the flatness to a few percent up into VHF and UHF so I need a very accurate signal source with very low source VSWR.
|
I set this up today. See the diagram below. This is a classic external levelling system as used professionally for metrology/calibration. The ALC feedback creates a voltage source (close to zero ohms impedance) at the input to the 11667A and this means that the source match to the device under test is defined by the quality of the series 50R resistor in the 11667A. A typical source VSWR would be < 1.02:1 at 500MHz. The level accuracy is defined by the accuracy of the power meter. Because the HP 8405A doesn't respond to Vrms, the harmonic content has to be really low to minimise uncertainty. I use a sig gen that typically has -55dBc harmonics up to about 350MHz. I also removed the two NXP diodes and fitted a couple of HSMS-282Z diodes in probe B. The results of the test are given below. You can see how good the original diodes are with the Channel A probe.? The Channel B probe has the two HSMS-282Z diodes in it and the results are quite good now. By 300MHz, the 1N6263 diodes would have gone off the graph so this represents quite an improvement. However, with the HSMS-282Z diodes, the input capacitance is just over 2.6pF and I really want to get this to just below 2.5pF to match the original probe used on channel A. I'm not sure how much closer I can get to the original response. I may have to start adding small metal shapes around the diodes to try and change the response. I can also try the BAT-62 diodes when they get delivered. The result below is probably good enough already. However, I also tried the same test on my other (really old) 8405A and it has original undamaged probes. The ChA to ChB tracking is really impressive on the older 8405A. Much better than the plot below. This has made me want to find some original diodes to fit to the newer meter.?  
|
I hope this isn't too much info, but see below for VNA derived plots of the parallel Rp and Cp of the HP 8405A diode when reverse biased by 3V. It's also compared to the 1N6263 diode across 10-100MHz. In reality, the Cp will be about 0.2pF lower than this because my test fixture has about 0.06pF (uncorrected) fringing capacitance and I had to test the diode without cropping the legs any shorter because I had to refit them back into the probe. So there could easily be another 0.15pF capacitance to free space in the test fixture. The main thing to do is look at the difference. 0.48pF for the HP 8405A diode vs 1.25pF for the 1N6263A diode at 3V reverse bias. The other plot shows the small signal dynamic resistance at 10mA forward bias. The earlier curve trace plot predicted about 11 ohms for the HP 8405A diode and the VNA plot below confirms this. It also confirms that the dynamic resistance of the 1N6263 diode is much higher at about 40 ohms. So this is another clear difference between these two diodes. I haven't bothered to put the NXP diode or the HSMS-282Z on the curve tracer or on the VNA as these diodes aren't close enough to the HP 8405A diode. However, I can do this at some point in the next few days if this would be of any interest. The BAT-62 looks like it could be a good substitute diode but I won't know until I get one delivered. I will definitely put this diode on the curve tracer and the VNA. I'm hoping it will be the best diode yet...  
|
Very nice work indeed! Thanks for making such careful measurements. That data is pure gold.
-- Cheers
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
toggle quoted message
Show quoted text
On 5/8/2024 10:29 AM, jmr via groups.io wrote: I hope this isn't too much info, but see below for VNA derived plots of the parallel Rp and Cp of the HP 8405A diode when reverse biased by 3V. It's also compared to the 1N6263 diode across 10-100MHz.
|
On Wed, May 8, 2024 at 07:27 PM, Tom Lee wrote:
Very nice work indeed! Thanks for making such careful measurements. That data is pure gold.
-- Cheers
Tom
Thanks! I had a play with a BAT62 on the curve tracer and it didn't behave as I expected. This is because I had initially misread the scaling on the IV chart on the datasheet and this diode actually has very high dynamic resistance at bias currents in the mA range. So (in terms of dynamic resistance) this explains why it didn't perform very well on the curve tracer. So I now have concerns it won't be right for use in the HP 8405A probes. Has anyone tried a BAT62 in the HP 8405A probes?
|
For completeness, see below for the IV curve for the BAT62 when compared to the HP 8405A diode. It's a very different diode in terms of the IV curve. My main concern is how the dynamic resistance will remain high, even when forward biased at a few mA. You can see that the BAT62 curve hits a knee in the IV curve at just over 100uA forward bias. Once this knee happens, the dynamic resistance tends to plateau for higher bias currents.? At 2mA bias, the dynamic resistance of the BAT62 could be in the region of 200 ohms according to the curve tracer. I could cross check this with the VNA, but I think it's best to keep looking for something even better than the HSMS-282Z as this is the best diode I've found so far. By contrast, the knee happens at just over 1mA with the HP 8405A diode so it will plateau with a much lower dynamic resistance and this is at about 11 ohms. at 10mA forward bias. To demonstrate this, I've included a prediction of the dynamic resistance for the HP 8405A diode below as well. This gives a fairly noisy curve but in reality, the curve should be very smooth. Excel creates the noise based on the way I compute the dynamic resistance. I look at the small signal change in current vs the small signal change in voltage at each pair of data points on the IV curve. The curve tracer takes just over 300 data points. If I'd taken more points the curve would be smoother. Hopefully all this info will help locate a suitable replacement for the HP 8405A diode. I don't know how much current passes through the diodes when the sampler turns them on. The on time of the sampling gate is going to be something like 300ps and this is about the same as the rise time of my fastest scope. So this isn't something I can easily measure. However, I think it's reasonable to assume that having a low dynamic resistance is a good thing here and there will be a forward current in the diode gate that achieves a reasonably low dynamic resistance.  
|
I may be getting close with the HSMS-282Z. I put one on the curve tracer and the result is as below. It is very similar to the HP 8405A diode in this respect. There is bound to be some spread between devices. It's also much warmer in my workroom this evening so this will possibly account for some of the shift to the left for the HSMS-282Z diode. There are lots of very similar Schottky diodes (to the HSMS-282Z) from HP that come in various packages so it might be worth exploring some of these to try and find one with slightly less capacitance. I only need a tiny reduction in capacitance and I think the agreement with the HP8405A will be much closer. The other thing to bear in mind is that the specification for accuracy vs frequency is quite relaxed for the HP 8405A meter. Using the HP8405A VVM datasheet, I've drawn a mask in the second plot below and the Channel A probe easily fits inside this mask. I think it could even be slightly better than this (below 200MHz) if I had used a power meter with a detector head with flatter efficiency. I use a 100kHz to 5.5GHz head and my other detectors are flatter than this one when used across 10MHz to 200MHz.  
|
jmr, I don¡¯t have an 8405A anymore so I haven¡¯t been following this thread too closely but today I was looking through my box marked ¡°HP parts¡± and found four diodes with the note ¡°5082-2835 for 8405A probe¡± and a package of 10 SMD diodes labeled ¡°Avago HSMS-2828¡± in the same ¡°8405A probe¡± bag. I remember replacing some diodes in the probes probably 20 years ago and I assume these are the extra parts I had ordered. Would they be of any interest to you?
toggle quoted message
Show quoted text
On May 9, 2024, at 5:04?PM, jmr via groups.io <jmrhzu@...> wrote:
?I may be getting close with the HSMS-282Z. I put one on the curve tracer and the result is as below. It is very similar to the HP 8405A diode in this respect. There is bound to be some spread between devices. It's also much warmer in my workroom this evening so this will possibly account for some of the shift to the left for the HSMS-282Z diode. There are lots of very similar Schottky diodes (to the HSMS-282Z) from HP that come in various packages so it might be worth exploring some of these to try and find one with slightly less capacitance. I only need a tiny reduction in capacitance and I think the agreement with the HP8405A will be much closer. The other thing to bear in mind is that the specification for accuracy vs frequency is quite relaxed for the HP 8405A meter. Using the HP8405A VVM datasheet, I've drawn a mask in the second plot below and the Channel A probe easily fits inside this mask. I think it could even be slightly better than this (below 200MHz) if I had used a power meter with a detector head with flatter efficiency. I use a 100kHz to 5.5GHz head and my other detectors are flatter than this one when used across 10MHz to 200MHz. <HSMS_282Z_IV_Curve.gif> <HP 8405A Accuracy Limits.gif>
|
Forgot to add, the 5082-2835 has a penciled note that says ¡°70v 150mw 2pf¡±
toggle quoted message
Show quoted text
On May 9, 2024, at 5:46?PM, Steve Krull <sw.krull@...> wrote:
? jmr, I don¡¯t have an 8405A anymore so I haven¡¯t been following this thread too closely but today I was looking through my box marked ¡°HP parts¡± and found four diodes with the note ¡°5082-2835 for 8405A probe¡± and a package of 10 SMD diodes labeled ¡°Avago HSMS-2828¡± in the same ¡°8405A probe¡± bag. I remember replacing some diodes in the probes probably 20 years ago and I assume these are the extra parts I had ordered. Would they be of any interest to you? Steve On May 9, 2024, at 5:04?PM, jmr via groups.io <jmrhzu@...> wrote:
?I may be getting close with the HSMS-282Z. I put one on the curve tracer and the result is as below. It is very similar to the HP 8405A diode in this respect. There is bound to be some spread between devices. It's also much warmer in my workroom this evening so this will possibly account for some of the shift to the left for the HSMS-282Z diode. There are lots of very similar Schottky diodes (to the HSMS-282Z) from HP that come in various packages so it might be worth exploring some of these to try and find one with slightly less capacitance. I only need a tiny reduction in capacitance and I think the agreement with the HP8405A will be much closer. The other thing to bear in mind is that the specification for accuracy vs frequency is quite relaxed for the HP 8405A meter. Using the HP8405A VVM datasheet, I've drawn a mask in the second plot below and the Channel A probe easily fits inside this mask. I think it could even be slightly better than this (below 200MHz) if I had used a power meter with a detector head with flatter efficiency. I use a 100kHz to 5.5GHz head and my other detectors are flatter than this one when used across 10MHz to 200MHz. <HSMS_282Z_IV_Curve.gif> <HP 8405A Accuracy Limits.gif>
|
Hi Steve, thanks! I think I have some 2835 diodes here but I can't find them. I do have an old curve trace of one though. I think I measured it at the same time I measured the 2826 diode used in the Marconi 2022 ALC system. I overlaid the 2835 curve trace with the 8405A diode and had a bit of a surprise. It looks really similar apart from a slight offset... see below! The capacitance is slightly lower than the HSMS-282Z? as well. So maybe this diode is even closer? It is only rated to 8V reverse bias though. I'm not sure how significant this is. I'd expect the P JFET to be more prone to transient damage than the diodes but I'm really just guessing. Are you sure you have 2835 diodes? They are 8V and 0.7pF parts and not 70V 2pF. I've also managed to find some HSMS-8202 diodes and some BAT17 diodes (both only rated to 4V) and some HSMS-2822 dual diodes that are 15V parts.? I may also try ordering some Skyworks 392x series Schottky diodes to try out. The other thing I need to do is spend some time making sure the rest of this HP 8405A meter is working OK. It is in excellent condition but I think I need to test the volt meter and the phase meter sections a bit more thoroughly than I already have. It is more stable than my older HP 8405A when tested for phase stability across a 10 minute test using a DVM to log the phase recorder output on the phase port at the back. Regards Jeremy 
|
?Hi Jeremy,
I need to take a closer look at the 2835s I guess. They are taped to a note card with the penciled in values in my scribbling. Glass body, I need to get a magnifying glass on it to see if there¡¯s any more information. I¡¯ve been called out of town for a few days, leaving shortly but will try to look at them before I go.?
toggle quoted message
Show quoted text
On May 9, 2024, at 6:52?PM, jmr via groups.io <jmrhzu@...> wrote:
?Hi Steve, thanks! I think I have some 2835 diodes here but I can't find them. I do have an old curve trace of one though. I think I measured it at the same time I measured the 2826 diode used in the Marconi 2022 ALC system. I overlaid the 2835 curve trace with the 8405A diode and had a bit of a surprise. It looks really similar apart from a slight offset... see below! The capacitance is slightly lower than the HSMS-282Z? as well. So maybe this diode is even closer? It is only rated to 8V reverse bias though. I'm not sure how significant this is. I'd expect the P JFET to be more prone to transient damage than the diodes but I'm really just guessing. Are you sure you have 2835 diodes? They are 8V and 0.7pF parts and not 70V 2pF. I've also managed to find some HSMS-8202 diodes and some BAT17 diodes (both only rated to 4V) and some HSMS-2822 dual diodes that are 15V parts.? I may also try ordering some Skyworks 392x series Schottky diodes to try out. The other thing I need to do is spend some time making sure the rest of this HP 8405A meter is working OK. It is in excellent condition but I think I need to test the volt meter and the phase meter sections a bit more thoroughly than I already have. It is more stable than my older HP 8405A when tested for phase stability across a 10 minute test using a DVM to log the phase recorder output on the phase port at the back. Regards Jeremy 
|
I had a look at the diodes before heading out of town. Definitely marked HP2 then 835 on second line and 714 on third. They were supposedly a matched set of 4. I think the note must have been for the 5082-2800 which I think we also tried.?
toggle quoted message
Show quoted text
On May 10, 2024, at 9:28?AM, Steve Krull <sw.krull@...> wrote:
? ?Hi Jeremy,
I need to take a closer look at the 2835s I guess. They are taped to a note card with the penciled in values in my scribbling. Glass body, I need to get a magnifying glass on it to see if there¡¯s any more information. I¡¯ve been called out of town for a few days, leaving shortly but will try to look at them before I go.? Cheers, On May 9, 2024, at 6:52?PM, jmr via groups.io <jmrhzu@...> wrote:
?Hi Steve, thanks! I think I have some 2835 diodes here but I can't find them. I do have an old curve trace of one though. I think I measured it at the same time I measured the 2826 diode used in the Marconi 2022 ALC system. I overlaid the 2835 curve trace with the 8405A diode and had a bit of a surprise. It looks really similar apart from a slight offset... see below! The capacitance is slightly lower than the HSMS-282Z? as well. So maybe this diode is even closer? It is only rated to 8V reverse bias though. I'm not sure how significant this is. I'd expect the P JFET to be more prone to transient damage than the diodes but I'm really just guessing. Are you sure you have 2835 diodes? They are 8V and 0.7pF parts and not 70V 2pF. I've also managed to find some HSMS-8202 diodes and some BAT17 diodes (both only rated to 4V) and some HSMS-2822 dual diodes that are 15V parts.? I may also try ordering some Skyworks 392x series Schottky diodes to try out. The other thing I need to do is spend some time making sure the rest of this HP 8405A meter is working OK. It is in excellent condition but I think I need to test the volt meter and the phase meter sections a bit more thoroughly than I already have. It is more stable than my older HP 8405A when tested for phase stability across a 10 minute test using a DVM to log the phase recorder output on the phase port at the back. Regards Jeremy 
|
This conversation reminded me that I have some HP matched hot carrier quads in storage. I went looking this morning and, surprise, surprise, they were where I thought they might be! The original box is marked 5082-6211 and it originally contained 10 sets of matched quads, and each envelope inside carries the same number. The individual diodes seem to be roll-marked:
HP
052
-3
I can't find any info online for this p/n but I didn't look all that hard either. Could these be of any use in this project?
73,
geo - n4ua
|
Thanks to both Steve and geo for the offer of the diodes. I'm in the UK so it might not be practical to send them to me?
I'm hoping to find some 2835 diodes at work on Monday. I've also got some here somewhere but I can't find them.?
The other thing is that the 282Z and 2835 diodes are long obsolete so it would also be nice to find a modern alternative that others can easily buy. I'm willing to try a few modern parts.
However, I will probably end up permanently fitting either the 282Z or 2835 diodes to this particular 8405A.?
|
Hello Jeremy,
thank you for this interesting thread on the HP8405A probes. I also got one of these VVMs but with two fried probes.
Having read W6RLC's article recently about his repair, I was thinking about ordering replacement FETs and diodes. PMBFJ176 are no longer available, but MMBFJ176 (40 ct @ Mouser) should work as well. The 1N6263 suggested from
W6RLC are also cheap but not an adequate substitute according to your measurements.
HSMS-282Z are unobtainium here in Europe, maybe I can get a hold on 5082-2835. Other suggestions ?
Looking forward to your next report.
Andreas
P.S. I managed to make replacements for the flaky meter scales of the HP8405A (laser marked with mirror cutout).
|
Hi Andreas Thanks for your reply. I've had to postpone my diode tests for a while, but I will revisit this stuff soon. One thing I recently found out is that the frequency response (up at UHF) might not be as good as shown in ?post ?#144234 ? This is because I wasn't using small frequency steps when creating that graph. If I use much smaller frequency steps, there is some additional ripple in the response up at UHF. I think this is because the 300ps wide sample pulses become increasingly significant wrt the period of the signal from my signal generator. This seems to affect the efficiency of the sampler and there is something like an additional +/- 0.3dB ripple up at UHF if I use much finer frequency steps.? My A probe has the original diodes and FET in it and it still shows this +/- 0.3dB uncertainty on finer step sizes. My older HP 8405A shows the same issue and it has original and undamaged probes. I don't think it's something obvious in the test gear as I've used this external levelling setup for many years up to many GHz. It's probably something to do with the 300ps wide sampling pulses and how they affect the efficiency of the probes up at UHF. Even a tiny change in frequency up at (say) 700MHz appears to cause up to a 0.3dB change in efficiency.? I know that the probes do leak energy from the sampler to the tip section but I don't think this will affect my levelling system. However, I haven't ruled this out yet. I do plan to set up the test kit to do a semi automated sweep via GPIB and this will allow fast testing using small steps (eg 2MHz) although I will have to pause it as required to allow the rotary lock control to be rotated on the 8405A to keep it in lock at all frequencies.? Regards Jeremy
|
Hi Jeremy and group,
any updates on the HP8405A probe repair ?
I've bought now a bunch of MMBFJ176 from Mouser and found a source for the 5082-2835 diode here in Europe. In browsing thru Avago Schottky diode data sheets, I found an interesting pair of common anode (HSMS-2823) and cathode (HSMS-2824) double diodes. From the data sheet their electrical characteristics look identical to the single 282Z you tested. Maybe they could make an equivalent circuit to the 4 separate diodes of the HP8405A sampler. What do you think ?
From the HSMS-282x series there is also a bridge and a ring quad (-P & -R) with identical (?!?) pin-out. But they have "wrong" orientation compared to the HP8405A sampler.
Any other suggestions ?
Regards?????? Andreas
|
Sadly, nothing significant to report from me. I tried putting a genuine 5082-2800 diode on the curve tracer and on the VNA and it is definitely a bit better (in terms of capacitance and dynamic resistance) than the modern 1N5711 made by ST. But it's not suitable for the 8405A probe.
I also tested the 2835 diodes I have here and I'm not sure they are genuine. The dynamic resistance isn't quite as low as it should be at 8mA and the capacitance is a bit higher than expected. So I didn't try them in the probe.
Since I fixed the drift issue in the 20kHz ref oscillator in my older 8405A I've been using it in preference to the newer unit. I also need to find some of the outer? threaded metal sleeves for the probes for the newer meter. The threads are damaged so I can't screw new probe needle tips to it. So it isn't as nice to use as the older meter as it's much harder to make reliable tip connections with the probes.
The older (made in 1967?) meter really is performing well now. It's also fully adjusted as per the service manual so it makes sense to use it instead of the newer meter. When I can find some more 2835 diodes here in the UK I'll have another look at this. I also need to find two new metal sleeves that screw on to the probes of the newer meter. The ideal scenario for me it to find an old scrap 8405A that I can salvage the probe sleeves from and maybe there may be some surviving diodes in the probes as well.
|
Hi Andreas. I think it might be possible to use the dual diodes and they might perform better than the single diodes with lower package capacitance? It's worth a try at least. I also have some dual Avago diodes here in SMD that I could try.
|
