Electolytics in signal path??

Greetings, gentlemen,

Would I be correct in thinking that no commercial enterprise, least of all one like the one-time gloriously sublime HP, would put a 2.2uF electrolytic cap in the signal path of a high-end RF board, surely?

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Cursitor Doom
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Why not? Size or ESL might be considered for really high frequency apps.

2.2u implies a really low frequency corner. What instrument might that be?
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John Larkin   Highland Technology, Inc   trk 

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John Larkin

It's an HP 8566B spectrum analyser 100Hz-22Ghz (but that is split into two ranges at 2.5Ghz). It's got the "YTO unlocked" error - a known issue with these otherwise excellent vintage instruments - which I'm in the process of chasing down.

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At higher frequencies that ESL might show itself.

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jurb6006

also depends on the age of the equipment - if it's old (okay all HP-branded test equipment is "old" to some standard in 2019 I guess, HP was been making equipment since the 30s or 40s?) an electrolytic might be all they have to work with as compared to wax paper caps, IDK if they even made them that large.

If it's "newer" I don't think good-quality monolithic SMT ceramics in that size were available in the 1980s and early 90s and a 2.2u through-hole or axial film cap is gonna run you what, $3 in singles? Even HP has to hit a budget

Reply to
bitrex

An electrolytic has a lot in common with a really dead battery, so I'd be concerned about temperature gradients causing voltage offsets.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

it's a nice unit that I estimate the chances of CD successfully fixing up at 'bout 0%

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bitrex

Bet they taught you all about HP stuff in art school. ;)

Cheers

Phil Hobbs

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Phil Hobbs

LOL! You got that right, Phil. :-)

Actually, I'm a pretty darn good troubleshooter and have fixed a lot of high-end gear in the past, although admittedly this one's probably the most complex unit I've undertaken to date. Given an oscilloscope and a schematic I can most times succeed in finding and fixing these faults. I'm way better at fault-finding than I am at designing, anyway. You have one of these IIRC, don't you, Phil? Have you ever had the "YTO unlocked" error? It refers to a YIG-tuned oscillator which I've never heard of before. I'm hopeful I can rectify the fault without having to spend time studying the minutiae of YIG-tuned oscillators first!!

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Cursitor Doom

Yup, it's by my elbow right now.

A YTO is a current-controlled oscillator, as opposed to voltage-controlled. There's a yttrium-iron garnet (YIG) cavity resonator, whose resonance is controlled magnetically via a coil.

Cheers

Phil Hobbs

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Phil Hobbs

Right, so that's this mysterious "main coil" the schematics are referring to I would imagine. You obviously haven't had the issue with yours yet. It's pretty common, so I'll be surprised if you escape indefinitely. The good news is it's almost always something very cheap and easy to fix that's gone awry; usually a trimmer pot or a fixed value capacitor so we're talking pennies. The tricky bit is identifying exactly *which* small, cheap component has failed. :)

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There's also often a vernier coil with much lower inductance (and so much less tuning range but higher speed) that's used for the actual phase locking. YIG-tuned oscillators have unbeatable close-in phase noise, which is why an 8566B can still run rings around any SDR-based system for phase noise.

I haven't had mine apart yet, except for swapping out a bad A3A4 display memory board, of which I happened to have one in stock. One fine day I'm going to declare Fix-It Week and get a bunch of sick gear back online. For instance:

HP 3585A DC-40 MHz spectrum analyzer (a fave of Joerg's that has an LO unlock problem)

Tek 466A analogue storage scope--worked fine till I turned on the storage function, and never since.

Tek TDS 694C 3-GHz, 10-GS/s scope that emits a piercing shriek from the CRT high voltage section for the first half hour of operation. Still in use, but needs an LCD transplant.

Tek P6249 4 GHz probe for the above, which oscillates at about 2 MHz--probably a dead cap someplace.

Plus a couple of RMS voltmeters (HP 3400A and Boonton 93A), a couple of HP 3456A 6.5 digit DMMs, and various smaller stuff.

Cheers

Phil Hobbs

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Phil Hobbs

I never understood how a magnetic field can be controlled so well as to result in that low phase noise.

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John Larkin

AIUI the inductance of the main coil is some henrys, so that isn't as hard as all that.

Cheers

Phil Hobbs

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Phil Hobbs

No surprise to me at all about that last point. You may have the advantage over me if you have a physical hard copy of the service manual. I have to scroll through 900 pages of pdf files of varying scan quality to find anything I want, which is a PITA, but I suppose I should be damn grateful for being able to even have that.

Oh me oh my that's going to be a busy "week" for you. :-D. I was able to rescue a 466B (I think it's the B IIRC) from a charity shop. It wasn't working but I got it going quite easily and am really pleased with it. There's something compelling about analogue storage scopes for people of my age (don't ask). I have a modern Tek DSO as well, but if there is something the 466 can handle it's my first choice now, as it's so easy to intuitively set up in under 2 minutes whereas if it's the Tek I have to RTFM every time, so 50 mins of set up minimum *if* I'm very lucky. I suppose it would be different if I were using it every day but I'm just a hobbyist and have so much test gear I can't possibly remember the quirks of every single instrument. I recall you have an impressive selection yourself so you know where I'm coming from on this no doubt.

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I've never understood how anyone could think an SDR-based instrument could equal an analogue device for spectral purity/low phase noise.

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My situation is a bit different as it's my day job. Also, I furnished my lab at IBM with the same or very similar equipment when it was new. For instance, I used an HP 8566A fairly intensively back in 1982-3 when I was doing satcom frequency control stuff.

Knowing the equipment is the main reason I've been able to assemble a very capable lab of my own for ~ $60k. It's probably more than $2M at the original list prices.

It's a great time to start out on your own.

Cheers

Phil Hobbs

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Phil Hobbs

It may happen. Modern frequency synthesizer chips are pretty amazing compared to the old single-loop analogue variety.

Cheers

Phil Hobbs

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Phil Hobbs

Not at my age! (don't ask). Mine too comes out at around $2.5m worth at new prices. Only worth a small fraction of that value today, but hey-ho.

Better go do some work I guess. :(

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As long as the number of turns of wire doesn't change, which goes without saying, and the current through the wire doesn't change, which is easy enough to ensure, the field remains more or less perfectly stable. There are hysteresis issues, but they don't affect steady-state operation.

The 8566 is nothing special in the phase-noise department, but they got the UI and the display right, and it is *stable*. I have several other SAs around here, some much newer and much more powerful than my 8566B. The 8566B will only be carried out of here after I am, while the others come and go.

-- john, KE5FX

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John Miles, KE5FX

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