Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!

Jim Ford

Great story, Tom, about Polarad and HP competing.

Polarad, I believe got bought by Rohde & Schwarz. In the mid 1980's I worked for the Varian Beverly Microwave Division north of Boston, and we had 2 nearly identical measuring receivers, one with a Polarad badge and one with an R&S badge. We used the two channels in them to measure the phase difference of the Constant Phase Limiting Amplifiers we were building.

Jim Ford

------ Original Message ------
From: "Tom Lee" <>
Sent: 12/8/2020 10:25:37 AM
Subject: Re: [TekScopes] SUCCESS! The "sick" 475A is now the "fixed" 475A!


I would not spend too much time looking at the 491’s docs — it’s a very primitive instrument architecturally, and today serves mainly as an example of how not to design an SA. The 492 is a completely different instrument. If you’re going to study a Tek SA, that’s probably the best starting point (along with the related Tek Concepts books).

Raymond pointed you to the HP Memory site, which has an amazing wealth of information, including many reminiscences of the design teams who created many of their groundbreaking instruments. Often they’ll utter a throwaway sentence that explains many mysteries (“Oh — THAT’s why they did it that way!”). You can spend many hours there. Way better than anything on Netflix; it’s Nerdflix, except no flix.

One of my favorite stories is about their first SA, the 8551A. It was revolutionary because they were able to tame a beast called the backward-wave oscillator (BWO), which is a vacuum tube oscillator that can be tuned over octaves. The BWO was, and is, infamous for having a maddening ability to burst into song at unwanted frequencies, but HP’s engineers figured out how to tamp down that tendency. I won’t burden this thread with a long explanation about why a broadly-tunable oscillator is a good thing for an SA, but just accept for now that More is Better here, and that HP’s solution enabled a kick-ass SA.

Polarad, which was then the dominant SA supplier, attempted to defend their franchise by underbidding HP for a huge DoD contract, and set about copying the 8551A. Polarad’s engineers could not solve the BWO problem, though, so they ended up fulfilling the contract by buying 8551As from HP at full price and rebadging them. HP made a fortune off of that.


Sent from my iThing, so please forgive typos and brevity.

On Dec 8, 2020, at 8:42 AM, Jeff Dutky <> wrote:


Well, my neural net seems to be quite open to calibration, but my eyes are another story

I have actually been running tests (quantitatively unsuccessful, but qualitatively enlightening) on the visual bandwidth of my scopes (feeding in a Z-axis signal and seeing how many discernible dots I can fit across the screen), and this has really driven home what the actual precision of the scope display is (it's not more than about 9 bits in either direction). I recall my father saying something similar about the 4014 terminals that he used: the resolution of the drawing commands was 4k x 4k, but the actual screen resolution was much lower (I don't recall the exact numbers, but i now expect that they couldn't resolve more than 512 dots across the small [12 inch?] screens, and probably not more than 1k across the large [20 inch?] CAD displays we saw once at a Goddard open house). By my calculation the 400 and 2200 series scopes aren't actually displaying more than 640x400 pixels, and only 320x200 resolvable, distinct lines. This severely limits the precision of any measurements you make by eye on screen despite the apparent smoothness of the waveforms.

I'm still a little mystified by exactly what concrete tasks I might need a volts or watts per frequency measurement for, but I'm happy to wait for enlightenment while I play with a new toy. I'm a very hands-on learner, even when I'm rigorously going through theory need to have concrete examples to test myself against and to illustrate academically presented topics.

I see that there were two SAs in the 400 series, one much more "analog" (and of much older manufacture) than the other, and at least two more in the 2700 series (they look like siblings to the 2200 and 2400 series scopes). They seem pretty rare on eBay, and are asking much higher prices than the comparable era scopes. At some point I will need to read through the manuals for the 491, just to see how the spectrum analysis was done in the days before microprocessors.

Everything I have learned since taking up this hobby has fed a keen awareness that, even when they are in perfect working order, your instruments are lying to you (if only by omission).

-- Jeff Dutky

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