Ping: Phil Hobbs

Another one was sold for about $3500 yesterday.

Thanks. It's actually the 694 I'm looking for--3 GHz instead of 2, and with the more responsive 600-series user interface, vs. the klunkier 700 one that I've got used to with my TDS744A. At IBM I had a TDS684C, which I quite liked, but it's not enough faster to be worth buying.

I'm still saving up for the next tuition payment, but once that's done, I'll probably be looking more actively.

I just got a spare HP 3403A true RMS voltmeter, because the 10 mV scale on my other one is hosed. Next on the list is getting my HP 8640A generator fixed. I'd also very much like a decent arb, but the need isn't pressing enough to shell out for one yet.

I've spent quite awhile lately trying to make a good noise source for calibrating really low-noise amplifiers. I was using small varactor diodes selected for conformance to the diode equation. Using simple theory, driving a diode with 0.5 mA should make a 50-ohm resistor with a noise temperature of 150 K (actually 0.5*T_j). Selecting a good one should get me a nice refrigerated noise source. Abrupt-junction varactors are the best, but in practice the capacitance is too big, even with a small one. It should work fine below 10 MHz or so, but not at

Next thing on the list is a SFH2400 photodiode (2 pF) across 25 ohms, in series with another 25 ohms. Turning on 2 mA of photocurrent will give me a 1.76 dB noise increase over the Johnson noise of 50 ohms, which should be good enough to use.

Cheers

Phil Hobbs

That's interesting Phil, So varactors have a 'non-ideality' factor that is close to one? And not the more typical value nearer two as for signal diodes?

George H.

A TDS694 that is.

Now, now ... do you also change vehicles when the ash tray is full? :-)

Can't it be fixed?

How fast of an arb do yuo need? And how much memory?

Just bring the amplifiers here:

:-)

Drats. I just bought a TDA744A. Is it that bad? I have a 510A and a

I do. Every now and then I use my wife's car. She then takes my car to the car wash and empties the waste bin :-)

This looks like a neat little gritter:

I like the secondary output. Specs aren't great but it still seems usefull.

Hmm, maybe I'll try that as well. Except I don't smoke :-)

That's exactly the one I have, except mine has another brand name. I think it's a Chinese design and when you look for the brand there isn't even a web site. But it works nicely.

I bought it here but they are constantly out of stock lately:

When you car gets dirty, aren't you supposed to buy a new one ?:-) ...Jim Thompson

le

tting my HP 8640A

Hi Joerg, So you still like your 'Quakko' DDS? We've been using/ reselling these cheap Protek DDS's here,

On many scales you can change the displayed numbers, but the frequency doesn't change... ughh. And at the last trade show, I found that if you left them alone they would sometimes stop outputing anything. Pushing a button brought them back to life... but still it took a bit to figure out where my signal went. I'd like to try something else.

George H.

Yes, I like it after they swapped out mine. The first one turned out to be defective, a bug masquerading as a firmware error that had them puzzled for a long time. They aren't terribly responsive to begin with but that's not too surprising with Chinese gear.

Now it works nicely.

Try the one I have, it's sold under various brand names. My source is out of stock but you could ask when the next shipment arrives.

One thing I like is the USB control. The generator sits way high on the equipment shelf. I can't reach it from everywhere on the bench but I can always reach the laptop.

Sure it can, and when I get motivated, it will. But I rely on it, so for under $200 it makes sense to have a spare. I paid $95 for the first one, but that was back in 2009 when stuff was cheaper. One of these times I'll declare it to be fixit week, and get all that stuff sorted.

Nothing that fantastic--say 250 Ms/s, 12 bits, 100k samples. Mostly something that I can use to play back recorded tone bursts in noise, stuff like that.

Thanks. It would have wound up being much cheaper to just buy a nitrogen dewar, but some of this wisdom will be saleable eventually, I expect.

Cheers

Phil Hobbs

Train one of your kids, make'em earn that tuition :-)

Ok, that puts it into the professional court. Don't know any good deals there right now.

I was always wondering, "Hobbs Scientific Instruments, Inc." could make sense. When you look at the sky-high prices of some of the Thorlabs stuff you could make a killing even if you sell at half price. A client once asked me if I could take a look at a FO-to-electric device because they wanted so much money to repair it. Took 10mins to fix it (with coffee). But when I opened it I almost screamed "WHAT?!". The profit margin must have been huge. Even their optics engineer who is used to high prices stood there in disbelief once I had it open.

The Lecroy LW400A series can add noise to the output signal. It has a seperate noise generator for that. You can also supply your own noise. The DAC is 8 bits though.

Thanks. I used to have a Tek one--AWG2400 or something--that I didn't like much, but whose specs were roughly like that. The software was a nightmare.

Adding my own noise is what I'm trying to avoid--in thresholding, for instance, it's useful to have a library of near misses and squeakers-under-the-wire, because it saves a lot of testing.

Cheers

Phil Hobbs

I've thought about it. JL and I have a collaboration going on some of those things, one of which has already seen the light of day, and I have a few other nibbles. Ideally I'd like to license stuff rather than build it, but if various family members express an interest, we might try doing one or two of our own. [I did go ahead and start learning Eagle. ;) ]

Cheers

Phil Hobbs

Same here. NY and CA are not biz-friendly areas, where it may be better to stick to pure consulting. The millisecond you sell merchandise of any kind the goons come. Unless you already run a merchant biz anyhow.

Just resist the temptation to do your own layouts unless it's all super-critical stuff. John has in-house resources for layout :-)

Yup. Out of one house, but still in the other. ;)

I do need somebody local to do PC layouts and BOMs--it's much more efficient to be able to do stuff together with the layout human, rather than tossing stuff over the wall back and forth.

Cheers

Phil Hobbs

Absolutamente, especially if it's hot RF stuff. That is why my main layouter is near 65th street, about 45mins from here if I avoid rush hour. Best thing is, he and I are politically on about the same wavelength :-)

No problem, I actually know it was the 694 you were looking for, I think I jumped when seeing the "94". Or something...

Missed it by THAT much!

Huh. I never noticed any difference. But I'm not a Tektronix user, I only repair and sell 'em - got to go with whatever sells, and Teks sell. I prefer Agilent scopes myself.