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Author Topic: Test Equipment Old Vs. New  (Read 13706 times)
KL0S
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« Reply #15 on: March 01, 2011, 04:42:41 AM »

I'd sure like to have a VNA (Vector Network Analyzer) on my test bench. But I don't have $40,000 laying around to buy one. :-)

You might want to check out DG8SAQ's VNA at:

http://www.sdr-kits.net/VNWA/VNWA_Description.html

Dino KL0S
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KA5IPF
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« Reply #16 on: March 01, 2011, 06:42:09 AM »

Hp 8754 for abt $600 on QTH. Oldie but a goodie...

Clif
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W0BTU
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« Reply #17 on: March 01, 2011, 11:42:34 AM »

Thank you, Dino and Clif. I see that there is also one designed by N2PK.

And the March QST I got today shows a review of one that Array Solutions sells for under $1500.
« Last Edit: March 01, 2011, 08:55:55 PM by W0BTU » Logged

W0BTU
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« Reply #18 on: March 01, 2011, 08:58:44 PM »

I'd sure like to have a VNA (Vector Network Analyzer) on my test bench. But I don't have $40,000 laying around to buy one. :-)

You might want to check out DG8SAQ's VNA at:

http://www.sdr-kits.net/VNWA/VNWA_Description.html

Dino KL0S

That looks like the least expensive one of all. :-)

Wonder how accurate, etc. it is, how it compares to others with similar freq. coverage?
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VE3LYX
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« Reply #19 on: March 02, 2011, 05:16:37 AM »

I have a Stark Signal generator (Tube type from the 40s). It was free. I have another one too but never needed it. I also have a pixel tube HP freq counter (also free) and a HP DB meter. I figure this stuff will still be working long after me. Cost $0. "just get it it out of here" stuff.  Checking the signal generator against a modern rx s digital readout find no descernable error . It is exactly where it is supposed to be. It is a very handy tool for me for HF stuff.
Of course I am still in the dark ages and think a vacuum tube voltmeter is an exotic piece of test gear. I hoping I wont have to change.
Don VE3LYX
« Last Edit: March 02, 2011, 05:18:22 AM by VE3LYX » Logged
G3RZP
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« Reply #20 on: March 05, 2011, 02:41:22 AM »

The problem with the VNA is that when you get far away from 50 ohms, a quite large change in impedance produces a small change in return loss. So going from 6000 ohms to 6500 ohms doesn't produce much change and you're looking to measure it accurately.  Years ago, there was an article in the HP Journal on the subject.
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W0BTU
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« Reply #21 on: March 05, 2011, 12:06:24 PM »

The problem with the VNA is that when you get far away from 50 ohms, a quite large change in impedance produces a small change in return loss.

Thank you. Are they all like that, even the expensive ones?
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W6RMK
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« Reply #22 on: March 05, 2011, 04:25:20 PM »

Yes they are, even the >$100k units from Anritsu, Agilent, etc.

It's inherent in how they work.

If you need to measure impedances that aren't near 50 ohms.. then get yourself an impedance measuring system, whether bridge/null or direct measurement (vector voltage).
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W0BTU
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« Reply #23 on: March 05, 2011, 05:17:14 PM »

Is 75 ohms near enough to 50 to obtain an accurate enough output from a VNA in most cases? That's about all I use here.
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G3RZP
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« Reply #24 on: March 06, 2011, 04:55:54 AM »

The errors really become appreciable at 500 ohms or 1 k ohm and above. At 75 ohms, with a perfect directional coupler in the device, you'd see a return loss of 14dB. If that was out by 1 dB, then your 75 ohms would come out as somewhere between 72 and 78 ohms: alternatively, anything between 72 and 78 ohms could come out as 75. A 4% error.

Now look at waht happens with a return loss of 1dB. That equates to about a 900 ohm load. If the 1dB is in error by 0.5dB, the load can be between about 600 ohms and 1750 ohms.

The examples are extreme values, but you can see that an error in measurement of even 0.05dB can have an appreciable effect with impedances of 2000 ohms and more. As W6RMK says, you need a different approach such as an impedance bridge of a vector measurement at these impedances. At high frequency, strays become appreciable, but resonance techniques can work well.
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W0BTU
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« Reply #25 on: March 06, 2011, 04:25:12 PM »

Thanks for the advice, which helped steer me in the right direction in some further research I did today. Looks like I'll be getting some different equipment before I get a VNA.
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KA5IPF
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« Reply #26 on: March 13, 2011, 09:49:27 PM »

And another apparent goodie. HP 8656B Sig Gen on the bid place # 190511146252 . It has a $250 buy-it-now. Looks nice.

Clif
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