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Old 10th January 2017, 10:11 PM   #1
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Default Attenuator DC-RF

I've got a couple of signal generator boards with 50Ω output, one that goes to 10MHz and the other to 65MHz.

The first can't output signal less than half a volt p-p, the other is better - down to 1mV but still not to zero.

Knowing pretty much nothing about higher frequency stuff I'm wondering if it's worth building my own variable attenuator with a few resistors and/or a pot or if it's better to buy a used HP box off the bay.

Any info, advice, or good stories welcome .

Thanks
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Old 11th January 2017, 12:31 PM   #2
skidave is offline skidave  United States
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Try this: http://www.elecraft.com/manual/E7401...%20Rev%20A.pdf

Good info within the document and you can build a version for yourself based off of the information.
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Old 11th January 2017, 01:42 PM   #3
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There are virtually no RF attenuators that will go down to zero RF output. There will always be some RF leakage getting past the attenuator. It is usually possible to get a low enough signal voltage for your particular application, but those last few dB may be hard to get or expensive. Even the top of the line HP signal generators of today start out around a tenth of a microvolt.

I spent 41 years of my life as a tech / engineer in a Motorola plant that designed and made two way radios and cell phones. Ten years of that were spent in the test equipment lab. It was a continuous problem until well into the 2000's that we had to modify and shield even the best signal generators that HP / Agilent / Keysight made.

In the 90's we sent a whole batch of HP8656A generators back to HP because the RF leakage was so bad that the radio would pick up the signal generator without a cable connecting them together. They sent an engineer to work with us to fix the issue. It required additional shielding, and redesigned attenuators. Around 2000 HP adopted our shielding methods as an option on their high end RF generators.

The internal oscillator in most RF generators runs in the 1 volt or so range. The sensitivity of our radio equipment at that time was such that a signal of 0.25 MICROVOLTS was clearly received. The leakage issue gets worse as the RF frequency is increased and by the 90's we were making radios and pagers at 900+ MHz.

The Elecraft design shown has a maximum of 41 dB of attenuation. It will turn a 1 volt signal into roughly a 10 millivolt signal. It would be possible to build two, or even three of these and cascade them. For this to work, each attenuator would need to be housed in its own RF tight box with RF connectors on each end.

I just finished repairing several HP RF signal generators. The good ones, HP8656A's from the batch sent back to HP, scrapped when Motorola closed down, use two separate 0 to 70 dB attenuators, each inside their own box, with both housed in its own shielded compartment inside the generator. The ultra high spec HP8664A's have three attenuators, one inside the RF generator module, and two in separate boxes inside the RF output module.

Tell me a little more about your application, maximum RF frequency, minimum and maximum RF level needed, and we can figure something out.
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Old 11th January 2017, 03:08 PM   #4
DF96 is offline DF96  England
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As Mr. Tubelab has said, making decent RF attenuators is hard for those who are real experts. It is harder for those who know less. As he said, it is a waste of time unless the sig gen is well screened. You can be reasonably sure that sig gen screening will not be good enough to let you get far below the minimum output already offered.
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Old 11th January 2017, 03:39 PM   #5
skidave is offline skidave  United States
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Tubelab is correct as DF96 said. I do nothing with RF myself, but I needed an attenuator to attenuate a 10 MHz signal for some older HP volt meter calibration. I used the pi calculator to make an attenuator that worked for my application. I installed it in an aluminum enclosure and it suits me just fine.

HP made a bunch of RF attenuators with models numbered 355. Here is a Google return for you: https://www.google.com/search?q=hp+3...24BRAQ_AUIBygC

You can find these on eBay for a reasonable price.

The HP 353a is a transformer coupled impedance matching attenuator. The actual attenuator section inside is a shielded unit and makes a great 600 ohm unit by itself.

The HP 350d is a 5 watt 600 ohm attenuator.
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Old 11th January 2017, 03:58 PM   #6
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The attenuator shown in the Elecraft link will probably work as designed up to 200 MHz or so, but may require an enclosure to be accurate above 100 MHz or so. The switches and resistors used DO matter.

Some leaded resistors become inductors in the VHF range. Any attenuator I made used SMD resistors. They are good to at least 1GHz if the board is laid out with 50 ohm runners above a ground plane. Contrary to popular belief ordinary FR4 will work up to 2.5 GHz with minimal losses if the layout is good.

The tiny toggle switches found on some of the Chinese web sites are good to 1 GHz as are some of the Panasonic tiny (non RF relays). For better RF performance RF specific reed relays are best.

At frequencies above a few hundred MHz the enclosure needs to have a separate compartment for each switch, especially 5 dB and above. Otherwise the RF will leak right past the resistors.

SMA or type N connectors are preferred above 500 MHz.

I use one of these. It's good to 1.5 GHz and you can't make one for $60. I just used this listing for example. I don't know anything about this particular unit, since it's too easy to blow these things with a radio transmitter.

KAY 837 Attenuator PMAX 3W DC-1.5GHz 50 Ohms | eBay
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Old 11th January 2017, 05:32 PM   #7
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Skidave,
Thanks for the link. I'd forgotten about Elecraft and it was interesting to see their stuff. The schematic was interesting as well.

George,
I was kind of hoping you'd appear. I actually had you in mind when I wrote the "good stories welcome" above.

The generator boards are from a Cleverscope. The CS700 is installed but there's also a CS701 that does sine wave only (though it's not installed and not sure if it works yet).
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Old 11th January 2017, 08:47 PM   #8
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I looked at the specs for both units. maximum frequency is 65 MHz and both units have a BNC output jack. There are two easy paths here.

The easiest is to purchase a few 20 dB fixed attenuators from a reliable source. Use one or more at a time connected end to end, each one will reduce the output voltage by a factor of 10. One attenuator will reduce 1 mV to 100 uV, two of them, 10 uV, three of them 1 uV.....Brand new ones cost $10 each from Mini Circuits, model # HAT-20+ They are good to 2 GHz.

I used the SMA versions at work, and there were no issues with any of them unless someone screwed one of them into a 5 watt radio and transmitted through them. I did this on purpose to emulate weak signal testing, and even still they don't blow, they just lose accuracy.

The other option is a step attenuator like the KAY (or HP) unit I shower earlier. Again each 20 db will drop the signal voltage by a factor of 10. The switches will eventually wear out, but it took several years in a daily use situation (with the Kay units) but they don't like transmitters.
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Old 12th January 2017, 02:05 AM   #9
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Thanks George.

The SMA's are clean and take little space but I know that if I have to put several in series I'll always end up standing there with one in hand, trying to figure out where I put the other two.

For a while I thought it would be cool to build my own until I saw this post on QSL.net where it shows the machined compartments for each stage of attenuation. That together with your story about the HP 8656A's put things in a different light.

If the Kay is what you use who would argue? There's a place that caries some surplus here in town I'll give a call tomorrow.
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Old 12th January 2017, 04:41 AM   #10
Frex is offline Frex  France
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Hello ,

Maybe you can look this :
http://www.qsl.net/k5bcq/Kits/Kits.html
(search " The Attenuator Kit #13 " on the page).

It's a 20$ kit but not sure if that would do the job up to 65M.

Frex
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