HP8640B Question

Discussion in 'Wireless & RF Design' started by lenc0135, Dec 23, 2013.

  1. lenc0135

    Thread Starter New Member

    Dec 23, 2013
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    I mistakenly applied a very short burst of RF, maybe 30 watts, to my Sig Gen. It does not have OPT3. It still seems to work. Is this normal for the 8640B? Does the unit stop working completely if damage has been done? Could damage be done to the calibrated attenuator output?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. lenc0135

    Thread Starter New Member

    Dec 23, 2013
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    Yes, I do have the manuals which seem to be directed mainly towards testing and performance assurance. This would seem to be just what I need to answer my question, but I find that I don't have most of the required test equipment to make informed determinations. I am looking for someone's experience with this reverse power application, i.e., what is/are symptom(s) that rev pwr was applied...does it mean complete failure and no output? Output with loss of calibrated attenuation? This sort of generalized symptoms.
    Thanks
     
  4. lenc0135

    Thread Starter New Member

    Dec 23, 2013
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    Posting my question again: maybe I did not explain my problem clearly? I was sure that out in this site's huge readership someone would be able to give me an answer to a possible reverse RF accident. Here goes...I accidentally transmitted an unmodulated UHF signal (30 watts) into the RF output jack of my HP8640B. This is a common, stupid, mistake and there is an option (OPT3) diode that will provide some protection against it. I do not have that option. My gen still works, the attenuator seems to be OK. How can I be sure that some damage has not been done to the output calibration? Does anyone out there have any experience with this sig gen and this problem?
    Thanks
     
  5. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    The problem is that your looking for a direct answer and not one person can answer that for you .. It is up to you to find the schematic and go through the circuit and test each part in order to find the problem ... You could also test the pieces and post your finding with the schematic so everyone can see any flaws or anomalies..



    Hope it helps...
     
  6. lenc0135

    Thread Starter New Member

    Dec 23, 2013
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    Hi, thanks for the input. Yes, I understand what you are saying. The problem is, I have the schematics but to determine if the generator is working to specs requires a Spectrum Analyzer and a high accuracy freq and RF power source. The lab I bought the sig gen from uses a GP disciplined Rubidium Oscillator accurate to 10 to the 12th power...I don't have that equipment. What I was hoping for is someone who did what I did and could say: "Well, my sig gen no longer does such, or, Well, it is OK because the output was not damaged because it would be doing...??." But thanks for your suggestion anyway.
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I fully understand but at best your just taking an educated guess based on someone else problem and throwing parts is no way to fix something but you might get lucky sometimes .. If you hang around for a while here you will notice that most everyone here are simply people and work with what have and not just blow money on high equipment ...
    Always remember it is made up of smaller parts that can be tested and gone through because It only takes 1 part for it to stop working ..
     
  8. vk6zgo

    Active Member

    Jul 21, 2012
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    Well firstly,think about what has happened:-

    An overload to the output of a Signal Generator is most unlikely to in any way affect the frequency accuracy of the Generator,so I can't see the need for "a high accuracy freq and RF power source"------you are fault finding,not calibrating.
    A Spectrum Analyser would be really useful,but you may be able to do without it.

    Do you have anything you can look at the output signal with?

    If you have an oscilloscope,you may be able to look at a frequency in the middle of the 'scope's passband,adjust your output attenuator,& calculate the change in dB,using 20log v1/v2.

    A 'scope has an accuracy of about +- 3%,which sounds terrible,but it is less than 1.0dB.
    You need to terminate the 'scope in 50Ω.
    If it isn't available as a switch position,use a through termination,or a BNC 'T" with the termination on the other leg.

    At a fairly low frequency setting,you should be able to determine the output impedance of your Generator--the voltage displayed on the 'scope screen will halve when the termination is applied to the 'scope.
    At higher frequencies transmission line effects will not allow this test.

    An RF probe & DMM may let you look at the higher output ranges,& check the attenuator,again terminating the output of the Gen in 50Ω & probing the voltage across that termination.

    What attenuation setting did you have on the Signal Generator?
    If it was set at full output level,you probably haven't hurt the attenuator circuitry.

    Remember,the best test instrument you have is the one between your ears!!:D
     
  9. lenc0135

    Thread Starter New Member

    Dec 23, 2013
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    OK, OK, I'm properly chastised for not trouble shooting as you all have recommended. The reason for that is that the HP8640B is a very high accuracy generator--even tho they sell for a reasonable price on eBay. An oscilloscope (none that I could afford) can measure an RF signal at 0.05 microvolt at a frequency of 1024 MHz. Yes, the generator is that accurate at near microwave region. BTW, to one comment, I am not "throwing money" at anything. All I was asking for was someone's experience--if they have that sig gen and made that mistake--what was the outcome? Was their sig gen damaged?
     
  10. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    You have not been chastised.

    What we were trying to point out was that any damage is very unlikely to be confined to the 0.05μV region,or at 1024 MHz,but will probably appear at more manageable levels & frequencies.

    After all,if you think there is a problem with your Lamborghini's brakes,you are unlikely to take a test drive at 250kmh.

    I don't have a HP8640B (or a Lamborghini,for that matter),& most of us probably don't,either,so the likelihood of us doing the same thing & sustaining identical damage is extremely remote.

    If you are so worried that the mistake has affected the accuracy of the instrument,the answer is plain.
    Send it off to somewhere that has all the gear to recalibrate it,plus enough technical knowledge to repair any fault,& be prepared to pay for it!:D
     
    Last edited: Dec 27, 2013
  11. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
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