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Spectrum analyzer

Discussion in 'General CB Services Discussion' started by kaos513, Aug 12, 2020.

  1. kaos513

    kaos513 Sr. Member

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    Aquired an hp spectrum analyzer 8590a any help would be appreciated how to setup to align my 29ltd thanks.what else would I need thanks.


     

  2. guitar_199

    guitar_199 Sr. Member

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    This is a favorite of mine. Shows using a spectrum analyzer to troubleshoot a "mixing/ if strip" problem using a spectrum analyzer.



    Also, I can not tell from your original post ... what you know about this already. Not intending to slight you in any way..........

    One thing to be SURE that you understand before you start using it... is the need for proper attenuation of the signal before you EVER feed it to your SA. Those things are EXTREMELY sensitive on the front end and are very easy to damage if you put too much signal in them. From what I have seen they generally like the neighborhood of 0 dbm input (1 mWatt over 50 ohms) with a max of 10 dbm (10 mWatt over 50 ohms) so you can't just drive the transceiver into it.

    Normally one pipes the transceiver into a dummy load with an RF Sampler of some kind between them that taps off a fraction f the RF to feed out to the analyzer and match the impedance of the analyzer (typically 50 ohms).

    I can dig up some reference info for you if you like!

    I wound up building my own RF sampler. I hand wound the toroid and built it into a project box. It worked about right.

    To know what I am talking about here.... I offer this video from YouTube. He builds a variable RF tap and demonstrates it's use on his bench.



    If you are not familiar with decibels in RF this video might help.



    Again, I do not know how much you already DO know .... so no harm is intended. Just wanted to include a smattering of things that I myself had to learn a year or so ago when I got my SA.

    Bob
     
  3. kaos513

    kaos513 Sr. Member

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    Well I know the unit passed no fails as I ran test
     
  4. kaos513

    kaos513 Sr. Member

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    I have a rf samplerl
     
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  5. guitar_199

    guitar_199 Sr. Member

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    Excellent.... you are far ahead of where I was when I got mine! I didn't even know I needed one!!!!! :LOL:

    The next lesson I have to get over is..... DON'T BUY CHEAP CABLES. I bought a handful of very very cheap RG-58 cables with BNC's. I didn't get my sampler built until just a few months ago.... so those cables were NOT used until I got to testing it.
    What I found was....I could key up my HT and see pretty much a full screen display of the output of my radio on the SA.

    To be honest, I don't know...maybe that will happen even with good cables....but ....I have NOW see the shielding on these cheap cables...... and it was a "face palm" moment fro me! "You get what you pay for" never rang more true.

    Just yesterday I was looking at a decent crimper kit to make my own out of quality cable.

    Have fun with that SA. Having one has been an eye opener for me. I understand that good work can be done without one....but I think it sure makes it easier.
     
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  6. guitar_199

    guitar_199 Sr. Member

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    I promise not to bore you forever.....but I found a couple of videos that show where an SA really "shines" (IMHO!). They underscore the value of "clean RF spectrum'. He clearly shows the effects of harmonic radiation and how a simple power meter will show "increase in RF power" BUT the SA will show that the increase is NOT in the band of interest.... so it is not doing you any good!!!


    "{Why not to "Peak" and tune, clip AMC limiters. It only causes distortion."



    "Golden Screwdrivers stop playing with and adjusting the 54 MHz trap filter circuit in CB radios."
     
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  7. kaos513

    kaos513 Sr. Member

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    I made a good rf sampler and I also have one I bought off of eBay
     
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  8. kaos513

    kaos513 Sr. Member

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    Here they are
     

    Attached Files:

  9. kaos513

    kaos513 Sr. Member

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    Noticed my rf port on spectrum analyzer says 75ohms how can I convert it to 50 ohms ?
     
  10. guitar_199

    guitar_199 Sr. Member

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    I don't know of a way. I'll step back and see if someone else does......
     
  11. NZ8N

    NZ8N Well-Known Member

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    The difference between 50 ohms and 75 in the sample line to your test equipment is negligible. You are not "transmitting" on the test equipment, only "receiving". So long as the throughput line from radio under test to meter to (dummy) load is still 50 ohms impedance, you should be good. I have used regular 75 ohm splitters to split a tap point to my frequency counter on one out port and my O'scope on the other.
     
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  12. blasphemy000

    blasphemy000 Active Member

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    Mini-Circuits makes a minimum loss input pad that will convert the 75R input of your spectrum analyzer to 50R with a 5.7db loss across DC-2GHz I believe. It's the BMP-5075R+. Here is a link to a place that has them.
    https://signalhound.com/products/bmp-5075r/
    I found them from a YouTube video by W2AEW, video #51.

    If the tap/sample port on your RF Sampler is designed to be terminated into 50R, terminating it into the 75R input of the SA will cause inaccurate readings. That input pad will fix the termination issue, but all on screen measurements will be 5.7db lower due to its design. There should be some kind of offset setting in your SA's options menus that would let you offset the 5.7db so that the measurements displayed on screen are correct. I guess it all depends on how accurate you need your measurements to be.
     
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  13. kaos513

    kaos513 Sr. Member

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    I ordered the loss pad now need to find a dc block would both work together for testing ?
     
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  14. guitar_199

    guitar_199 Sr. Member

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    I definitely want to follow this. I have seen DC blocks mentioned...and seen that they were considered essential..but I never could find a small inline one like I see in videos. Rather they were large gigantic things that were $700-$800 each.

     
  15. guitar_199

    guitar_199 Sr. Member

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    Nevermind. At the time, I didnt know enough to "search properly".

    Hope the link works....

    [EDITED] it didn't .

    If you go to Amazon and enter a search for ..... inline DC block 50 ohm BNC

    you will get some results.....

    However the only one that pops up is cheap...but is from China. If you take the same search string and just Google it.... you get a lot more choices......and perhaps a better product.
     
    #15 guitar_199, Sep 2, 2020
    Last edited: Sep 2, 2020

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