Input voltage spec and Return Loss Bridge questions

Discussion in 'General Technical Questions and Answers' started by N2NJO, Oct 28, 2008.

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  1. N2NJO

    N2NJO Ham Member QRZ Page

    The input spec for a Spectrum Analyzer that lists maxium dc voltage, how imprtant is this? Most scopes I have seen are in the range of 25-50vdc. There is one that lists it aa "0 vdc". I don't understand how it can be "0" anything other than that tells me there can't be any dc voltage present.
    How important is this spec and where/when would it come into play?

    Next, I need a "Return Loss Bridge" for taking 'Distance to Fault' and VSWR readings. The device from the scopes manufacture is ridiculously priced $1500).
    Anyone have suggestions of something much cheaper?
    Last edited: Oct 30, 2008
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    If the scope has AC coupling, you aren't going to pass any DC. (I wonder what brand of scope has that spec, though?)

    If all you are doing with the return loss bridge is measuring DISTANCE to the fault, this can be done very cheaply with a TDM circuit...just a pulse generator and an oscilloscope. There was a poor man's TDM described in QST recently.

  3. WA9UAA

    WA9UAA Ham Member QRZ Page

    Hi OM,
    The DC protection specification tells you whether the front end of the analyzer is protected from DC, for example, in signal tracing on live circuit. If the DC level is exceeded the front end of the analyzer will be destroyed.

    On the second part of the question, maybe an MFJ 259B would do the job.
    Rob WA9UAA
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    0 Vdc as the rated input for a spectrum analyzer is a common rating. A spectrum analyzer isn't a scope, and usually is DC coupled and has a 50 Ohm resistive input attenuator made of very small chip components. If the input could handle 100mW, at 50 Ohms resistive input this is only 2.23 V -- pretty close to "zero."
  5. N2NJO

    N2NJO Ham Member QRZ Page

    The SA is a Aeroflex 2399A/1 with a tracking generator. They wanted $1500 for the bridge.
  6. AI3V

    AI3V Ham Member QRZ Page

    Try Mini Circuits for the RLB.

    I have used this one with good results. Oops, can't get linky to work.

    Mini Circuits ZFDC-20-5+ about $100 delivered

    Remember, any measurement below 1.22:1 VSWR with a 20 db coupler is a guess.

    If your worried about DC, Mini circuits makes nice DC blockers.


    EDIT: this rlb is rated to two ghz, probally will work ok a bit above that, if you need 2-3 ghz you are probally better looking for a used NARDA or General Microwave RLB, usual is 2-4 ghz.
    Last edited: Oct 28, 2008
  7. AI3V

    AI3V Ham Member QRZ Page

    $1,500 - OUCH!, Not without a Kiss !:eek::D

  8. AG3Y

    AG3Y Guest

    Almost all the Tektronix and other brand SA's that I have ever used have a "Zero Volts DC" specification on the front end. If you put ANY KIND of DC into them, you might fry the input amplifier, which can be a VERY EXPENSIVE repair job !

    Remember, the DC component would take away from the total amount of power that the front end could handle, so as in Steve's example, if you were putting 100 mW of RF into the SA, you would be strongly advised to NOT have any DC riding in on top of that RF signal!

    It is not at all unusual to see SAs rated at "ZERO DC" input levels. There are special couplers designed to pass RF through with almost zero attenuation, or at least a well calibrated amount of attenuation as well as a load ( like 50 or 75 ohms ) while blocking any DC offset, to be used for measuring transmitter output power, perhaps directly at the output stage of an amplifier, where there could very conceivably be a high level of DC voltage !

    Hope this helps. 73, Jim
    Last edited by a moderator: Oct 29, 2008
  9. AC0FP

    AC0FP Ham Member QRZ Page

    :D:D Or more than a kiss! I also agree this is to expensive. I also have a Mini Circuits bridge (directional coupler) with BNC connectors. It is rated down to 5 MHz but I have tested it down to 3.5 MHz and it works just fine.


  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    It is very easy to damage a spectrum analyzer mixer or input attenuator
    by applying DC, hence the zero DC input rating. Few analyzers are DC-blocked.

    In order to have "distance to fault" measurements, a special software package is needed, which is an [expensive] option with most modern analyzers.

    It remains to be seen if the return loss bridge offered provides any special properties to be compatible with the Aeroflex software. If not, any return loss bridge should work within its bandwidth or directivity limitations.

    Finally: "Say after me, there are no 'c:s' in Tektronix" :)


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