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Service equipment question....

Discussion in 'General CB Services Discussion' started by guitar_199, Oct 12, 2021.

  1. guitar_199

    guitar_199 Sr. Member

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    This is..... by far..... the most technical forum that I belong to. For that reason, I want to try a question here to see what kind of opinions I get and ... more... what kind of backing information on "why".

    I have a spectrum analyzer. It's input impedance is 50 ohms and it's max input signal is +10 dBm (10 mWatts). believe me..... I have that on an RF Tap because I KNOW that if I probed a CB radio....even at 4 watts.... direct...I would smoke the daylights out of the SA's input. My thinking is that this is because it is 50 ohms impedance.... so the instant I touch it to the 4 watt signal .....the SA is going to try to pull HALF of that wattage right on in BECAUSE it is low impedance.

    But let's talk about a frequency counter.....with an impedance of 1 MegOhm and a max signal input of 150 VRMS.

    I think that a great many .....STILL say that this should be put behind an RF tap to keep from damaging it.

    My question is..... why?

    With an input impedance of 1 Meg this is a whole different animal from the SA. At 1 meg the SA is not going to pull any meaningful current from that transceiver's RF output.....it will ONLY see the voltage. RF voltage at 10 watts would only be about 22 volts or so max which is well within the range of this counter.

    With this in mind..... what is a good reason that this counter HAS to be protected by an Rf tap/attenuator.... when it looks as though the counter should handle it?



    IF IT TURNS OUT that it is "for safety's sake only" I can accept that! I believe in protecting one's investment. I am just wanting to understand IF it is necessary...and IF So.. why?

    Thanks in advance!

    Bob
     
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  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    For one thing a freq counter can be used while probing around a circuit and therefore it should be high enough impedance so that it does not cause loading of that circuit which can result in detuning. That is one reason freq counters have a high input impedance function. Also it is only concerned with freq and not the relative strength of the signal like a spectrum analyzer is. The S/A must compare frequencies all across the spectrum and the amplitude relation of them. It is also VERY much more sensitive because it needs to see signals just above or even down in the noise level. S/A's are often used simply connected to an antenna for monitoring of the RF spectrum hence the 50 ohm input. I am sure others will come up with more reasons.
     
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  3. guitar_199

    guitar_199 Sr. Member

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    Thanks for the response.... all of that....I pretty well understand......

    Breaking it down to a question.......
    If my freq counter is 1 MegOhm input impedance......and
    It can take 150 VRMS max input level.....

    Should I be afraid to touch it to the center pin of the coax connector while the radio is keyed up?

    let's say my radio is 10 watts.... then keying it into a 50 ohm dummy load should be a little more than 22 VRMS which is well within the 150 VRMS limit.

     
  4. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Well - what do you want to measure...

    The ACTUAL RF signal or it's spurries COMBINED.

    I'm seeing your post from several different approaches...angles - degrees of separation ... e&c...

    1M input IMPEDANCE does not EQUATE to ATTENUATION nor Rejection.

    Just what it will look as to the source - a very big resistor - internally, the intrinsic (Detection and sampling going on inside) of the Very Big Resistor is a different story.

    This is not just about safety - it is also proper matching of input to TRANSFER the signal you wish into the counter - not just how it appears - as a load -to the source of power.

    So you should make a "tap" from the source, to MEET the Expected impedance of the device/detector - you would need some form of transformation to reduce the level of input from the source to match the Device as a load to the source - to the source - it doesn't matter - it's the Admittance of power into the LOAD - transformation of signal level - is what you want to properly use so the device can operate properly.

    So you know the SA inputs handle 10mW - you'll need to make a resistor attenuator pad so you can properly source the signal into the Load = SA.

    So if you have a 4W carrier - you need to not only balance out the ohmic impedance from the Source (50 ohm) but also provide enough power into the LOAD (SA) - so they claim it's only going to appear as a 1M Ω load, but also remember too, termination of the signal INTO that load - if not done right generates an impedance bump and potential reflections GENERATING a termination mis-match - a mixing or intermodulation issue.

    It's why you see - - back to your original post...
    Then when you look at your output - don't overload it, develop a tap for it to "sip" the "hot tea" the signal is...
    upload_2021-10-13_7-53-51.png
    You can look at this several ways...
    The above is a SIMPLISTIC view of how to allow
    a STRONGER signal can get looked at/sampled by
    your equipment...
    The weaker the input signal is, the better the selectivity you provide for sampling.
     
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  5. guitar_199

    guitar_199 Sr. Member

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    I absolutely have an RF tap (-40 dB) for my SA and i know it works.

    My main wonder is the frequency counter. IT is 1 Meg input with a max input of 150 VRMS.

    As far as where I would like to probe.... I can see using it internal to the radio at various points.... crystal oscillators, VFO output.... but would also like to know that I can probe the signal pin on the coax if I want to.

    And that is where my thinking currently is..... that if my counter is 1 Meg.... it is not going to draw any kind of power off of the RF......so I would not THINK that it would suffer any damage.... PLUS since it can take 150 VRMS.... looking at a 10 watt RF output ...which is about 22 VRMS... should not hurt it either.

    THAT is kind of the basis for my question........ is my thinking wrong???

    Thanks as always HA!

    Bob


     
  6. Dmans

    Dmans Sr. Member

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    guitar_199,
    For what reason would you ever need to key a radio directly into a Frequency Counter?

    Your 10 watt RF output/22 VRMS may be sound math and may not hurt your Frequency Counter but without the output of a radio terminated into a 50 Ohm load, can you believe what your Frequency Counter is telling you?

    I have 2 different Frequency Counters. One is a pass through type with an input and output S0-239. The other is used with an RF sampler. Both of these "sample" a portion of the RF for measurement. In my opinion, the pass through type is not all that accurate (6 digits vs. 10 digits on the other type).

    I built a sampler based on the information from W5QN (in the attachment below). I did measure the coupling factor and it was indeed 50db. It was easy to build (though my enclosure is slightly different but same physical size) and I trust it completely. It is the only thing I use now with my Spectrum Analyzer and Frequency Counter.

    It just seems without a properly terminated load, the measurement can't be trusted. Though I may be wrong!

    73
    David
     

    Attached Files:

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  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I remember times when there used to be "sales" on specific O-scopes that used 72~75Ω termination resistors - designed for TV/ASTM RF Lo-band VHF - could never figure out why they cost $400 to as much as $1,000 more - than a less "bandwidth" Scope...only now they went for the same price as their "standard 50ohm Types"

    Bring in the termination Resistor...

    TV and Cable used 75/300 ohm impedance cables - the rest of us used 50 ohms.

    So when I see...
    I tend to agree!

    SWR Reflections due to a mis-match and common mode currents can wreak havoc with the instruments.
     
  8. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    A typical 4-watt carrier - is about 40V P-2-P - RMS is 70% (thereabouts) to keep it simple...

    So where are you arriving to the 22 VRMS value?

    In unterminated - or as in an antenna - the antenna itself be DC or otherwise is reactive.

    So it would show 40V/w/RF on the antenna end and even at the Radio end - with some value in-between (if a standing wave exists) of voltage.

    So you're kinda playing "chicken" with the effort of sampling RF directly - and with no way to terminate INTO a load unless your coupling is open line feed. (Proximity)

    Gotta' fix dinner but this is an interesting enigma...
    :)
     
  9. Dmans

    Dmans Sr. Member

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    HandyAndy,
    I may have jumped the gun here. Re-reading Bob's post, he stated "would also like to know that can probe the signal pin on the coax if I want to".

    From the inside of the chassis, he could probe the signal pin of the S0-239 while outside the chassis, the S0-239 may be hooked to a dummy load, antenna, etc.
    A scope probe, being capacitively coupled, could be used in this situation without causing damage-been there, done that.

    73
    David
     
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  10. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Which was why - in being cautious and careful - showing the cap in there to "break" any Sort of current loop that could form from close coupling.

    In light of these - this also reminds me of the uses of the Isolation Transformer they did have to prevent electrocution hazards for TV sets - still can't avoid the HV dangers but the current loop was broken by the adding of an isolation wind or twos to lessen the chance of receiving a direct charge from the probe leads and such.

    The reason for my concern is from the "Ground potential" - the differences between Chassis and True Earth Grounds - you can get a poke which may startle you, but the equipment doing the Sensing will certainly react to it.
     
    #10 Handy Andy, Oct 14, 2021
    Last edited: Oct 14, 2021
  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Sure you could probe directly the coax cable on the radio's output jack but chances are that the counter will be displaying the frequency before you even make contact. Even at 4 watts there is enough RF to trigger the counter with the probe held very close to the cable and not even making contact. In my 22 years of servicing commercial broadcast gear including VHF/UHF mobile radios and point-to-point radio links I have NEVER probed directly to the coax jack or anywhere along the transmission path. I always used a "sniffer". The dummy load had a toroid pickup on it for contactless measurements and if I was in the field without one I would use a small piece of wire as a probe or even a very low value capacitor such as a 0.001uF.
     
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  12. guitar_199

    guitar_199 Sr. Member

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    That is exactly what I meant.... that it WOULD be connected to a 50 ohm load. Dummy load would definitely be hooked up!!!! SO the radio would be transmitting into that load.... that load would safely handle the output of the radio..... I would just like to touch my freq counter to the signal pin and see what the transmitter is putting in to the dummy load.

    Bob

    So
     
  13. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Frequency will NOT change anywhere along the transmission path. It will be putting the same frequency out to a dummy load as it does into an antenna as it does straight out of the final O/P transistors. There is NO, ZERO, need to probe the output connector. None at ALL. Imagine the probe tip slipping off the centre pin of the jack and sliding across it until the tip hits the chassis. You just put a dead short across the output. You seem obsessed with doing something simply because you want to test either yourself or your test gear. Do as you wish but be prepared to accept any and all consequences regardless how unlikely they are to happen.
     
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  14. guitar_199

    guitar_199 Sr. Member

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    Okay.... so if I take your meaning........
    If what I want to see is the frequency that the radio is putting out....
    You are in favor of measuring "inside the radio" at a point further back in the transmit strip...... rather than just reaching over and touching the center pin on the coax?

     
  15. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    No, my meaning is that there is NO REASON WHATSOEVER TO HAVE TO PROBE THE ACTUAL OUTPUT. Once that 27.??? signal is generated waaaay back at the mixer stage it WILL NOT CHANGE along the output path. As I stated above your counter will undoubtedly trigger before physical contact is made to the RF output. The output freq. at the coax jack will be the same at the output of the finals, back at the output of the driver stage, back at the output of the pre-driver stage and anywhere else after the final mixing stage. If you insist on monitoring the final output then use a pick-up coil or even a piece of insulated wire laid near the output.
     
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