When you can see RF with a Scope?

Discussion in 'Wireless & RF Design' started by Art, Nov 23, 2013.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi,
    I just got my first scope :)
    Somewhere between the tuned frequency end, and input to the first preamp,
    is it possible to view an RF waveform, or only possible after it's rectified,
    and the RF component is discarded?
    Thanks :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Depends on the frequency of the RF and the bandwidth of the scope.
    A 100MHz scope will cover a major portion of the broadcast band.
    I can view RF from my crystal radio with a 60MHz analog scope.
     
  3. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    It's a 1976 Dual channel 25MHz, and first connected it to audio of course :)

    [​IMG]

    It was cheap from a retiree.
    AM radio is down to kHz, is that suitable?
    Is it just that with RF, before any pre-amplifiication, you have to set the scope to a low voltage?

    I am grasping it, I understand how you could look at a fast logic signal
    for a microcontroller project.
     
  4. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    It's not a joke, I haven't done anything faster than this even with micros.
    I went out for a box of valves, but this was being sold cheap too.

    I'm kicking myself for letting a couple of "Thing Boxes" go.
    I should have just got them, and investigated them. That could have been the real fun stuff.

    One only had an antenna input and output and a dial.
    I can't think of much other than RF amp for that.
     
  5. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Nice old BWD!

    You may be able to see the RF input to the first converter stage on a Superhet Receiver.
    You definitely will be able to see the Local Oscillator signal,& the resulting 455kHz IF signal.

    Using a probe,just have a poke around in that area with the BWD on 'AC coupled",otherwise if you probe a spot with HT volts on it,the display will go off scale.

    Some Oscilloscopes have a "DC Offset" adjustment,but this model BWD doesn't have that.

    Most of these signals will be visible with a X10 probe,which is the preferable way of doing it,but very low levels may require a X1.

    If you have an original BWD probe or a Tektronix one,you should have little problem with spots with HT on them.
    Check the "maximum voltage" specs on "after market" probes,though.

    The large,early Tektronix probes,which were designed for the Tek 545 & similar,are not popular as they are limited to around 40MHz bandwidth,& are too "clunky" for small components & PCBs,but may be ideal if you are working on vintage radios.

    The BWD doesn't automatically rescale when you use X10 probes,so you need to multiply the v/div by 10 to get the correct voltage reading.
     
    Last edited: Nov 24, 2013
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
  7. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi VK, and greetings from Australia :D
    BWD made here :)

    I haven't looked into what the different probes are for.
    I thought just bigger ones were for high voltage.

    Will def check out your links, this is what it's all about for me.
     
  8. vk6zgo

    Active Member

    Jul 21, 2012
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    Vk6zgo is a Ham callsign----I'm in West Oz!:D
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    What I see in the photo is RCA audio cable plugged into BNC to RCA adapters.
    If that is the case you will lose the RF signals.
    Get proper BNC scope probes.
     
  10. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Yes I am going to be a VK4F, but I cant have VK4FART, or even VK4FRT!

    It did come with probes, but with repairs, they are more homebrew than original :D
    I think I'll have to chase a new set down.
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    When you have a good set of probes, you can see RF signals from about 20 mV.
    Keep in mind that the given 25 Mhz is most likely the - 3dB point of the range.
    If you want to have a look at signals of a higher frequency, a detector might help to see that amount of RF without seeing the real RF.
    A RF detector could be something like in the following link:
    http://www.qsl.net/n9zia/wireless/pics/rfprobe.gif
    Lower the 10 nF capacitor to say about 100 pF if you want to follow the RF signal very quick.

    Bertus
     
  12. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Fortunately this is about AM broadcast 550-1600 KHz,
    even the one and only shortwave band on the radio of interest, is 6-18 MHz.
    So I shouldn't be mistaken that the equipment is appropriate?

    I don't understand the RF probe circuit exactly,
    but is it dropping AC as the first TRF circuit in a radio does?

    "Maximal Rated Power" is it protecting the device under test, or the Scope?
     
  13. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Looks like I also have a vintage RF probe.
    Whoops. I can fix the part I wrecked.

    This one has no diode.
    The variable cap and resistor are both connected across the input and output of the probe,
    and a hole through the probe provides access to the variable cap.

    I don't know what he was looking at with the scope to know what values were selected for the resistor capacitor,
    but he would have been filtering something outside of a particular desired frequency,
    or tuning a desired frequency, and selected values to suit as I understand.


    [​IMG]
     
  14. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    RF Probes are basically an untuned “Crystal radio”


    They come in rwo main forms:-


    (1)“Series diode”


    &


    (2)“Shunt diode”


    (2)Is the most common,but the reverse voltage rating of the diode is critical
    If you use it with too high an RF voltage,you will destroy the diode.


    The following links may be interesting.


    http://n5ese.com/rfprobe1.htm


    http://www.zen22142.zen.co.uk/Circuits/Testgear/rfprobe.htm


    http://www.qsl.net/g3oou/simplerfdetectors.html


    http://www.swlradiostation.org/heathkitcatalogs/heathkit-rf-probe-model-309-c.pdf
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    This is not necessarily an RF probe but looks more like a frequency compensated 10x probe.
    One would use a square wave input and adjust the capacitor trimmer until the tops and bottom of the square wave were as flat as possible.
     
  16. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Yes, that's right MrChips, it's a reproduction of the "BNC Box" part of a probe.
    That original part I also have and it's broken, so the homebrew one makes sense.
    I can toss that one now, I got a probe kit today, which made it more obvious.
    It's a pity it's broken, it makes the new one look like junk (even when broken).

    Thanks Guys, I will read up, I managed to find some recognisable signals from
    my cheap LCD radio, rather than sticking it into a valve radio with high voltages.
    The most interesting was actually one side of the watch crystal that is driving the real time clock part of the radio.
    Some real fun stuff going on there... I already knew one side of the crystal
    is different from the other.. wow!
     
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