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Hello I want to use my new oscilloscope, but i have questions

jtrouter

Active Member
Jul 7, 2015
100
41
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So i got my new Oscilloscope and want to see how it works. I was told all i needed to do was to hook it up
to my dummy load and i could test it on my CB. Then i was told no that could/would burn it out. Then i was
told i could hook it to my antenna switch to port 3, port 1 has my antenna, port 2 my dummy load and port three is
unused. Then a Ham radio user told me i had to use a RF sampler, which looks like it would be smarter? Part of the problem
is i am worried about running out of time it it is not working right as its from Amazon and i have already lost a couple of
weeks due to you know life and family taking time away from hobbies you know how unfair of my family just kidding. So i did look
around the forum and did see older postings about samplers but the pictures of them or schematics were to old to be viewed now.
Is it safe to hook my scope direct to stock wattage CB to test? Is it safe to hook to a dummy load? or is it safe to use the open port
of my antenna switch with a legal 4 watt radio? If i must use a RF sampler is this one from Ebay look ok to use it this is the only safe
way to use my scope?
Or is this one a better option?
Sorry for so many questions i am so wanting to check the Oscilloscope out but i do not want to smoke it right up the first time i use it.
I thank you for reading and await your wise responses.
 

Jay Mojave

Sr. Member
Nov 17, 2015
319
597
103
69
A simple way to see your transmitted signal is to wrap a single wire around the coax say 4 to 10 turns and connect the scope leads to the wire ends. This isolates any direct damaging rf energy going to the scope. Using slower sweep time you should see the AM RF Signal Envelop, the carrier and the modulated envelop.

Let us know how this works out for you.

Jay in the Great Mojave Desert
 

Lkaskel

Well-Known Member
Aug 4, 2017
250
187
53
58
So i got my new Oscilloscope and want to see how it works. I was told all i needed to do was to hook it up
to my dummy load and i could test it on my CB. Then i was told no that could/would burn it out. Then i was
told i could hook it to my antenna switch to port 3, port 1 has my antenna, port 2 my dummy load and port three is
unused. Then a Ham radio user told me i had to use a RF sampler, which looks like it would be smarter? Part of the problem
is i am worried about running out of time it it is not working right as its from Amazon and i have already lost a couple of
weeks due to you know life and family taking time away from hobbies you know how unfair of my family just kidding. So i did look
around the forum and did see older postings about samplers but the pictures of them or schematics were to old to be viewed now.
Is it safe to hook my scope direct to stock wattage CB to test? Is it safe to hook to a dummy load? or is it safe to use the open port
of my antenna switch with a legal 4 watt radio? If i must use a RF sampler is this one from Ebay look ok to use it this is the only safe
way to use my scope?
Or is this one a better option?
Sorry for so many questions i am so wanting to check the Oscilloscope out but i do not want to smoke it right up the first time i use it.
I thank you for reading and await your wise responses.
JT, there are many ways to connect your scope to the output of your radio to see its transmit envelope and Jay's suggestion would work. I use the RF Sampler you have in your question. I am assuming that you have a scope probe. You could connect a piece of coax to your switch and the scope probe to the coax. DO NOT switch it to the 3rd port. Have the switch go to your dummy load. The scope probe will likely pick up the "resonance" signal not the direct signal. One other thought is to get the service manual for your radio. There are a few areas of the alignment that you would be instructed to connect a VTVM or scope to a specific test point or speaker or even the leg of a transistor and adjust for maximum. You will connect the scope directly to what ever they recommend. You will get a reading on the scope if you follow the instructions. It may say to inject a signal for receive or just have it in a specific mode (AM SSB) in the case of the PLL/VCO alignment.
 

loosecannon

Sr. Member
Mar 9, 2006
4,194
3,585
273
using some sort of sampler that keeps the transmitter from being directly connected to the input of the scope is always a good idea if just for safety in case you forget that it's connected and do something crazy.

Here is a simple idea that won't cost you any money, and should work well.

can you take the covers off of that antenna switch?

if so, you can make one of the output connectors into your sampling port.

just unsolder the wire going from the center pin of the chosen SO-239 to the switch.

now get yourself a short length of insulated wire and solder one end of it to the center pin of the SO-239 that you just disconnected.

now wrap that wire around one of the other SO-239 center pin wires about 5 or 6 times. (do this on the SO-239 that you plan on using for the dummy load switch position)
then solder the other end of the wire to the shield of the SO-239 (really any ground lug inside the switch will work).

now label your three position switch as:

1: antenna (this is the SO-239 that you didn't mess with)
2: dummy load (this is the SO-239 that you wrapped the wire around)
3: sample port (this is the SO-239 that you removed the stock center wire from and soldered on your inductive coupling wire)

so now when you have the switch in position 2, you can connect your scope to position 3 and you'll be sampling the signal going to the dummy load without actually connecting it to your scope.

hope this makes sense.
LC
 

jtrouter

Active Member
Jul 7, 2015
100
41
38
62
Thank you, and yes it made sense. I do like this idea. I did try wrapping about 12 turns of hookup wire around the cable from the radio to a 3 way power/swr/mod meter but was not able to pick up anything on the scope yet. So i need to take baby steps and watch some very basics on oscilloscope use. Seems i thought it would be simple to figure it out by myself.....nope.
 

brandon7861

Well-Known Member
Nov 28, 2018
201
222
73
Northern MN
Whether you can or not depends on the kind of scope. If it is a digital scope, it is probably limited by max voltage. Some scopes like old CRT ones have a maximum current rating and connecting to a CB would certainly blow it up. If you know the watts and you know the ohms you can find the volts (which will be higher than the 12v going into the radio).

A safe method is to connect the radio to the load by running it through an antenna switch or tuner and connect the oscilloscope to the unused port (making certain that port is NOT selected). Enough of the signal will be on the unused port for the scope to see, even if that port is switched to ground when not selected.
 
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Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Most scopes have a "calibrate" and scale so they can check for signals as low as uV (Micro) mV (Milli) with Micro 1,000X smaller (weaker) than Milli.

1645892460150.png
1645892488808.png
- so if you picked it up used, I wanted to advise you that you may have got one that has suffered some damage from excessive inputs. Especially when you can't see any signal on the scope when you try to get a signal to look at.

1645886140611.png

So, before you go too far - try to do up a Probe to help you get a grip on today's radios.

Because you should check to see if the Scope can even see input signals and not have a blown input buffer.

Most of the time? These things are very basic and use a resistor array set up like a precision divider to allow the SCALE of the scope to work with the proper amount of attenuation the resistor array provides to give the scope that ability to even offer a range of test settings.
  • So, if you don't get a reading, it may just be a blown input resistor and you just have to track it down internally or just use a different scale setting

Some scopes have a "calibration" signal present on the front panel that lets you preset and test and calibrate the scope.

Example...
1645886323484.png
You might want to grab that owner's manual
and find the procedure to see if the Scope can
function on the bench.

As far as the Test Probe?
1645893112571.png
Get one of these to tap around the radio with...
and some of these to help make your probe...
1645893230548.png
The ones with clips
like this....
1645893443578.png

And if you're creative, use that "tester" as the case and as a method to probe and use the clips to gather and hold one those bare leg leads to run your tests.
 
Last edited:

Jay Mojave

Sr. Member
Nov 17, 2015
319
597
103
69
I went back and tried the wire wrapped around the coax, it didn't work on bare foot power but did work on dead key 150 watts. Had the scope on 5 Mv scale and was able to seen the envelope and carrier. This method had approx a 80dB isolation factor which isolates the scope from harmful input to the scope . Works ok running a little steam.

Jay in the Great Mojave Desert
 

Dmans

Sr. Member
Jan 22, 2017
1,273
1,550
173
Georgia
I have posted this attachment before. I have built myself one (or 2:unsure:). I believe my total cost was about $7.00 (for the solid coax) and using junk box parts.

Also attached are a few pictures of my example. This was scrap sheet metal and a few screws. Measurement of my sampler did show a coupling factor of 50dB.

It works well and I have used it on both my scope and Spectrum analyzer with 1500 watts output into a dummy load. It also displays a 4 watt CB signal as well.

73
David
 

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nomadradio

Analog Retentive
Apr 3, 2005
5,314
7,634
573
Louisville, KY
www.nomadradio.com
Back in the day, the frequency response of a 'scope was called "widebanded" if it went up to 5 MHz. Naturally, a 27 MHz signal won't be visible if you feed it into the vertical input. A diode detector with resistors to cut down the RF voltage would 'rectify' the RF and act as an envelope detector. The resulting audio waveform would be clear to see on even the cheapest 'scope.

Fast forward to today, and the vertical-channel frequency response issue no longer matters to a lot of the 'scopes you can buy new. 50 MHz response is a lot cheaper now than 30 or 40 years ago.

Trouble is, the time base will try to trigger from the RF waveform, and not the modulation envelope you're interested in. Makes the audio waveform tough to stabilize on the screen, sliding back and forth in a blur.

Andy's RF probe circuit above would yield a good audio waveform from your RF signal. The advantage is that the 'scope will now trigger on the audio, and making it more stable on the screen and a lot easier to sort out what you're seeing.

A similar circuit is built into the old Heathkit "Cantenna" dummy loads. With a couple of component changes, it serves this function here every day.

73
 

jtrouter

Active Member
Jul 7, 2015
100
41
38
62
Back in the day, the frequency response of a 'scope was called "widebanded" if it went up to 5 MHz. Naturally, a 27 MHz signal won't be visible if you feed it into the vertical input. A diode detector with resistors to cut down the RF voltage would 'rectify' the RF and act as an envelope detector. The resulting audio waveform would be clear to see on even the cheapest 'scope.

Fast forward to today, and the vertical-channel frequency response issue no longer matters to a lot of the 'scopes you can buy new. 50 MHz response is a lot cheaper now than 30 or 40 years ago.

Trouble is, the time base will try to trigger from the RF waveform, and not the modulation envelope you're interested in. Makes the audio waveform tough to stabilize on the screen, sliding back and forth in a blur.

Andy's RF probe circuit above would yield a good audio waveform from your RF signal. The advantage is that the 'scope will now trigger on the audio, and making it more stable on the screen and a lot easier to sort out what you're seeing.

A similar circuit is built into the old Heathkit "Cantenna" dummy loads. With a couple of component changes, it serves this function here every day.

73
I wish i knew this a day before, I broke down and bought one off of EBAY for to much$35.00 and to throw salt in the wound i do have a Heathkit cantenna in my stash. And on top of all that burning i own a Sencore CB42 super duper CB repair box and i broke down a reread the manual and it has this device that converts the signal so that all you need is a 1mhz Scope to view 11/10 meter waveforms.
But hey its only money...
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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But, with what you have, you now have a workable solution to many different problems using the equipment in many different ways.

IF you wish, you can "modify" the RF probe to help you with RF/IF tuning and peaking by simply removing the diode used in the probe, you swap it with a 10K resistor to offer a high impedance but a better balanced load to the Hi-Z input the scope would have if the RF you are detecting generated a reflection (Strong signal input) - it would make the signal look like junk - because of the termination and subsequent reflection of the signal back to the probe tip and with this type of noise - would make the tuning difficult - changes the loading on the circuit. Too high (no termination resistor) you can get reflections, too low, the circuit doesn't operate in "free-form".

To show how bad it can get, the IF sections of some AM-only CB radios have outputs close to 50mW - able to transmit in the room to be detected in another radios IF noise - located opposite side of the room.

Uniden and Cobra oftentimes use a cap across the IF output to reduce the signal level so the IF strip can actually see the incoming RF converted to the IF the strip is to use. Allows for it to actually receives signal - not just hear its own IF carrier.

If you have time - look at the Foil Side of the Cobra 25 or a Uniden PC-68 - you might see a little 22 to 33pF disc cap soldered across the OUTPUT of the 1st IF MIXER (TP3) (From L16) - the one that uses the ~16MHz to 17MHz signal to make the 10.7 MHz Image - it's to reduce the signal level so the noise generated from the IF the PLL is making for the 1st IF - is not drowning out the signals arriving from the antenna you want to hear.

On a Cobra 29 / PC-76-78 series, it's TP3 - same frequencies - but output is "tailored" The FET and its support - different values are used to handle the output IF signal from L19 - a little less noisy. (FET1)
 

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nomadradio

Analog Retentive
Apr 3, 2005
5,314
7,634
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Louisville, KY
www.nomadradio.com
For future reference, we make some small mods to the DC output jack on the Heath Cantenna. The diode gets turned around to provide a positive voltage. The disc capacitor on the RCA jack from center pin to ground gets changed from .01uf to .001uf. A 10k resistor gets added from the RCA jack center pin to ground.

The Cantenna's DC jack was meant to use with an analog DC voltmeter as a relative-output feature. Wasn't meant for audio, and you'll find the audio frequency response of the stock circuit is terrible.

73
 

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