Modulation looks fine on dummy load, not the antenna.

[LeapFrog]

Something is wrong with my setup, maybe the sampler/BNC jumper.

I'm having an issue first noticed while testing a few different transceivers on my test bench.
Using a dummy load, then antenna I noticed a discrepancy with regards to the sampled rf/modulation.

The waveform looks great on the dummy load, when I switch over to the antenna after the alignment is complete, I noticed the sampled RF looks extremely distorted (the modulated carrier anyway) like a child attempted to draw a sine wave!
If I wrap an oscilloscope probes' wire around my BNC jumper cable (between the sampler & 'scope) and select the channel with the probe attached I can then set the volts per division and see the sine wave/clear modulation (as if I was back on the dummy load and selected the channel with the BNC jumper attached).

This has been observed on four of my radios, not all of them do this.
(My SWR on the antenna is about 1.7, almost too high for comfort, I know)
At first I thought it was reflected rf (maybe it is?) This happens on only some of my Cobra radios, and yes they all have the 54Mhz TVI trap intact.

Maybe common mode currents, issues with coaxial rfi?
Has anyone experienced this before?
Any thoughts, comments, or concerns are welcomed.
Thanks, 73.

-LeapFrog

 

[543_Dallas]

Is it just some flat topping or pinched carrier that can be fixed by turning down the modulation or is it something out of the ordinary? Any pictures?

I have saw collector modulated cb radios have more or less modulation when the swr changes. Like when switching from antenna to dummy load or turning on an amplifier. To minimize the effects I tune the antenna the best I can and make sure the input tuning on the amplifier is right. Final tweaks are made with everything in line as it will be used.

What radios have this issue and which ones don't ?

 

[LeapFrog]

I have 2 Cobras, 2 Uniden and 1 Galaxy radios all doing the same thing.
The other Cobras & Galaxy rigs I have do not do this when in the same setup.


It's not flat topping or pinched carrier, but rather some "twisted mushroom trip" type of distortion. the sampled rf looks almost sideways, I will get a picture posted.

EDIT: Yeah just tested my setup with another radio, and this doesn't happen to every radio so I'm a little stumped. But on the dummy load any radio looks fine, when I test "on-air" the modulation sample looks weird. I may just shoot a video to demonstrate.

 

[JoeDirt]

Stupid scopes finding problems you didn't know you had!!!!

Most of the time the increase in swr can cause an radio to osolate. Might check your radio alignment, most likely the transistor bias.

 

[LeapFrog]

I could perform a transmitter alignment I don't have a signal generator for the receiver side but how do I go about adjusting transistor bias on a Cobra 29? lol I understand the process for the export radios far as I know there is no bias adjustment on the AM only Cobra radios.

JoeDirt, how would I go about checking to see if the radio is "oscillating" & I don't mean the 10.240 megahertz reference oscillator?

Learn something new every day I suppose!

Thank You JoeDirt
73

 

[Captain Kilowatt]

What are you using for a pickup when connected to the antenna? I prefer a toroid with the line run thru the middle because a troid only responds to current thru the core and rejects everything outside it like radiation from the antenna itself. You could be having issues with RF comjng from more than one source or it might a ground loop somewhere between the radio,power supply, scope or antenna.

 

[543_Dallas]

A cobra 29 is collector modulated and operates in class C so no bias adjustment....unless someone has put a mosfet in it.

Oscillation will look fuzzy on the scope. Causes a lot of harmonics to be transmitted and usually makes the swr meter read higher when using an antenna.

 

[LeapFrog]

A cobra 29 is collector modulated and operates in class C so no bias adjustment....unless someone has put a mosfet in it.

Oscillation will look fuzzy on the scope. Causes a lot of harmonics to be transmitted and usually makes the swr meter read higher when using an antenna.

How can I track down an oscillation problem in the (tx stage?) radio?


Post Statement Edit: (& Way unrelated to post) I have to say the "collector modulated" comment got me thinking; I need to study amplifier design, because I want to "stack two finals" together on an etched piece of FR-4 w/ a proper bias circuit and power requirements met to turn the Cobra 29 LTD into an AB Biased (All about the Audio!!) and you'd have dual finals so low level modulation wouldn't be unfeasible in this AM only radio, then convert the rig over to use a balanced modulator with a clipper circuit, Direct Inject your Audio and maybe a relay switched SDR to take advantage of your "newly expanded" transceiver. Only thing left after that is to take it 10 Meters with my ticket (when I take the test). Just some wild pipe dreams..

I love to talk about and learn more of the radio field, but honestly I'm not yet technically advanced enough to do the things I talk about & described in the above post statement.


Best Regards
-LeapFrog

 

[LeapFrog]

What are you using for a pickup when connected to the antenna? I prefer a toroid with the line run thru the middle because a troid only responds to current thru the core and rejects everything outside it like radiation from the antenna itself. You could be having issues with RF comjng from more than one source or it might a ground loop somewhere between the radio,power supply, scope or antenna.

I hope it's this and not that 5 of my radios are throwing spurs/harmonics &/or having foreign oscillation problems!

Thank You Captain Kilowatt
73

 

[543_Dallas]

I haven't had a lot of oscillation problems with radios so no method of tracking them down. Amplifiers yes but those were usually bad design, tuning or components breaking down.

High level class C transmitters are good things, modern versions of plate modulated rigs. Low level has its benefits though. since you mentioned this are you using a CB mic or external gear? I've had a lot of weird issues with RF in the rack gear.

 

[LeapFrog]

What are you using for a pickup when connected to the antenna? I prefer a toroid with the line run thru the middle because a troid only responds to current thru the core and rejects everything outside it like radiation from the antenna itself. You could be having issues with RF comjng from more than one source or it might a ground loop somewhere between the radio,power supply, scope or antenna.

Captain kilowatt, I am still using the homemade RF pickup I posted about in another thread discussing rf samplers, I believe it was the Bob Heil schematic that I settled on, I found this in a HAM Nation episode I watched on YouTube.

* I use a crappy 75 ohm BNC jumper lead from China with little to no shielding and no toroid (But i will try this).

* The dummy load is a simple one quart paint can DIY deal with resistors, in get this.. olive oil (temporarily), male-male PL259 barrel between dummy load and samplers' SO239.

* My bench/station does not have any attention paid to dc grounding.

* I'm using a Pyramid PS-4K this power supply has no earth connection on the chassis nor a 3-pring earthed mains plug for the receptacle.

* 3 Foot 50 ohm Jumper between DUT and meterering equipment.

* The antenna (Hard mount k40, 571/4" Steel whip baseload) does not have any special attention paid to it either, it's on an isolated steel patio table about 8 feet away from my house with the factory 18' "Custom-Made RG-58A/U" coax. (I had to shorten it a few inches but I doubt this is the problem, but I don't know. I have 8 seperate & equally spaced ground radials extending out from the circumference of the tables edge. VSWR is around 1.6-1.8 and doesn't change much around the citizens band. The length of the radiating element is positioned so that it is almost tuned "well enough". SWR was 1.5 at one time using a different radio, and this was before the coax got damaged in the window, Oops!
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[photo="medium"]4351[/photo] .
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LeapFrog

 

[543_Dallas]

I shared this in another thread that you may have saw. My scope pick up is a bnc connector and piece of blue wire mounted in the final amplifier. Simple and works where more complicated pickups have given me issues.

 

[LeapFrog]

Simple and works where more complicated pickups have given me issues.View attachment 19595

That's it a great idea, I will try many wraps because it's going to be a much lower power scenario.

 

[543_Dallas]

That's it a great idea, I will try many wraps because it's going to be a much lower power scenario.

That pickup works fine with a barefoot radio at 10 watts pep. I wouldn't get too carried away with the wraps. The scope is in another room so the rg58 jumper is about 70 feet long.

 

[Captain Kilowatt]

Try this idea. Instead of connecting a jumper from your RF pick up directly to the scope input use a scope probe instead and insert just the tip into the output of the pick up. Do not use the ground of the scope probe. This will show if the problem is a ground loop involving the scope.