Sweeping your Transmitter

Submitted by Tom K1JJ


I've received a few Emails with questions about audio sweeping an AM TRANSMITTER.  I guess we sometimes take for granted that new AMers understand the concept. But, there's really nothing on the license exam about this stuff... so here goes:


It's real easy!
Basic gear needed to check  AM transmitter frequency response and determine "general" linearity: 
1) Audio sweep generator, 1-20,000 hz or higher. Most any will do, but the old tube HP 200CDR type works especially well and has a pretty clean sine wave.  Make sure it is "flat"...the output amplitude varies VERY little as you tune the frequency from 40-10,000 hz.  Some guys use triangle waveforms, more revealing...do both - whatever you have.
2) A decent oscilloscope, 25-50mhz minimum, preferably dual trace.
3) A capacitive pickoff probe for the scope to sample RF from your transmitter output 50 ohm coaxial line. Look for the article I wrote on this site entitled " Scope  Pickup"  for details on how to build a simple one.    Once built, you can use this capacitive probe as your main permanent station probe while on the air monitoring your own signal in the future. Do not use a coiled loop. They tend to pick up harmonics and other "fuzzies".
Before running any tests, inject a 1kc audio tone into the scope and study...REALLY STUDY what a pure and clean sine wave looks like. Notice the peaks are sharp but rounded, the rise from low to high is nearly straight and the cross over through the middle or zero line is also straight.  You will be looking at a waveform that is mixed with RF, but should still maintain this smooth look. Your eye can see subtle distortion products in a problem rig as low as 1% and less IF you really know what a perfect sine wave should look like.
To sweep your AM transmitter, simply reduce the audio generator to minimum and plug it's output into the mic jack. Bypass all EQ's , compressors, etc.  Just straight unmodified audio. Tune the rig into a dummy load and slowly increase the audio drive until you
see about 95% modulation on the scope.  Now SLOWLY sweep the audio generator from 40hz up to 7khz and closely note the change in amplitude as well as the subtle distortion in the waveform on the scope.
The AMPLITUDE level on the scope should be as "flat" or constant as possible. Peaks and dips in amplitude are indications of potential rig problems and can sometimes be corrected through negative feedback and other solutions beyond the scope of this discussion.
Look especially at the "form" of the sine wave. Does it start slanting to one side as the audio frequency is increased? Does the crossover zero point have a hump or funny looking curve at certain frequencies ?  Do the peaks start rounding/squaring off even though you increase audio further?  Is the rise from low to high straight like the  perfect sine wave or does it exhibit a slight "S" curve?
Increase and decrease the audio generator level and look for anything popping out from the norm.
Also try over-modulating to 120%-140%. Look for the RF carrier zero point to form a thin line while the peaks keep getting higher and higher with perfect sharp rounded peaks.
You will find that at 40hz and lower (depending on the rig, of course)  the waveform will start to fall apart and get bumps, and all sorts of funny shapes. This is normal for saturated modulation iron as well as for other reasons. 
At high frequencies you may start to see a "right or left triangulation"  or slanted look caused by phase distortion and non-linearities above 5-7 kc. Hopefully this will not occur below 5kc for you.  Any departure from the perfect sine wave is an indication of things to be corrected.
You will want to keep the scope hooked up all the time when on the air as an invaluable indication of what's happening in real time with your rig and voice. After awhile you will find that you can almost set you equalizer just using a scope and your voice. It's all about developing a trained eye for waveforms, whether they be a sine wave or your own voice.
The final step is hooking the scope to the I.F. output of your hi-fi receiver and looking at other AMers on the air. Be sure you have spent time studying your own voice before giving out reports. A scope and a tape playback can tell another Amer quite a lot of information about his rig.
We may discuss sweeping your RECEIVER next time...more later -
Tom, K1JJ

                                                Back to East Coast Sound          Home