[AMRadio] Monitoring Modulation Accurately


W2XJ w2xj at w2xj.net
Fri Nov 15 13:06:31 EST 2013


The rectified and un rectified method does not take full advantage of the trapezoidal representation. Taking audio from the modulation transformer will indicate the linearity of the modulated RF stage. Better yet, taking an audio sample at the TX input and an RF sample at the output will display the linearity of the entire TX. It is also a great tuning aid. 

Sent from my iPad

> On Nov 15, 2013, at 9:46 AM, Bill Guyger <bguyger at yahoo.com> wrote:
> 
> Bry
>  
> Here are several pages I scanned out of the '55 ARRL Handbook. They show grabbing an audio sample off the Modulation Transformer to be sent to the Horizontal input of the scope, and a RF sample off the tank circuit for the vertical inputs, but the circuit I sent to you yesterday provides both by rectifying the sampled RF for the Horizontal input and giving a direct RF sample to the Vertical input so 2 birds with one stone and you don't have to do a sample circuit for the audio. Just place the probe where it can get a good RF sample.
>  
> FWIW just following the thread, you being the sales guy that you are, might be able to come up with a broadcast Mod Monitor somewhere. I personally have a Belar AM-3A which is spec'd to operate up to 160 Mhz. There have been some older RCA (etc.) units on Ebay but stay away from units made by TFT. They cut off at 1700 Khz.
>  
> Bill AD5OL
> 
> 
> On Friday, November 15, 2013 10:30 AM, Bry Carling <bcarling at cfl.rr.com> wrote:
> I am still wondering what differences in result I would look for when using reduced carrier system AM. Oscilloscope and mod monitor results would seem to be different compared to full carrier systems.
> Can anyone comment?
> 
> Bry Carling
> http://af4k.com/
> 
> > On Nov 15, 2013, at 11:17 AM, W2XJ <w2xj at w2xj.net> wrote:
> > 
> > With proper processing it should not be a problem. I always trust a scope before a mod monitor. Properly clipped negative peaks are easy to see and the absolute value of positives are not important. In broadcast it is common practice to modulate as much positives as the TX can take and in amateur operation a peak power measurement is the only thing that really counts. 
> > 
> > Sent from my iPad
> > 
> >> On Nov 15, 2013, at 5:15 AM, Steve WA1QIX <wa1qix at piesky.com> wrote:
> >> 
> >> Using a scope to monitor your modulation gives you a good indication of the shape of your waveform, but the human eye is just not fast enough to catch the peaks nor can you really determine accurately your actual percentage of modulation unless you are using a very modern peak storing scope.
> > 
> > Sent from my iPad
> > 
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> <Modulation Monitor001.PDF>
> <Modulation Monitor002.PDF>
> <Modulation Monitor003.PDF>
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