[AMRadio] New Topic - moniitoring modulation help please


Rob Atkinson ranchorobbo at gmail.com
Thu Nov 14 12:57:12 EST 2013


And if  you mind  having a computer or have one but don't want to mess
around with it, you can do the easiest thing (I think it is pretty
easy) and get an oscilloscope and connect the vertical input to an
unused coax switch position jack.   Everyone who uses coax for
feedline usually has a switch somewhere in the line.  Assuming you are
using a good one with 60 dB isolation or more, if you have an unused
position, you can connect a feedline to it and the other end to the
scope.  Now, obviously, you don't want to transmit into the scope.
I've been doing this for years, and never made that boo boo, but if
you think you will, you can also take a small project box, put
feedline jacks one on each end, connect them with a copper wire or
tubing run, then put a bnc jack on one side of the box and connect its
center pin inside the box to a short wire stub to act as a probe.  Run
a coax line from the BNC to your scope vertical input and use the
other jacks to insert the box in  your transmit feedline.

this will allow you to use the oscilloscope to look at your modulation envelope.

by lowering the trace line so only the top half of the mod. envelope
is visible, and adjusting the v./div setting, you can see where your
positive peaks are topping out.  if dead carrier above the trace line
is taking two vertical divisions and the voice peaks are hitting above
four vertical divisions then the positive peaks are over 100%.

Read this for a better idea (but note it is geared towards
broadcasting the the AM broadcast positive limit law of 125%)

http://www.thebdr.net/support/xmtr/maint-ammod.pdf

Rob
K5UJ

On Thu, Nov 14, 2013 at 8:02 AM, Larry Szendrei <ne1s at securespeed.us> wrote:
> On 11/14/13 7:20 AM, Bry Carling wrote:
>>
>> I would not mind buying a commercial signal monitoring meter for my
>> station if it was
>> reasonably priced.
>>
>>
> Bry, if you don't mind having a computer in the shack, the REA MM-SD1 is an
> excellent choice. Price is $200 (minus the 'pooter). Brought to you by none
> other than our friend and listmember WA1QIX. See:
>
> http://www.radioassociates.com
>
> Although I haven't sprung for one (yet!), I have seen it demonstrated
> multiple times and you can trust what it tells you.
>
> 73,
> -Larry/NE1S


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