[GreenKeys] scope

[Ralph Mowery]

----- Original Message ----- 
From: "Eugene Hertz" <eugene at hertzmail.com>
To: "greenkeys" <greenkeys at mailman.qth.net>
Sent: Sunday, March 18, 2007 10:01 AM
Subject: Re: [GreenKeys] scope


ok - so here's the fundemental question.

I am using the scope to tune the radio. There are two approaches I can see 
taking, which one is right?

1. Using my function generator, pump 2125 and 2295 into the TU and using a 
grease pencil, mark two lines on the scope that go though the major axis of 
each ellipse. Then, when I tine the radio, I tune so that the ellipses are 
lined up with the pencil marks. This would ensure that I have tuned the 
radio such that the mark and space frequencies are 2125 and 2295.

2. using the function generator, find whatever frequency makes the st6 meter 
point to the max number (in tuning mode) then mark the scope?

Having written this email, it seems #2 is the right approach.

So it sounds like I shouldn't care about the orientation of the ellispes as 
long as I can reproduce it.

Thanks everyone!
Eugene
++++++++++++++++++++++++++++++++++++++++++++++++++++++


The quick way is to use the function generator and adjust it for the two 
tones and mark the scope.

 If you look where the maximum amplitudes are , you will know how wide the 
shift actually is and if it is shifted up or down form the niminal 2125 and 
2295 tones.  If you are going to feed the audio signal into the mic input of 
a transmitter on the low bands, adjust your tone generator so the tones, 
especially the mark tone matches the st-6 mark filter evenif it is not 2125 
hz.  If running afsk on vhf FM then do not do this, but adjust the tones to 
the 2125/2295 settings.  On the low bands it does not mater what tone pair 
you use as long as they are 170 hz apart and the mark tone is over about 
1500 hz so the second harmonic will fall outside the passband of the 
transmitter filter.

The "best" way is to inject the mark tone and change the capacitance or 
remove a few turns on the mark filter and then do the same for the space. 
Tune for maximum voltage comming out of the filters where you connect the 
scope.  A good voltmeter will probably be esier to tell the peak than the 
scope.  Thie should give you a near vertical and horizontal eliipse.  The 
filters in the st-6 are not very sharp and that is why you get the fat 
elipses. They were designed that way.   It is more of an FM type 
discriminator than sharp channel filters.  Also make sure you adjust the pot 
that balances the voltage going to each filter.   If yo understand how a FM 
receiver's discriminator works, that is the way the st-6 works up to that 
point.  The stage after the filters (slicer stage) is looking for a voltage 
change around zero volts.  Just a small voltage either way and the output 
will change states.  Hoff stated the st-6 could detect just a few Hz shift 
(cycles in his day) .

If your st-6 has the meter on it, the frequency it peaks at for the mark and 
space is where the filters are tuned at.