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RE: Transystems Modifications



Howie DeFelice wrote to me privately:

	I just measured mine with a signal generator. On a unit using the
2278 LO
	without the front end filter, but using the standard pre-mixer
filter, the
	image of 2401 comes out at 2155. In my case, the gain at the image
frequency
	is significantly better than the gain at 2401. I have the stub
inserted.

Wayne replies:

You measured without the front end filter?  You removed a filter that is in
the stock unit?

If 2100 MHz and 2400 MHz signals both pass equally through the front end
filter, wouldn't the IF noise level drop only slightly when I tune to the
frequency of the 2400 MHz stub?

The IF noise level of my unmodified unit goes down about 3 S-units when I
tune from 145 MHz to 123 MHz.  That suggests that the noise at 145 MHz IF is
dominated by the 2400 MHz signal, since it goes WAY down when I tune the IF
to a frequency where the 2400 MHz stub kicks in.  That is, the front end
bandpass filter seems to attenuate the 2100 MHz signal quite a bit.  It also
suggests that the noise figure may be improve if I cut the stub to ensure
that the noise from 2401 Mhz is much stronger than the noise from 2155 MHz.

Too bad there is no easy way to lengthen the stub to move the notch to 2155
MHz.

Is there something wrong with my analysis?

Wayne Estes W9AE
Mundelein, IL, USA
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