[CQ-Contest] 756 pro: definition of 'ratty'

jljarvis jjarvis at keithley.com
Sat Dec 1 15:53:18 EST 2001

Tony, K1KP's comments centered on three effects:

1) AGC pops: late to catch up to a big signal, if used on fast.
	Probably exactly what's happening.  Why not avoid the
	pumping sound, and use mid?  What happens if you adjust
	the time constant?  Is there a sweet spot, or just progressive

2) Flutey edges to cw characters, attributed to AGC.  Could this
	be an artifact of the ADC?  Is it possible the leading edge
	has components which are fast enough to violate Nyquist,
	and result in aliasing?  Perhaps this is an AGC-alias interaction?
	Using manual gain, and reducing the levels may remove the AGC dynamic.

3) Key clicks:  I have to wonder if those clicks are actually present, as
	Icom claims, and all existing receivers are fundamentally low-pass
	filters, which obscure them....OR, if those clicks are actually
	aliases produced when the rise time of the waveform is digitized.
	The implicit bandwidth of the rising edge of the waveform is well
	beyond 2x the sample rate of the ADC.

	Tony's observation that if you use the filter-set menu, the filters
	have softer slopes, and the clicks go away would suggest that
	the softer slope acts as a low pass filter, and doesn't permit the
	rise time to exceed the Nyquist criteria.

So, with that speculation in hand....what would have to be different in
	the 756PRO, for us not to suffer the same abuse?

Jim Jarvis
Keithley Instruments
Essex Vermont
802 872 5830 voice
802 872 5831 fax
440 498 2991 paging/messaging

"If the only tool you have is a hammer, you tend to see every problem as a
nail."  Abraham Maslow

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