> Ya had me going till the FT 757. I had one of those for many years. It was
> the worst rig I've ever owned for dynamic range and phase noise. It's
> frontend is just crushed under even moderately busy band conditions. How
can
> it be as good as the near perfect TS-830s? Not even in the same league in
my
> book.
Hi Tony,
Phase noise and dynamic range/overload problems have any effect in a test
like Earl's. We won't hear phase noise unless a strong close by signal is
present, just as we won't hear overload.
I've found ability to dig out weak signals with noticeable antenna noise is
a lot different than a bench test with only a signal generator on the
receiver input.
I suspect manufacturers and filter vendors almost never worry about group
delay errors, which are critical for copying signals near noise. This
problem results in a variable time for "things" to pass through the receiver
system, with lower pitched frequencies we are hearing taking a different
amount of time to pass through the system than higher pitch signals. What
happens is one sideband of the CW signal is delayed a different amount than
the other sideband, resulting in what sounds like ringing or excessive
softening of the rise and fall. Noise pulses are also subjected to the same
uneven time delays with respect to frequency in the passband we hear,
stretching out noise pulse duration.
How many filters have you seen rated for group delay errors, or how often is
it mentioned? I'd bet almost no one looks at it, even though it can be
critical. My 751A totally blows away a stock R4C or 75A4 for selectivity and
overload by close signals and looks much better for noise floor, but the
250Hz filters are unusable for working noise floor signals if there is any
rough noise present. The 500Hz filters in the 751A are barely acceptable.
Because of that, I never use the 751A for working weak signals. All the
sensitivity in the world does no good when the filters ping on every noise
pulse, and CW sounds mushy.
Aside from "ringing" or group delay errors, some receiver detectors seem to
blend noise and signal together making it impossible to copy noise floor
signals. I am not positive, but I *think* much of this is detector
non-linearity or intermod. I always observe receivers with a lot of headroom
in detectors and audio systems work better for me in receiving weak signals.
It would be great to come up with a lab test protocol for weak CW signals in
noise!
73 Tom
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