[TenTec] Argo, Jupiter, Pegasus, RX320 overload

[George, W5YR]

Jack, I wonder if some unintentional typos got into your posting.

Adding any loss ahead of the receiver *increases* the noise figure by the
amount of the loss in dB and raises the minimum detectable signal level by
the same amount.

The noise figure is increased one dB for each dB of loss introduced. In
fact, by definition the noise factor of a lossy passive element is simply
the loss, which when expressed in dB becomes the *increase* in noise figure.
The MDS is raised the same amount. Example: a lossy coax cable run from the
antenna to the rig on VHF/UHF.

This increase in NF degrades weak signal performance although it raises the
TOI and increases the dynamic range. To that extent, it is desirable.

Changing the gain of a pre-mixer r-f amplifier stage is one way to enhance
strong-signal handling capability at the expense of weak-signal detection
capability. Your suggestion for reducing gain by introducing negative
feedback by eliminating the bypass capacitor is novel and might have merit
in some designs. I am not sure that I would agree that reducing the gain has
little effect on noise figure since the NF of the stage depends upon several
factors in addition to just its gain.

But any gain reduction affects weak-signal performance while it enhances
strong-signal-handling capability. If it is done in such a way that
linearity is improved, so much the better. But reduced gain is reduced gain,
and that increases the effect of all noise sources ahead of that stage, per
the classic theory of cascaded noisy two-ports.
Note, however, that theory is based upon the assumption of linear elements
in the overall system. Once an active stage is driven into non-linearity,
conventional linear systems theory begins to stray from reality.

Actually, from a practical point of view, HF operation is so totally
dominated by noise external to the radio, that what we do in the radio to
influence its own noise figure is largely academic. So, our principal
weapons are achieving linearity in all stages to the economic extent
possible and applying appropriate filtering and gain distribution, including
AGC operation, to finish the job.

Interesting stuff, huh?

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr at att.net>





----- Original Message -----
From: "Jack Bryant" <ke4id at bellsouth.net>
To: <tentec at contesting.com>
Sent: Wednesday, July 23, 2003 7:34 AM
Subject: [TenTec] Argo, Jupiter, Pegasus, RX320 overload


> Good morning.
>
> George, thanks for your comments about removing the  source bypass
resistor (I should have said CAPACITOR)
> from the 1st IF JFET amp in my RX-320 to cut down the front end gain.
>
> A resistive attenuator in the antenna input will do about the same.
Anything
> that lowers the front end gain or input signal level will boost the IMDDR3
> by the same amount.
>
> 73/72, George
> Amateur Radio W5YR -  the Yellow Rose of Texas
>
> I have used the attenuator approach, but it reduces the noise figure or
minimum discernable signal
> of the receiver.  The big advantage of reducing the gain with the approach
that I mentioned is that while the
> dynamic range and 3rd order intercept are improved, the noise figure is
not degraded much.  While the
> use of the front end attenuator will increase the 3rd order intercept, it
degrades the noise figure by the amount
> of attenuation and does not improve the dynamic range.
>
> Regards,
> Jack, KE4ID
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