Tom, you wrote:
"Rolling a pre-amp out of discrete components allows the designer to
add negative feedback to reduce gain, and that
improves dynamic range and noise figure! Solid State Design for the
Radio Amateur covers this, as did Ulich Rhode in some of his RF Design
articles in the late 70's and early 80's."
One mistake, adding negative feedback (either shunt or series) will
never improve noise figure. The addition of series feedback will give
you low frequency stabilization, raise noise figure, and the ability
to move S11 closer to the optimum Noise Figure match, which will
improve your input match while trying to source match for Noise
Figure. Shunt feedback will again improve stability, reduce gain
depending on the amount of feedback, and raise noise figure depending
on the amount of feedback.
Peter
W5PS
______________________________ Reply Separator _________________________________
Subject: Re: [AMPS] Re: TopBand: Followup Beverage Box
Author: w8jitom@postoffice.worldnet.att.net at internet
To: <topband@contesting.com>
Date: 9/23/97 6:01 AM
> From: philk5pc@connect.net (Phil Clements)
To: <topband@contesting.com>
> Date: Sat, 20 Sep 97 10:03:00 +0000
Hi Phil,
Thanks for the info.
> Hi Tom,The ZJ boxes I have are not broad band. They are either single-band, or
> 1.8-4.0 mhz.
> The tuned circuits in the pre amp are very good, as a 50kw AM station 4 miles
> away
> cannot be heard at all on my EWE.
IMHO, it's better to use a IMD-immune device rather than filter
the input/ output of a device that by itself offers marginal
performance. Remember it's usually not the level of ONE signal
that is the problem, it's the total energy of many signals combined
that causes most overload problems. The larger the number of signals,
the less level that can be tolerated from any individual signal
before the amplifier "falls apart". Even if BC stations are
eliminated, the accumulation of signals in any two or three
MHz wide spectrum can cause problems.
Add a good filter if necessary, but its best to start with a good
device. A less than a dollar CATV type transistor will produce a much
better noise figure and overload long after most receivers.
The cheaper MMIC's are not wide dynamic range low distortion
devices, nor are they rugged.
> > A single LT1001 or cheap 2N5109 is much more immune to damage, has
> > better IMD performance, and a better noise figure. MMIC's are simple
> > to use, but that does not make them good devices.
>
> I have no idea why they chose these devices in such a hostile
environment.There
> must be some advantages.
The only advantage I know of is the MMIC has "designed-in" bias and
feedback circuitry. In other words, it eliminates a few external
resistors and capacitors and does not require any design effort. The
disadvantage is cheaper MMIC's have poor IMD performance and
are particularly poor with even-order harmonic distortion. You are
also stuck with the gain and bias settings they designed in the
device.
Rolling a pre-amp out of discrete components allows the
designer to add negative feedback to reduce gain, and that
improves dynamic range and noise figure! Solid State Design for the
Radio Amateur covers this, as did Ulich Rhode in some of his RF
Design articles in the late 70's and early 80's.
> In my case, the utilities
> are all underground and even using 9913 coax, there was 60hz pickup in the
> shack mounted pre amp which was amplified along with the signals. By putting
> the pre amp at the EWE and adding a line isolator at the ZJ box output to keep
> the
> 60hz out of the ZJ box, all noise was eliminated.
Other than with pre-amp construction or design shortfalls, 60
Hz hum should never modulate the signals. That's always a sign of
poor internal supply line filtering, or a poorly designed input/
output system that has unwanted LF AC coupling into a non-linear
device. It could be caused by core saturation in the input/
output transformers too. If AC ground loop currents pass through
the windings and cause core saturation (or even a permeability shift)
the AC will cross-modulate signals inside the transformers
themselves. MCL transformers saturate very easily with any winding
current. If the input is dc or LF ac coupled to the primary, or if
the same is true at the secondary, I'd expect trouble with induced
hum in some systems.
Any modulation of signals by ac hum means something is NON-linear.
Non-linear is not good in DX receiving systems, and the line isolator
is a band-aid for the real problem...the real problem being a
non-linear device somewhere in the system.
> > Even at the quietest location, a Beverages noise will
> > greatly exceed cable losses. Install the cable properly, using
> > reasonably good feedline (CATV drop line is excellent) and GOOD
> > connections at the connectors, and the preamp might as well be right
> > next to the receiver
>
> See above....not in ALL cases!
You're right Phil. I should have included the qualification the
system must not have non-linearity. If the Beverage receive system is
properly designed and installed and the feedline has less than five
or ten dB loss, even the quietest location would never require a
remote amplifier.
73, Tom W8JI
-
---
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
-
---
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|