Thanks so much for explaining the amplifier choice.
The choice of a thru-hole part is completely understandable
in light of the positioning of this system as a kit.
The choice to use unity voltage gain minimizes BCB distortion.
Of course, unity voltage gain actually represents substantial
power gain because of the big difference between input and
output impedance. The KC1XX experience is an important
benchmark for this design.
Is the decision to discontinue the kit related to the
recent explosion of FT8 activity? Software beats hardware?
73
Rick N6RK
On 11/6/2018 5:40 PM, John Kaufmann wrote:
I designed the YCCC preamp so I can answer the question about the AD8055 op
amp.
As used in the YCCC preamp, the linearity is actually very good. We use a
YCCC receiving array at KC1XX in a multi-operator, multi-transmitter
environment with 20 foot receive verticals. That's a pretty severe RFI
environment. When listening on the array on either 160m or 80m, there is
absolutely zero interference or other noticeable degradations from
out-of-band transmitters. When listening with an in-band transmitter, there
is a lot of noise near the transmitter frequency, but most of it is phase
noise from the transmitter itself.
If you want more technical detail, here are some of the considerations
behind the choice of the AD8055. First of all, the YCCC preamp is a kit.
Based on that consideration, I ruled out surface mount op amps that
inexperienced kit builders might not be comfortable with. In truth there
are some truly outstanding surface mount parts with much better performance.
The AD8055 comes in a standard 8-pin DIP package that's easier to work with
and it was the best 8-pin DIP part I could find. It is intended
specifically for use in RF systems.
Second, the preamps must exhibit very accurate gain and phase matching to be
used in a multi-element active array. The YCCC preamp meets this
requirement because it operates with unity voltage gain in a negative
feedback configuration that virtually guarantees extremely accurate gain
setting. There is no trim adjustment needed at all for gain. I have done
VNA measurements on many of these preamps built by different people and
every single one of them, when assembled correctly, exhibits a gain accuracy
of +/-0.1 dB gain and better than one degree of phase matching accuracy
across 160 and 80. The large amount of negative feedback used at unity gain
also contributes directly to low distortion.
I don't have test equipment to perform accurate distortion measurements.
However, the AD8055 datasheet does have data on second and third order
harmonic distortion figures that look very good at 1.8 through 4 MHz. In
fact these distortion levels are similar to what you might see in a high
quality stereo audio amplifier.
Having said all that, prospective system builders should be aware that DX
Engineering has just discontinued sales of the YCCC preamps as the entire
inventory of these kits has been sold out. Once their inventory of phase
combiner and direction switch kits is sold out, those will also be
discontinued. YCCC, as a club, has no intention of selling kits.
73, John W1FV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Richard
(Rick) Karlquist
Sent: Tuesday, November 6, 2018 3:17 PM
To: donovanf@starpower.net; topband reflector
Subject: Re: Topband: Which compromise receiving antenna
I am puzzled by the choice of the AD8055 for the amplifier.
It seems to be unprepared for strong BCB signals. Has
anyone successfully used this near a BCB station? I am
6 miles from a 50 kW station. It seems to me there are
much better amplifiers available. Or why not a FET follower?
Rick N6RK
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