What matters is the signal-to-noise ratio (SNR). Take the canonical
example of an ideal 2-port Wilkinson power combiner with in-phase
coherent signals of 10 Vrms applied to each input along with 1 Vrms
random thermal noise from the respective element amplifiers applied to
each input (i.e. each input signal has a 20*log(10 Vrms/1 Vrms) = 20dB
SNR).
The power loss of the combiner is 3.01 dB [i.e. 10*log(2)], so voltage
of each signal is attenuated by 1/sqrt(2) = 0.707. Thus, the components
of each input signal appearing at the output are 7.07 Vrms each and
0.707 for each of the noise inputs.
The signal components add coherently at the combiner output yielding a
total signal voltage of 14.14 Volts rms. The noise voltages are
incoherent, so they add as root-sum-square at the output of the
combiner. This yields a total noise voltage of sqrt(0.707^2 + 0.707^2) =
sqrt(1) = 1.0 Vrms. Thus, the combined noise voltage is unchanged, but
the signal voltage goes up by sqrt(2).
The SNR of the combined output = 20*log(14.14Vrms/1Vrms) = 23dB, a 3dB
improvement.
The same things holds for an ideal N-way combiner with equals noise
components at each input. The noise power at the combined output equals
the noise power of any of the equal input components (i.e. 0dB gain).
73, Mike W4EF..................
On 3/11/2020 7:22 PM, Lee STRAHAN wrote:
Hello John and all,
Concerning the adding the noise in a typical array. If the noise was
coherent or exactly the same signal from each element/amp the summed noise
would indeed be 8 times. However circuit noise is always random and incoherent
which causes the summation to be a single noise power times the square root of
the number of elements assuming equal levels from each amp. In the case of 8
elements 4.5 dB increase which is no small matter as well. In the case of the
three elements the noise summation would be about 2.4 dB higher than a single
element.
Lee K7TJR OR
As the designer of the YCCC high impedance feedpoint amplifier, let me address
some issues related to the design of the YCCC amplifier as well as feedpoint
amplifiers in general. If you don't want to read a lot of technical
gobbledygook, please disregard this message.
The YCCC uses an AD8055 RF amp as the gain element. As Lee, K7TJF, points out, there are most
certainly better op amps out there. However, the AD8055 was the "best" part I could find
in a DIP-8 package. The "better" op amps are all SMT parts but given that the YCCC
preamp was a kit, I intentionally limited the selection to DIP-8 parts that kit builders could work
with relatively easily on a PCB. Not everyone is able to do a competent job soldering tiny SMT
parts.
Within the universe of available RF op amps, tradeoffs must be made in terms of
noise, linearity, and bandwidth. The AD8055 is not the lowest noise part but
it has excellent linearity and plenty of bandwidth for HF use. At my QTH there
is an AM BCB station 3 miles away, which makes it a somewhat challenging EMI
environment. The decision to run the op amp in a unity gain configuration
comes down to linear dynamic range. It is easy to design for more gain, but it
is also easily demonstrated that you will begin to suffer in terms of unwanted
intermods. With the YCCC preamp, I get absolutely zero BCB intermods or
distortion products in the 160m band at my QTH.
In general I do not like to use an outboard preamplifier between the output of
the phased array combiner circuit and my receiver because it degrades the
linear dynamic range of the system. The YCCC system user's manual (Section
12.1) does specify several outboard preamps that could be used. In a low EMI
environment, I think they all work fine. However, at my QTH, with the nearby
AM BCB station, all of them, without exception, generate increased distortion
and intermod, which I find unacceptable.
It is always desirable to apply RF gain with a roofing filter in front, which
is becoming common practice in high performance receivers. With my K3S
receiver, the use of a unity gain antenna feedpoint preamplifier is perfectly
fine if you also turn on the preamp in the K3S. This gives the best overall
linear dynamic range with a preamplified short vertical system.
There is no loss in noise performance because the noise on 160 and 80 is
totally dominated by atmospheric noise. In measurements I made at my QTH, the
internal noise of the YCCC preamp is about 10 dB lower than my daytime
atmospheric noise on 160m when using a vertical about 20 feet high.
You must also consider the number of active elements in an amplified antenna
array when evaluating overall system noise performance. This is because the
amplifier circuit noise power of all the feedpoint amplifiers is added together
when the elements are phased up in a combiner. If you have N elements in your
array, the effective circuit noise contribution gets multiplied by N. The YCCC
array has 3 active elements at a time. However, the YCCC design is somewhat
unusual in that maximum RDF is achieved when the signals from the elements are
combined in unequal ratios. As a result the effective amplifier circuit noise
contribution is less than 3 times (or 4.8
dB) the noise of a single amplifier. In fact because of the unequal combining
ratios, the actual effective noise goes up by a bit less than 2 dB compared to
a single amplifier. An array like the Hi-Z array with 8 active elements
combines the elements in equal proportion so the effective amplifier circuit
noise of the system is 8 times (or 9 dB) higher than the noise of a single
amplifier. For this reason, the YCCC array can tolerate noisier amplifiers
without degrading system noise performance. The objective is to keep circuit
noise well under atmospheric noise.
On the subject of op amp noise specs, you must consider *both* input voltage
noise and input current noise because, in general, both contribute to the total
output amplifier noise. It is not good enough to pick an op amp with low input
voltage noise without also considering the input current noise.
For a good noise analysis, download a copy of the datasheet for the CLC425 op
amp: http://www.elektronikjk.pl/elementy_czynne/IC/CLC425.pdf. Refer to pages
8-10. (The CLC425 is a very good RF op amp but has been obsoleted by newer
parts). I put the noise equations into an Excel spreadsheet, which allowed me
to compare many different op amps in terms of total noise performance, using
their input current noise and voltage noise specs.
Not all op amps publish specs on linearity. It is safe to assume that if no
specs are given, the linearity is not particularly outstanding. Look for
harmonic distortion (HD2 and HD3) as well as TOI (third-order intercept) data.
You do have to be careful in interpreting the data because the linearity is
directly tied to the amplifier gain configuration.
If I were to recommend a particularly outstanding RF op amp, it would be the LMH6622, an
inexpensive but very high performance SMT part. It comes as a dual op amp package but I
only use one of the op amps. There is no single op amp equivalent part. The noise is
very low and the linearity specs are outstanding. It is intended for use in RF systems
with very stringent linearity requirements. I have built a "beta" version of
an antenna feedpoint amplifier using this op amp in a very unique configuration (not a
high impedance design). The effective circuit noise floor is about 8 dB lower than the
AD8055 preamp with similar linear dynamic range performance and about 8 dB higher RF
gain. I am still working some tradeoffs in this design, so I'm not ready to go public
with it just yet.
73, John W1FV
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|