Mike and all,
Well stated Mike. It's been a long time since we have conversed. The
modifier to this is when the signals coming into the combiner are no longer in
phase or coherent. This as a result of delay lines and time of signal arrival
at the many elements. Most often in our small portion of a wavelength low
frequency arrays the combination of signals is subtractive to form a given
pattern per array dimension. This then lowers the signal to noise ratio. It
gets pretty complicated to arrive at a noise figure. The only way we have been
able to do this with amplified arrays is to simulate the array in NEC being
excited with a known signal many wavelengths away. We can extract the actual
amplitude and phase of these multi element array signals and then combine them
mathematically as you have done by example to arrive at a signal gain number
from signal combination. The noise gain is easy. I say we because I have a
retired very smart Ham friend in Finland that has helped me through this. It h
as caused me to rethink gain distribution in some of my arrays.
Lee K7TJR OR
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
>
>
>
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