Topband: Hi Z amplifiers for 160m

Thu Mar 12 07:13:48 EDT 2020

Lee,
I think you are confusing voltage and power.  For incoherent sources like amplifier noise, the voltages of multiple incoherent sources add in a root-sum-squared (RSS) fashion.  The voltage of the sum of eight incoherent sources is square root of eight times a single noise source, assuming equal combining ratios.  However, because power is proportional to the square of voltage, then the *power* of the combined sum is the sum of the individual noise powers.  This is well known in the theory of random processes, which is the basis of communications theory.  So, what I said earlier is correct.  For a system with eight amplifiers, the effective total noise power in the sum is eight times the individual noise powers when the sources are combined with equal weights.  The YCCC array does not use equal weights, so the powers have be weighted when combining them to get the total noise power.
73, John W1FV

-----Original Message-----
From: Lee STRAHAN <k7tjr at msn.com>
To: topband at contesting.com <topband at contesting.com>
Sent: Wed, Mar 11, 2020 10:22 pm
Subject: Re: Topband: Hi Z amplifiers for 160m

  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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