Value chain · CLM-1058
In plain terms
The often-repeated claim that the magnetron is 188 decibels quieter than its carrier traces to a conference abstract in which the tube maker said "180 to 190" for hand-picked tubes and said his instrument could not see below that outside a 60 megahertz window.
The claim as recorded
The magnetron noise figure that Rockwell quoted at second hand in 1981 as "at least 188 dB per Hertz at frequencies outside of a 60 MHz band" is Brown's own statement at the April 1980 Program Review, given as a range with no tube named and no conditions beyond the notch: "Making use of a special measuring technique in which a high-power, narrow-band notch filter rejects all but one part in 100,000 of the carrier signal to permit a spectrum analyzer to be exposed to the full level of the noise output, signal to noise ratios of 180 to 190 dB/Hz in selected tubes have been measured. The measurement sensitivity is still limited at frequencies outside of a 60 MHz band centered on 2450 MHz by the residual noise level of the spectrum analyzer", from which "an 8 gigawatt SPS transmitter would radiate less than 2.5 milliwatts of noise for each megacycle of the frequency spectrum".
Evidence behind this: T3, a tube developer's conference abstract: a noise range measured on selected tubes at the instrument's floor, with no tube number or conditions.
Scope and limits
An extended abstract: no tube number, no drive, no heater state, no bandwidth, no figure. "Selected tubes" is Raytheon's own qualifier. The 60 MHz is the width of the instrument's residual-noise limit, not a property of the tube.
What it assumes
The 1 in 100,000 notch is 50 dB; the 2.5 mW/MHz follows from 8 GW at -185 dB/Hz (8e9 W x 10^-18.5 x 10^6 Hz = 2.5e-3 W), which is the mid-point of the range.
Where it comes from
PDF p.358 / printed 336, render read, normalised: as quoted in the claim. Same page: "The maximum efficiency that has been achieved from a klystron is 75% while the efficiency that has been achieved by both magnetrons and amplitrons is in the 83 to 85% range. Top efficiency from a klystron after a substantial development program is expected to be 85%. A similar effort could increase the crossed field device efficiency to 90%." PDF p.359 / printed 337, normalised: harmonic levels JPL "-55 dB, -65 dB, and -68 dB for the 2nd, 3rd, and 4th harmonics" and Raytheon "-71 dB, -85 dB, and -86 dB"; "These levels are lower than those expected for klystrons but are still orders of magnitude above what would be acceptable without making special frequency allocations for these harmonics or making extensive use of filters"; "The ferrite materials in the circulator are not suitable for high temperature operation in space"; lifetimes "tens of years" and "fifty or more years".
Basis: dB/Hz signal to noise; MHz band; mW per MHz at 8 GW
Related records
- Answers question
- QST-001 not published in this release
Cite this
energyaperture.com/c/clm-1058
CLM-1058, Energy Aperture, as of 1980-04.
Connected records
Other published records linked to this one. A list, not a judgement.
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