Quiet Amps DC to 45 GHz For NMR/MRI Testing.

Transcription

1 Quiet Amps March 2007 DC to 45 GHz For NMR/MRI Testing. Noise Blanking Square Waveforms RF Gating Low Distortion No Droop Low Noise rf/microwave instrumentation

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3 Noise Reduction Between s. Various methods are used to reduce the noise power output between RF pulses. Solid State Amplifier 75AP250 and 75AP250A. The solid state amplifiers use two RF switches in the amplifier. The first switch is located at the input to the driver amplifier. The driver amplifier provides most of the amplifier gain and power to drive the output stage. The second RF switch is located between the driver amplifier and the output stages. Both switches are in the high isolation mode during pulse off time. The noise output of the amplifier during the pulse off time is a function of the gain of the output stage ( ~ = 20 db) and the noise figure of the output stage during pulse off time. TWT Amplifier 200T1G3A, 200T2G8A, and 200T8G18A. The TWT amplifiers' TWT beam current is turned off during the period between pulses. This turns the traveling wave tube off, reducing the noise power output. Tube Amplifier L Series - 5,000LP and 10,000LP The AR tube amplifiers gain is reduced between pulses by removing the screen voltage during the period between pulses. This reduces the amplifier gain and reduces the noise power output.

4 Quiet amps let you detect even the smallest More Powerful Than the Competition. Able to Deliver Power and Not Foldback. AR RF/Microwave Instrumentation amplifiers have power and abilities far beyond those of ordinary amps. Whether you need low power or high power, you get what you pay for with AR. And you pay less, watt for watt. Because you get more real power for your dollar. Power that holds up in the toughest test environments. Researchers thinking about the purchase of an RF amplifier for NMR/MRI studies would be well-advised to consider the performance features critical to test outcome. More than 35 years of experience designing and building amplifiers for experimental NMR/MRI imaging applications has shown us that: Fast noise blanking (gating) Very fast RF rise and fall time No-droop pulse shape Class A operation, and Wide bandwidth are the amplifier features crucial to clear, accurate test results features we've made part of our line of amplifiers for NMR/MRI imaging. Fast blanking reduces noise to near thermal (Thermal Noise dbm/hc). So you can track elusive transients easier than ever. We invented noise blanking the feature and the term. We know how difficult sample detection in NMR/MRI imaging can be. Difficult, but not impossible. Because received signals from NMR/MRI samples are prone to rapid decay, an amplifier must reduce noise amplitude to an acceptable level for the probe to be able to read the signal during its optimal detectable duration. This is usually accomplished by blanking circuitry within the amplifier. Blanking circuitry is an integral feature of most AR amplifiers for NMR/MRI studies. The 75AP250 and the 250AP250A offer a form of high-speed blanking circuitry that all but eliminates noise at the end of an RF pulse, letting the effects of applied RF be detected with the greatest sensitivity. In some amplifiers, isolation diodes may supplement the blanking function. In others, electronic switching reduces noise to an acceptable level. Blanking 101. A theoretical example shows a 10-watt RF pulse from the amplifier (solid line) along with a trace from the NMR/MRI probe which both excites and senses the nuclear spin at the Larmor frequency. Depending on the decay time of the received signal, the RF noise must be blanked by the amplifier's internal circuitry and/or isolation diodes to enable a meaningful reading. Power dbm (50 ohm system) Class A with gating keeps waveforms square. Rise and fall times rapid. Fast pulse applications and recovery are also essential to NMR/MRI sampling: the effect of our gating circuitry is fast rise and fall time less than 1 microsecond in the 75AP250 and the 250AP250A which quickly brings the pulse down to a level the blanking circuitry can manage. AR Amps are Class A Amps. Droop is minimized. You hear true blue effects. Any departure from the pattern of pulses delivered to the amplifier is distortion. And when what comes back is distortion, the quiet reflection you're listening for is probably lost in the spurious muddle. All AR amps are rated, and operate Class A. Class A operation implies the highest degree of linearity, offering the greatest freedom from both harmonic and phase distortion. Desired RF output is not diminished by power-pilfering harmonics, and the amplifier readily provides rated output well below saturation. A look at what your scope or measuring device has captured will reveal pulses of extraordinary flatness, with little detectable droop. AR amplifiers continue to operate in both pulse and cw modes at full rated power without damage or shutdown, even in the face of severe load mismatch. Envelope of RF pulses (amplifier output) Sense probe input/output, after diodes 10 watts fall time to 10% signal modulation off (35 db down) signal gated off (65 db down) Time noise blanked (125 db down) Diode RF isolation Envelope of detected signal (viewable only after the detection system recovers from RF pulses) Rise Time Overshoot 10% Model 250AP250A 90% Amplitude Width P

5 transients. Amplifiers DC to 45 GHz. Model 75AP250 Broad bandwidth gives you faster RF rise and fall time for higher resolution. The RF envelope profoundly influences the accuracy of results obtained by your sensitive NMR/MRI receivers; as bandwidth increases, the RF envelope becomes ever sharper, as does the resolution of your NMR/MRI sample. No matter how narrow the frequency range of interest today, the broadband amplifier provides flexibility and headroom for exploration. The combination of features offered by AR's amplifiers for NMR/MRI experimentation assures you sensitivity today, plus the capacity to accommodate your needs in the future. 20,000 10, Model Frequency d Output Power Power (Watts) 5000LP Gain Min (db) 10,000LP Blanking 30W1000B MHz no 50W1000B MHz no 100W1000B MHz no 150W MHz no 500W1000A MHz no 1000W1000C MHz no 150A100B 10kHz-100 MHz no power ratings are at 3dB nominal compression. All models have ±1.5dB gain flatness and operate with 1.0 milliwatt maximum RF input for rated output. Front panel gain adjustment standard. Blanked Output Noise Level Floor (Typ) 200T1G3A GHz yes 140 dbm/hz 200T2G8A GHz yes 140 dbm/hz 200T8G18A GHz yes 140 dbm/hz 250A250A 10kHz-250 MHz no 500A100A 10kHz-100 MHz no 1000A100 10kHz-100 MHz no 75AP MHz yes 130 dbm/hz 250AP250A MHz yes 130 dbm/hz 5000LP MHz yes 125 dbm/hz 10,000LP MHz 10, yes 125 dbm/hz Fall Time Period 90% 10% Droop Bounce The finer points of RF wave formation. The ideal pulse would be completely square zero rise and fall time, no droop, no overshoot, no bounce. Broad bandwidth and gating circuitry on AR amps keep them as close to ideal as possible. RF Power (watts) A100A 150A100B 250A250A 250AP250A 75AP A W1000B 50W1000B 1000W1000C 500W1000A 150W W1000B T1G3A 200T2G8A 200T8G18A Frequency (GHz) Frequency (MHz)

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