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1. FAQs
Amplifier systems
1. How does the “auto-CW” function operate in Tomco pulsed amplifiers?
Specifications for pulsed amplifiers typically include a figure for maximum pulse duty-cycle (20% for example), and also a figure for maximum pulse width (10 milliseconds for example). Such amplifiers usually include some form of duty-cycle and pulse width limiting which serves to protect the amplifier and any equipment connected to it, from excessive average power.
However, in many pulsed RF applications it is necessary to also be able to produce low-power bursts of continuous RF without any limitation on pulse width or duty-cycle.
All Tomco pulsed RF amplifiers are capable of CW operation at low power.
In many pulsed amplifiers it is necessary to switch between pulsed and CW mode (either manually or via a control interface) in order to permit CW operation, but with Tomco amplifiers this is not necessary as the switching is performed automatically. Provided that the RF input to the amplifier is sufficiently low, the duty-cycle limiter is disabled and CW operation is possible. When the RF input level is raised above a certain threshold (typically corresponding to about 10% of the full drive level), the duty-cycle limiter becomes active and the specified limits for pulsed operation are asserted. In other words, all you need to do to produce a CW output is reduce the RF input level and hold the gate input high.
As noted above, the threshold is typically about 10%. For example, a Tomco amplifier that is rated for 1kW pulsed will be able to produce around 100W of CW.
2. What is the difference between a pulsed amplifier and a CW amplifier? (or, can a CW amplifier be used for pulses?)
The most obvious difference lies in the ratio of peak to average power capability. A good quality pulsed amplifier that is rated for say 1kW pulsed power at 10% duty-cycle, will contain just as many power transistors as a 1kW CW amplifier. However, the power supply and cooling requirements for the pulsed amplifier are much less, since it is rated for only 100W average power.
Apart from that, RF amplifiers that are designed for pulsed operation include many features and specifications that will not be found in CW amplifiers. Pulsed amplifiers include high-speed noise gating, which reduces the output noise level to almost zero in-between pulses. This is important in applications such as NMR or radar, in which the inter-pulse signal levels of interest are extremely small.
Pulsed amplifiers generally must be capable of reproducing pulses with very fast rise and fall times, while amplifiers designed for CW operation often do not have that capability.
3. What protection do Tomco amplifiers incorporate?
Tomco RF amplifiers are protected against damage due to the following conditions:
- Over-temperature
- Over-drive
- Excessive duty-cycle
- Excessive pulse width
- Excessive load SWR
Note: SWR protection is not included in Tomco’s “brick” amplifier modules.
Amplifier modules
4. Can you recommend a power supply for use with the Tomco amplifer module, BTM00250-AlphaSA?
For pulsed operation we would recommend the S-240-48 from Meanwell. For CW operation the rail must be reduced to 28V or less. If this functionality is required, two S-240-24 supplies can be used in series to give the otpino of 24V and 48V voltage points.
Alternatively, any benchtop varibale supply can be used provided it can be set to the required voltage and can deliver the required current. Required current depends on the maximum duty cycle required in pulsed mode. Worst case is full power at 20% which would require just over 3A at 48V. Npote that external cpacitors are almost alwa6ys required as pre the amplifier module application note.
5. How are pulsed and CW operation implemented in the Tomco amplifier module BTM00250-AlphaSA?
At DC voltages up to approx 30V, pulsed and CW operation are available. Once the DC voltage is greater than about 32V protection is activated which limits the maximum duty cycle to 20% and the maximumpulse width to 100ms. If you apply more than +32V and try to run in CW mode the amplifier module will shut down due to the over duty protection activating. Once the duty cycle/pulse width return to normal limits the protection resets and the module continues to function. So, above 32V DC voltage, pulsed mode only is possible.
6. What effect does the level of DC voltage applied have on the output power of the Tomco amplifier module, BTM00250-AlphaSA?
Output power varies as the square of the DC voltage applied. If +28V gives 50W CW output, then 14V will give 12.5W output for the same drive level. Hopwever, if the DC voltage is greater than approx 32V and a duty cycle of greater than 20% is applied or a pulse width of greater than 100ms is applied, the module will shut down. +50V is the upper limit of DC voltage that can be used. Damage may be casued if you apply greater than +50V DC.
2. Definition of terms
| Rated power |
This is the minimum RF output power that the amplifier will produce, when the full specified input level is applied to it. The full specified input level for Tomco amplifiers is usually 0dBm (1 milliwatt).
The rated power specification for Tomco amplifiers is a guaranteed minimum. |
| P1dB |
This is the minimum RF output power level at which the amplifier’s gain will have dropped by 1dB compared to its gain at one-tenth output power. It is a measure of the amplifier’s linearity at high power levels.
The P1dB specification for Tomco amplifiers is a guaranteed minimum |
| P0dBm |
This is the output power from the amplifier when an input level of 0dBm is applied (0dBm is the standard maximum input level for Tomco amplifiers) |
| Pulse rise/fall time |
Pulse rise and fall times are measured between the 10% and 90% output voltage levels. The measurement is made using a “pre-gated” RF input signal so that the gate risetime and gate delay are not included in the figures |
| Gate delay |
Gate delay is measured from the rising edge of the applied gate pulse, to the point where RF just begins to appear at the amplifier output. The measurement is made using continuous RF applied to the amplifier’s RF input. It is typically less than 1 microsecond for Tomco pulsed amplifiers |
| Harmonics |
The RF output of a power amplifier always includes some internally-generated harmonics of the RF input signal. The harmonics are measured with the amplifier running at its specified P1dB level. They are expressed as “dBc” figures: for example, a third harmonic of -20dBc at 1MHz means that if you apply a pure 1MHz sinewave to the input of the amplifier, the output will include the amplified 1MHz signal plus a 3MHz signal that is 20dB (100 times) smaller |
| Gain flatness |
Gain flatness is a measure of how the gain of the amplifier changes with frequency. To ensure that the figure doesn’t include any false “improvements” due to gain compression, the gain flatness is measured at low signal levels – usually at one-tenth of the rated output power. For a broadband high-power amplifier, a gain flatness figure of +/-2dB is generally considered good.
It is important not to confuse gain flatness with rated output power; a Tomco amplifier that is rated for 1kW output will be capable of producing at least 1kW across its full frequency range. However, the RF input level required to produce that 1kW output will vary with frequency |
| PEP |
PEP stands for Peak Envelope Power. It is equal to the product of the RMS voltage and RMS current when the signal is at its maximum level.
The concept of PEP is useful in situations where the amplitude of an RF signal varies rapidly with time.
In mathematical terms, the product of RMS voltage and RMS current is called the “average power”. Unfortunately this is easily confused with the average power of a varying signal level measured over a period of time.
For example, imagine an amplifier that is outputting a constant CW level of 100W. The average output power is obviously 100W, and the PEP is also 100W. Now imagine the same amplifier running at the same input signal level, except that the input signal is switched on and off rapidly such that it is on for only 10% of the time. In other words, it is pulsed at 10% duty-cycle. The average output power is now 10W, but the PEP is still 100W.
This example suggests that the difference between average power and PEP is somewhat dependent on time scales, and in that sense the distinction is slightly arbitrary. In terms of RF power amplifiers, the distinction basically relates to the capacity of the amplifier’s cooling system, so the timescales involved are of the same order as the thermal time constants of the RF power transistors and heatsinks. These time constants very enormously from one amplifier to another, but typically the transition from a “pulsed” specification to a “CW” specification occurs at pulse widths between a few tens to a few hundreds of milliseconds.
For example, an amplifier that only needs to run at 100W for one millisecond every ten milliseconds would be a pulsed amplifier rated for 100W PEP, 10W average. But an amplifier that must run at 100 watts continuously for one day every ten days would not be called a “pulsed” amplifier, and would need to be specified for “100W CW”, not “100W PEP, 10W average”.
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Phase performance of Tomco Delta amps (100-600MHz)
Performance measurements for a BT01000-Gamma amplifier (5-200MHz, 1kW) |
