Instrumentation

Engineering

Limited

Attenuators
Description

Attenuators

Attenuator Applications

  • RF Factory test
  • VSWR generator for PA testing
  • RF path simulation
  • Front-end protection for field test gear
  • IMD reduction in multi-coupled systems
  • Circulator load allowing instrument access to RF installations
  • Stability addition for broadcast systems when used between exciter and PA

Attenuators are used to reduce incident power, control impedance between RF stages, and as loads if the attenuation factor is sufficiently high. Place the attenuator between RF producing equipment stages or before the antenna to reduce the incident power, as these do not suffer from OEM instabilities when power must be reduced below the designed power output of the equipment. Control impedance with attenuators between power amplifier stages to maintain stability or in place of a load for testing.

Attenuator FAQ

RF loads are usually required when testing RF transmitters. The reason for this is that the transmitter has been designed to work into an impedance that is usually equivalent to 50 ohms resistive. Modern transmitters (solid state Bipolar or Mosfet) will not produce their rated output power unless they see a reasonably good load impedance.

A good rule of thumb for estimating forward power from solid state PA’s is- ***Forward power is bounded by 1/S and 1/S2 where S is the SWR number (S:1). Typically the rated output power is held constant by the internal control and bias circuits for SWR values up to 1.3:1 or sometimes as high as 1.5:1 (this problem is caused by the load lines for the active devices in the output shifting). At this point the forward power starts to turn down as SWR increases. Typical antennas present load impedances with SWR values as high as 3.0:1. In such cases the forward power could be as low as 1/9 of the value presented into a good dummy load. The true power is 25% less than the 1/9 value due to the reflected power of the 3.0:1 SWR. (25% of forward power is reflected to the transmitter by a 3.0:1 SWR.) Therefore the worst case true power delivered to the load is approximately 11dB below the power delivered to our good dummy load by our transmitter***.

Most transmitters will work into a load SWR of up to 3.0:1 without damage. An SWR of 3.0:1 is equivalent to a load return loss of 6dB. A 3dB attenuator also presents a return loss of 6dB if the end away from the transmitter is open or short circuited or shunted by a pure reactance.

Dummy loads (excluding expensive precision types) have typical measured return losses of between 15 and 20dB (SWRs of 1.15:1 and 1.05:1) with cheaper units specified at SWR of 1.2:1 ( a return loss of approx 13.5dB).

From the above it can be seen that an attenuator of greater than 3dB can be used as a load that will not damage the transmitter. However if transmitter output power is to be measured with reasonable accuracy then attenuation values of greater than 8dB should be used which will give a result accurate to ± 0.12dB.

Examples of attenuators as a load use.
  1. From *** above. If we connected an attenuator of 4dB in series with the antenna the transmitter would see a return loss of 14dB (made up of 6dB from the antenna and 8dB from the attenuator.) The transmitter would then supply full power minus 4dB to the antenna, a theoretical gain of 6dB. However because of its SWR, the antenna has a mismatch loss of a further 1.25 dB. We are still ahead by 4.75dB in transmitted power than without the attenuator.
  2. By putting an attenuator onto an isolator (circulator with dump load) instead of a load, access to the transmission system can be had to check deviation, carrier frequency, and condition of the antenna / feeder. If the attenuator was 10dB in value this would provide 20dB of return loss to the circulator and reflected energy from the circulator would be measured at one tenth actual value. Advantage of this is that all this can be done with out taking the transmission system out of service.
  3. Checking antenna SWR. When checking antenna SWR (return loss) the antenna should be isolated from the output impedance of the transmitter. This is due to the fact that most commercial land mobile transmitters do not have a 50 ohm output impedance. By putting a 10dB attenuator on the transmitter output the measuring bridge (which is designed for 50 ohm source and load) will measure the actual SWR of the antenna with reasonable accuracy. Without the attenuator it is possible that a good SWR indication is had when in fact the true SWR indicates the antenna or feeder has failed.
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