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Outboard sensors give RF power meters versatility

Posted: 16 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Alex Mendelsohn? Anritsu? ML2490A? RF power meter? IEEE-488/GPIB?

Not mentioned in Anritsu's release notes for its ML2490A Series RF power meters are that they are IEEE-488/GPIB (General Purpose Interface Bus) instruments, and they can also communicate over RS-232 serial lines.

The ML2490A series meters also give you dual display channels, and each channel's set-up can select or combine sensor inputs. You can also configure an ML2490A to view one display channel or two, and you can switch between them using a channel selector hot key on the unit's front panel.

You can also set automatic test limits, and use an internal limit editor to create and select limit profiles. For pulsed measurements such as radar signals, a time-varying limit line can be set to test pulse profiles.

A ML2490A power meter can also be set up to indicate Pass or Fail, and it can hold a measurement if there's a failure. That can help when you're trying to locate intermittents.

You can also choose to view a measurement as a graph profile or as a numerical readout. You can also use the box's video output jack to feed a 1/4 VGA signal to an outboard monitor so others can watch the results.

Outboard sensors
Regardless of how you drive them, key to applicability for the graphics-based ML2490A power meters are Anritsu's many sensor options, as briefly referred to in the press statement. These outboard sensors are what make these boxes sing.

Anritsu offers no less than seven families of power sensors, each family designed for a class of applications. These sensors are based on either thermal converters or diode detectors.

The diode detectors are half-wave or full-wave rectifiers using zero-bias Schottky diodes. The rectifier outputs are then low-pass filtered to comprise envelope detectors.

The RF power meters themselves can operate in a pulsed modulated mode or a CW (continuous wave) mode. The tradeoff between the modes is dynamic range (increases of bandwidth are traded off for a reduction in sensitivity).

Your choice of sensor is dictated by frequency, dynamic range, and modulation type you're dealing with. In any case, you want to choose the risetime of a given sensor to match the risetime characteristics of the RF signal's modulation you're dealing with.

It is also worth noting that Anritsu's MA2400 Series sensors include EEPROM-stored factory calibration data. What's more, up to nine user-cal factor tables can be stored during on-site calibration. The EEPROMs can also store extra frequency points or compensation data for associated attenuators or couplers.

Sensible sensors
For example, Anritsu's MA2490A and MA2491A sensors are dual-purpose wideband and CW sensors. They exhibit 18ns risetimes so they're suitable for the ML2490A's pulse modulated mode. They permit the meter to make average power, peak power, and crest power measurements on signals with rapid amplitude changes. Such signals are found in WCDMA cellphone systems, WLAN and WiMax network gear, and in radar.

The MA2490A and MA2491A sensors also have a bandwidth of 20MHz. In CW mode, a FET switch chops the signal from the sensor at low power levels when CW mode is selected on the power meter.

For its part, Anritsu's MA2411B pulse sensor has the widest possible bandwidth and fastest risetime. It doesn't use the FET switch to detect low-level CW signals. The MA2411B can be used to make rising-edge measurements and wideband measurements on signals such as OFDM (orthogonal frequency division multiplexed) multi-carrier signals.

Anritsu also has MA2470D Standard Diode Sensors for the ML2490A power meters. The MA2470Ds exhibit high dynamic-range, and are best suited for CW and TDMA (time division multiple access) spread-spectrum power measurements.

The MA2470D sensors exhibit 90dB of dynamic range and have linearity specs better than 1.8 percent. Their risetime specs are fast enough for power measurements on GSM and similar TDMA systems that use GMSK (Gaussian minimum shift keying) modulation.

Wide dynamic range
You can also use MA2440D diodes with the power meter. These diode sensors are recommended where the best measurement accuracy is required over a large dynamic range (such as when measuring amplifier power). The MA2440D diode sensors have built-in 3dB attenuators to keep input VSWR nice and low. The MA2440D high-accuracy diode sensors have a dynamic range of 87dB (compared to 90dB for standard diode sensors).

The MA2480A sensors are billed as universal types. They're true-RMS sensors with 80dB of dynamic range. Typically, they're used for average power measurements on multi-tone or WCDMA signals. These sensors comprise three pairs of diodes. Each one is configured to work in its square-law region over the dynamic range of the sensor.

Anritsu claims this 3-stage approach leads to a faster measurement, as the SNR (signal-to-noise ratio) is better than predecessor 2-diode sensors. Anritsu supports these sensors with an option that provides TDMA measurement capability. The option calibrates one of the diode pairs to achieve linearity over a wide dynamic range.

Fast thermal sensors
You can also fit a ML2490A RF power meter with fast thermal sensors. Anritsu's MA2420D thermal sensors support power measurement over 50dB of dynamic range, and they're fast. These thermal sensors measure true-RMS power, regardless of input waveform, so they can measure the power of continuous waveforms such as WCDMA, multi-tone signals and CW.

The MA2420D sensors also exhibit good linearity and ensure low VSWR. You can also get a MA2420D sensor without DC blocking; that can let you make power measurements at frequencies down to 100kHz.

Multiple gates
The release notes briefly mention Anritsu's use of multiple gates. The gates are what let you capture information from a signal under test, and the measurement gates are also crucial to the ML2490A Series's signal processing. You can trigger on a radar pulse or sequence of pulses, for example.

The ML2490A instruments support up to four independently-set gates, or eight gates that are repeated in a pattern. In use, the instrument's wide bandwidth and high-speed A/D (analog-to-digital) conversion positions the gate accurately within a signal's profile.

You can then choose between several measurements performed within the gate, such as average power, peak power, crest power and maximum/minimum power. As for the latter, maximum and minimum power data is also time-stamped. That lets you store the position of these signals so you can record over-shoot and under-shoot of pulsed signals.

- Alex Mendelsohn

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