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RF calibrator crowds out multiple single-function instruments

Posted: 10 Jan 2005 ?? ?Print Version ?Bookmark and Share

Keywords:rohde and Schwarz? fsmr measuring receiver? rf power meter? spectrum analyzer? fsmr3?

When working with RF, it's easy to find yourself with a variety of signal generators and sources, both in the lab and in production. Swapping these in and out when periodic calibration is required can easily put a crimp in tight design or production schedules. If you do in-house calibration, you'll likely need a precision wideband spectrum analyzer to assist in calibration, too.

That's where Rohde and Schwarz's new FSMR measuring receivers come in. For calibration of signal generators and attenuators, they're multi-function instruments that can serve not only as calibrators, per se, but as modulation analyzers, audio analyzers, RF power meters and spectrum analyzersall neatly rolled into one box.

As mentioned in the press statement, three versions are available. An FSMR3 spans 20Hz to 3.6GHz (when DC coupled; 10MHz at the low end when AC coupled); an FSMR26 goes up to 26.5GHz; and an FSMR50 goes up to 50GHz.

Significantly, FSMR boxes offer a high level of stability over time and temperature, attributes every instrumentation engineer looks for. The FSMR's parameters are all traceable to national standards, and their own calibration procedures are fully documented.

Automation hooks
Not mentioned in the company's press statement is that FSMR receivers are also operable across the venerable IEEE-488.2/GPIB bus. In that capacity, they can be set up to do things such as display limit lines for Pass/Fail evaluations. That's a boon in automated cal labs.

These receivers can also make as many as 70 measurements per second in GPIB operation. They also obey the SCPI (Standard Commands for Programmable Instruments) command set.

FSMRs are also capable of saving screen images to a floppy disk, a PC's hard disk, or into USB flash memory sticks. These boxes also support RS-232-C serial communications. Finally, they have VGA output connectors so that you can display screen images remotely.

Calibrate every parm
In use, an FSMR can calibrate virtually every parameter of a typical signal generator; it can test a generator's frequency accuracy, modulation depth and deviation, and modulation frequency response and distortion levels. When used as a spectrum analyzer, it can measure a signal's harmonics and (with an optional software package) can measure things such as phase noise.

The FSMR's integrated modulation and audio analyzer capabilities support AM, FM, and phase-modulated modulation modes, with audio parameters measured on a demodulated signal or on a signal applied to the instrument's audio input connector. For amplitude modulated signals you can see modulation depth and modulation frequency, and, optionally, you can measure average modulation values.

Demodulated audio signals can also be displayed in the time domain, and also as an RF spectrum generated by means of an FFT (fast Fourier transform), with harmonics and any spurious modulation (such as hum) displayed selectively.

For THD (total harmonic distortion) and SINAD (signal, noise, and distortion) measurements, the FSMR is automatically tuned to the fundamental. The THD function selectively measures all harmonics within the FFT spectrum. This rich array of capabilities means that no outboard instruments are required for calibrating modulation settings and a modulation generator.

At the other end of the spectrum, although a basic FSMR doesn't have sufficient image rejection above 3.6GHz (the YIG filters normally used to achieve this would impair level linearity), Rohde and Schwarz offers an optional YIG (yttrium-indium-garget) pre-selection filter. The YIG pre-selector extends the FSMR26's spectrum analysis functions well into the microwave regime.

Exceeding industry standards
Under the hood, the instrument's linearity and stability is largely determined by its ADC, but you can also optionally order a low-aging crystal oscillator reference.

Also, any of the system's modules or components that can drift (such as the YIG filter) or nonlinear level response components (such as crystal filters) are switched off during level calibration. As a result, the FSMR touts linearity values that are equal to or better than industry standards.

For signals within the instrument's selectable measurement bandwidth, level measurements aren't affected by frequency drift, deviation, or residual FM. Over its selectable measurement bandwidth extending from 100Hz to 10MHz, the FSMR is insensitive to frequency offset or residual FM of the generator to be calibrated. Various audio filters, weighting filters, de-emphasis filters, and detectors are available for audio analysis, to speed residual FM measurements.

Very small levels are also measured using a special narrow detector scheme. The narrow detector determines the signal power within the measurement bandwidth using an FFT at a reduced noise bandwidth. The technique yields improved SNR (signal-to-noise ratio), without increasing the effect of frequency offset or residual FM.

The FSMR is also automatic in many ways. For example, when measuring power (both absolute power or reference power) the FSMR's power meter can be connected to your signal generator's output, or it can be connected in parallel with the analyzer input using a power splitter. For the latter, the FSMR can automatically correct for the power splitter's frequency response as well as its insertion loss. It does that based on a stored correction table.

Similarly, to make measurements over the full level range of your RF generators, you can switch the FSMR receiver's RF attenuator or adjust its IF gain (the FSMR eliminates any level errors that result due to range switching by calibrating the adjacent range prior to switchover). That means that you benefit from the FSMR's linearity (within 0.015dB + 0.005dB deviation/10dB) across its level range. Moreover, level calibration demands only a few steps.

External power meter support
The FSMR can also be used with a variety of power meters, not only ones from Rohde & Schwarz. That can help preserve your investment in existing instrumentation. In use, the FSMR instrument controls the power meters across the IEEE-488/GPIB bus, permitting whatever power meter you have to be operated from the FSMR's front panel.

Rohde & Schwarz also offers a number of options other than the YIG filters for its FSMRs. For storage, you can add a removable hard disk. And you can add a second hard disk to increase local storage.

On the RF side, you can opt for RF pre-amplifiers (one, for the FSMR26) goes up to 26GHz. You can also install a tracking generator option; it spans 100kHz to 3.6GHz, and you can fit it with an electronic attenuator option.

- Alex Mendelsohn

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