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RF sniffer is a recording spectrum analyzer

Posted: 03 Feb 2005 ?? ?Print Version ?Bookmark and Share

Keywords:rf radiation? microwave frequency? narda safety test solutions? instrument? fcc?

With renewed interest in the possible deleterious effects of non-ionizing RF radiation on the human body, particularly at cellphone microwave frequencies, Narda Safety Test Solutions' new handheld instrument is a timely product. Especially so now that the FCC and OSHA are in the RF safety exposure game.

Weighing in at just over four pounds, Narda's SRM-3000 selective radiation meter is a battery powered portable that's essentially a precision handheld spectrum analyzer. It's priced at about $15,500.

Spanning the frequency range from a low of 100kHz all the way up to 3GHz, this unit has low inherent SSB phase-noise and a high degree of oscillator stability. At 30kHz carrier spacing, phase noise is less than -85dBc (1Hz), and at 100kHz carrier spacing, phase noise is less than -105dBc (1Hz); at 1MHz carrier spacing, it comes in at less than -120dBc (1Hz).

In terms of stability, initial reference frequency deviation is less than 1.5ppm, aging at a rate that's less than 0.5ppm/year. Likewise, thermal drift is lower than 2ppm.

It's probe-tolerant

To its credit, Narda doesn't limit you to its probes only. The Li-ion-powered SRM-3000 (it also comes with an ac power supply module) can be used with probes available from Narda, as well as with probes from other manufacturers. That can be cost-effective, especially if you already own some RF probes.

In any case, Narda's own tri-axial probe lets you make isotropic measurements in the range from 75MHz to 3GHz. Measurements are typically made by using the probe directly affixed to the handheld, so that you carry just one unit, and don't have any trailing cables to mess with. That can be very handy in situations such as tower climbing, where snags can be dangerous.

However, you can also make measurements through a probe that's connected to the unit via a cable. You can even hang an SRM-3000 from your belt and poke the probe by hand. You can also mount your probe on a tripod, for example, and connect it to the SRM-3000 using a supplied cable.

Once the probing is accomplished, the unit stores your findings. However, results can also be exported to a PC for downstream analysis.

Crowded RF environs
According to Narda, even in a complex field environment, and in the vicinity of powerful broadcast signals, its SRM-3000 can be used to measure individual transmit channels. What's more, the unit can also integrate over its entire frequency range and then display a total value of RF (it can read out either an absolute value or one as a percentage with respect to a given limit).

In its spectrum analysis mode the SRM-3000 gives you an overview of all of the frequency components, with their field strengths. All you have to do is set the frequency range. The SRM-3000 will then only accept values that don't exceed the frequency limits of the connected probe.

The spectrum is then displayed, and markers and a zoom function can be used to quickly make a field-strength assessment. Peak tables show all of the field strength values above a certain value, along with their frequencies.

You can also integrate over a desired frequency range, letting you display the power of a transmit channel with a bandwidth greater than the RBW (resolution bandwidth) of the measurement.

A cellular example
For example, for GSM cellphone system measurements, the SRM-3000 can use a 200kHz RBW and measure the field strength of an individual control channel that always transmits at full power. It will then estimate the field emissions that all of the traffic channels would produce under full load. By using a 5MHz RBW, the unit can also detect an entire UMTS frequency block. It can also demodulate the pilot channels of individual UMTS base station cells.

All of functions are set using so-called softkeys and/or a built-in thumbwheel. Once set up, the unit computes field-strength level, or percentage of permissible exposure level, from either a single source or a single channel, or from a list of sources or channels. It can then reveal the RF contribution from a given telecom service, or the contribution from all services and their percentage of total field strength.

EEPROM correction

The SRM-3000 also recognizes antenna factors when converting probe output voltage into field strength values. Through its control cable, the unit automatically recognizes the appropriate correction factors based on data stored in an EEPROM in the probe. If the probe is connected to the unit using a Narda extension cable, the SRM also recognize any frequency-dependent loss factors for the cable. Very neat.

When using probes and cables from other manufacturers, you can input your own correction factors, too, sending them across a 115kbps serial connection from your PC. That's done with supplied software. Once that's set up, all you need do is select your probe and cable type from a configuration menu. This degree of user-friendliness can be a boon, especially in the hands of field personnel.

Exposure limits
Exposure limits are also stored in the SRM-3000, so you can select these limits from a menu, and also transfer sets of limits to the unit across the serial interface. User-editable tables are supported in the software package, so that a service table listing operators (by name), along with upper and lower limits of assigned frequencies, can be referenced.

These kinds of user-friendly features and tables let the SRM-3000 help you correlate of results with various telecomm services, such as GSM, UMTS, or TETRA. In its Safety Evaluation mode, the unit essentially gives you automatic computation of contribution of individual services to overall field exposure.

The P-CPICH option to be

Similarly, Narda's optional UMTS P-CPICH Demodulation mode will make a worst-case extrapolation of UMTS Node-B base station emissions. Unfortunately, Narda hasn't yet put a price tag on this option, so it's not quite available.

Nonetheless, when it does become available, and is priced for sale, the unit's UMTS P-CPICH demodulation mode will decode the primary common pilot channels (P-CPICH) of each UMTS cell detected in a 5MHz UMTS frequency block.

It will then provide the results in a table arranged according to the scrambling codes used to identify the various cells. The SRM-3000 will display the results as percentages of a limit value, or as absolute values in V/m, A/m, or W/m? and mW/cm?.

In addition to the sum of all the demodulated field strength results, the unit also determines the actual overall analog channel power level of the UMTS frequency block being evaluated.

For each cell, it simultaneously displays the current values and the maximum values that occurred since a last reset. The SRM-3000 then applies a user-defined extrapolation factor to the individual and total result values in order to calculate the worst case values that would occur if all the traffic channels were operating at full load.

- Alex Mendelsohn

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