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- Applications, Measurements
What is the fundamental difference between the EFS-105 and field probes common in the field of EMC?
- Compared to the EMC probes, the EFS-105 is much smaller. Furthermore, due to its working principle, it allows for a large variety of measurements.
Measurements of electric fields in time domain, measuring the amplitude and phase of the field: These are possible now with an unprecedented precision.
There are no limits in operating time, as the probe head is optically powered and does not contain any batteries.
Measuring RF electric fields over a wide range of frequencies is possible at a very short measurement time. The speed is only limited by the network analyzer used.
A typical sweep with some thousand frequency points usually takes no more than a few seconds.
Even at this high measurement speed, the sensitivity is orders of magnitude higher compared to the conventional EMC probes. This enables experiments at lower field strength.
The internal noise of the EFS-105 at a resolution bandwidth of 10 kHz limits the sensitivity of the probe to a field strength larger then 1mV/m.
This sensitivity is a factor of 1000 higher compared to conventional probes (sensitivity approx. 1V/m).
The advantage of this high efficieny: Many measurements can be performed at lower field strength, reducing the need for expensive amplifiers to generate high field strengths.
What is included with the EFS-105? Do I need additional equipment like optical fiber cables to perform measurements?
- The probe system EFS-105 includes the base unit, the probe head, the optical fiber cable connecting both and an AC adapter (see Power Supply of the EFS-105).
The (electrical) output signal is available at the N-connector at the front of the base unit.
Depending on the measurement task, an oscilloscope or a spectrum or network analyzer is required for aquisition and processing of the measured signal.
You don't need additional optical fiber cables or optical receivers for using the EFS-105 - it's all included.
How do I power up the EFS-105?
- The base unit is equipped with a wide voltage range input (9-30V DC), e.g. it can be supplied by a car battery (12V or 24V).
Due to the low power consumption of the probe system (2W), long operating times are possible using batteries.
For the use in the lab, a separate AC adapter is included (100-240V AC input).
Is it possible to use a longer fiber cable between the base unit and the probe head?
- The standard lengths of the fiber optical cable between the base unit and the probe head are 5m and 10m.
Other cable lengths are available on request.
Is there any influence of the fiber length on the performance of the probe system?
- No, the probe's performance is independent of the fiber cable length.
Time Domain Measurements: How to optimize the signal-to-noise ratio (SNR) of the probe system?
- Periodic signals
If you are measuring periodic signals, the easiest way to improve the signal-to-noise ratio (SNR) is to perform an averaging.
Please note that for optimum performance an independent triggering signal has to be used. If you use the measurement signal of the probe as the trigger source,
the noise added to the signal will cause some jitter. This will distort the result of the averaging.
On the other hand, if you can use the averaging of the measurement signal, even for very small signals with poorly SNRs to be measured very clean and noise-free results can be obtained.
The SNR improves with 10×log10(n) dB, where n is the number of measurements.
Further improvements can be obtained by using both the averaging and the band-pass filtering (see next paragraph).
Single event signals
If you want to measure single-event signals or there is no possibility to make use of averaging, the SNR of your results can be enhanced by band-pass filtering of the signal,
i.e. by suppressing the probe noise outside the frequency range of the signal. You'll need to have an idea of the signal bandwidth.
If there is no separate trigger signal available it is advisable to perform the filtering by the digital oscilloscope you use and prior to the trigger detection in order to minimize the jitter.
I use two EFS-105 in my measurement setup: one for a reference signal (fix position) and the other one for scanning the electric field.
Even if both probes are located close together their output signals show a phase difference. Why?
- There is always a slight difference in signal propagation delay from the probe head to the output connector between one EFS-105 and another.
However this difference does not vary over frequency or by bending the fiber - for two EFS-105 it is constant.
Usually it can be compensated by the oscilloscope or network analyzer (calibration / offset settings).
Is it possible to measure in water or other liquids?
- The standard version of the EFS-105 is not water proof. A modified version, which is suited for measurements in liquids,
is available on request.
Is it possible to use the probe as a transmit antenna?
- No, the probe only receives the field.
What is the basic principle of the probe? Does it use an electro-optic crystal like Lithium Niobate (LiNbO3)?
- The EFS-105 contains a laser diode inside the probe head for the signal transmission, the laser diode current is modulated by the RF electric field.
That's why it's called an active probe.
Passive probes using an electro-optical crystal like Lithium Niobate are much less sensitive, they exhibit a very strong noise. Furthermore the electro-optic probes usually are larger.
But the principle as seen by the user is similar - a (small) antenna with an optical fiber instead of the coaxial cable.
How does the energy supply of the probe head work?
- The probe head is optically powered. Therefore the main unit is equipped with a high-power laser diode, whose optical energy is transferred to the probe head using an optical fiber.
In the probe head, the optical energy is transferred back into electricity.
Is it safe to work with the probe system?
- The EFS-105 is qualified as a laser class 1 device - comparable to a CD-player. No additional safety requirements are necessary during operation.
In case of mechanical damage of the optical fiber, the laser diode will be switched off immediately, due to an advanced safety control circuit in the main unit.