The difference between
“Test Instruments” and “Indicators”
There are many digital
field meters on the market.
But only a few can be
called “Test Instruments”. Smart Fieldmeter (TM) is one of them.
Other field measuring
devices are only “Indicators”. The basics for that distinction are explained
below.
For most test
application the test instrument is intended to measure the value of electric
component E of the electromagnetic field at a given point. The signal frequency and orientation of
vector E is not typically known.
Therefore, good field
meter must have the following features:
Table
|
Parameter
of the Field
Meter |
Required
Value and Desired Accuracy |
Smart
Fieldmeter (TM) |
Notes |
|
Frequency Response of the Probe |
Flat in
the test range. +/- (0.5
– 3) dB |
+/- 1 dB
(0.2 MHz - 1 GHz), +/- 2.5
dB (1 - 3 GHz) |
Requires
probe detached from the meter metal body (to avoid parasitic coupling) and
small broadband non-resonant antenna. |
|
Isotropicity of the Probe |
Isotropic. +/- (0.5
2.5 dB) |
Better
than +/- 1.5 dB, Typically, +/- 0.6 dB @100 MHz |
Requires
probe with 3 (X, Y, Z) orthogonal antennas. |
|
Calibration Accuracy |
NIST
Traceable. +/- (0.5
– 1) dB |
NIST
Traceable. +/- 0.5
dB |
NIST
traceable calibration guarantees device correct measurements. |
|
Field Strength Linearity of the Meter |
Linear in
the field test range. +/- (0.5
- 3 dB) |
Better
than +/- 1.5 dB. Typically,
+/- 1 dB. Any range
10-150% of full scale, 0.2 - 600
V/m. |
Requires
linearization technique depending on the type of probe detector is used. |
|
RMS (Root Mean Square) Measurement Range |
RMS in
whole test range. +/- (0.5
- 3) dB error in the whole field strength range. |
RMS. <0.5
dB (for fields less than 60 V/m), <2.3
dB (at 600 V/m) |
In
multi-signal environments the errors may be higher. |
|
Response Time |
Typically,
0.5 - 2 sec. Some
applications require much faster response. |
Three
modes of operation: Average:
2 sec, Pulse:
0.1 sec, Peak: 50
ms/14 sec. |
Pulse
mode is good for close loop field control during the EMC testing. Peak mode helps to identify the AM
modulated signals. |
|
Zeroing |
Automatic
or user activated even in the presence of strong field. |
Automatic
or user activated even in the presence of strong field. |
Absolute
must in the field. |
From the Table above we
see that in order to qualify as test instrument, field meter must have at
least:
|
- Special detached probe with three small broadband antennas |
|
- Linear RMS meter with
wide dynamic range |
|
- NIST traceable
calibration |
Based on that, being
“digital” doesn’t make good test instrument,
unless major test
instrument features are present, so
most of the low cost
“Field Meters” on the market are simply “Field Indicators”
because they have one or
few of the following drawbacks:
|
-Don’t have the
broadband isotropic probe |
|
- Have antenna built
into meter body |
|
- Have directional or
long telescopic antenna |
|
- Don’t display the
RMS value of the field |
|
- Require manual
zeroing |
|
- Don’t have (or can’t
have!) the NIST traceable calibration |
Smart Fieldmeter (TM)
doesn’t have any of those limitations,
and is a true “Test
Instrument”.
For detailed comparison
of Smart Fieldmeter to some industry leaders,
see Application Note
AN-005.