On Fri, 19 Jan 2007, John Miles wrote:
There are a couple of differences. Thermally-driven sensors can be somewhat
harder to burn out accidentally, because they use a resistive element
instead of a tiny diode chip. You won't tend to fry them with ESD on the
workbench, for example. For the same reason, they can theoretically look
like a better 50-ohm load than a diode sensor, although I don't believe the
difference matters much in the real world.
I would take issue with you on this, there is a greater 'safe
area' on the 8484A diode sensor (measures -70 to -20, max safe
input +20dBm) than there is on an 8481A thermocouple (measures -30
to +20, max safe inut +25dBm). Of course, you have to put a 40dB
pad in front of the 8484A, but hey, that's not a bad idea!
The biggest drawback to thermocouple-based sensors that I've personally
noticed is that they don't work well at low power levels, where a zero-bias
Schottky diode excels. With a resistor-thermocouple sensor, it can take
several seconds for a power reading near the bottom end of the range to
settle down. They really don't do well below -20 dBm.
This is true, diode sensors are good to below -60dBm.
A thermal sensor will also give true-RMS power readings for all waveform
shapes, for obvious reasons, while readings taken with a simple diode
detector will be most accurate for clean sinewaves.
Thermistor sensors, the 8478A etc. have less dynamic range and are
less stable, but have the unique advantage of being transfer
standards, i.e. can be calibrated at DC (given the right power
meter).
I *believe* the 435/436 wattmeters can use both types of elements, but don't
quote me on that...
Yep, all those I have seen can, as can the 437 and 438. None of
them work with thermistor sensors though, for them you need a 431
or 432.
BTW, you may want to add GMC and Narda to your list.
Geoff
--
Geoff Blake G8GNZ located near Chelmsford, Essex, U.K.
Please reply to: geoff (at) palaemon (dot) co (dot) uk
Using Linux on Intel & Linux or NetBSD on Sun Sparc platforms
Please avoid sending me Word or PowerPoint attachments.
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