I remember reading several blogs and some article in the ARRL handbook (James Hontoria, W1JGH) about the Mini-Circuits power detector series ZX47. This power detector is useful within a frequency range from 10 MHz to 8 GHZ with a typical dynamic range of 60 dB. The actual dynamic range depends on the model of the detector:
Model Number |
Frequency Range
(MHz) |
Dynamic Range
@±1 dB Error (dBm) Typ. |
Output Voltage
Range (V) Typ. |
VSWR
|
|
click for
data |
Low
|
High
|
|||
10
|
8000
|
-40 to +20
|
0.50-2.10
|
1.20
|
|
10
|
8000
|
-45 to +15
|
0.50-2.10
|
1.20
|
|
10
|
8000
|
-50 to +10
|
0.50-2.10
|
1.40
|
|
10
|
8000
|
-60 to +5
|
0.50-2.10
|
1.7
|
|
(direct link to product selection page of ZX47, opens a new window)
The cost of one of these detectors is about 90 EUR, which is still quite affordable considering the alternatives.
Unfortunately, these detectors output an analogue voltage so that it is necessary to provide some kind of voltmeter or ADC circuit to do the actual data acquisition. In the past, I just hooked the detector output up to my bench multimeter and recorded the data via the GPIB interface. Needless to say, it would be nice if I could just connect the device to a USB port and talk to it via SCPI or similar so that I can use it in LabVIEW without the need to occupy my main bench meter.
The solution I found was to use an LPC1768 embed and have LabVIEW object libraries map to the mbed API via the RPC protocol. There is a special RPC firmware readily available on the embed webpage under the following links:
http://developer.mbed.org/cookbook/Interfacing-Using-RPC
http://developer.mbed.org/cookbook/Interfacing-with-LabVIEW
After a bit of trial and error to get LabVIEW to talk to the mbed, I managed to write a nice virtual instrument that provides some basic logging functionality:
Apart from displaying the instantaneous power and some basic measurement statistics, a second ADC channel is used to display the current temperature of the power detector (it has an integrated temperature sensor). An idea for future additions would be to have automatic temperature compensation and provide means to select the frequency of interest, gate time, power offset and resolution.
For anyone interested, I wrote a short paper that shows the LabVIEW code and does some basic assessment regarding accuracy. The document is available as PDF from my Google drive. The LabVIEW *.vi files are available on request.
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