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 Use Utilite to Measure Microwave RF Frequency 
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Joined: Mon Jan 09, 2012 3:16 am
Posts: 351
Post Use Utilite to Measure Microwave RF Frequency
I recently had a need to measure the output frequency of an X-Band microwave source. The specified frequency for that source is 10.525 GHz, but I had indications that it was transmitting off-frequency. Since I was not willing to spend thousands of dollars for a microwave counter, I needed another method. Fortunately, Utilite in conjunction with some other simple equipment provided a good solution.

The System

The budget priced system I arrived at consists of a horn antenna, an X-Band Mechanical Cavity Frequency Meter, an X-Band Waveguide Diode Detector, and my Utilite running an oscilloscope application. With the exception of Utilite, the pieces and parts were obtained from Ebay and assembled as depicted. A diagram of the system is shown below.

Attachment:
Freq Measurement System Diagram.png
Freq Measurement System Diagram.png [ 100.05 KiB | Viewed 2898 times ]


Attachment:
Freq Meter System.JPG
Freq Meter System.JPG [ 107.15 KiB | Viewed 2898 times ]


Attachment:
Horn-Freq Meter-Detector.JPG
Horn-Freq Meter-Detector.JPG [ 108.18 KiB | Viewed 2898 times ]


Theory of Operation

The horn antenna component of the system captures the incoming X-Band microwave signal and provides sufficient gain so that the signal is more easily detected.

The Hewlet Packard frequency meter is a tunable cavity device which varies the size of an internal microwave cavity as it is mechanically tuned. It absorbs RF power when it is tuned to resonance. Don't scoff at the idea of using a mechanical device for making this measurement as the wavemeter used has a specified accuracy of 0.05% - which is very good - about +/- 5 MHz.

The output of the microwave source - horn antenna - frequency meter is received and converted to a voltage using an X-Band diode detector which consists of a 1N23 diode housed in a short section of WR90, X-Band waveguide. Since the output of the microwave source is pulsed instead of continuous, the ouput of the detector is also pulsed. The output of the diode detector is sent to the audio input of the Utilite which displays the detected signal using an oscilloscope application running in Linux.

The frequency of the X-Band source is determined by tuning the frequency meter until a quick reduction of the detector signal is noted on the Utilite based oscilloscope. That reduction, which is often referred to as a dip, is on the order of 1 dB when the frequency meter is tuned to resonance. It is easily seen and can be pinpointed by tuning the meter up and down until the minimum signal level is seen on Utilite. Once this is done, the frequency is read from the mechanical dial of the Frequency Meter.


Last edited by hassellbear on Fri May 09, 2014 2:05 am, edited 2 times in total.



Fri May 09, 2014 1:20 am
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Joined: Mon Jan 09, 2012 3:16 am
Posts: 351
Post Re: Use Utilite to Measure Microwave RF Frequency
Results


The frequency meter is tuned until the amplitude of the pulses shown by Utilite are minimized.
Attachment:
X-Band 2.png
X-Band 2.png [ 31.62 KiB | Viewed 2898 times ]


Attachment:
X-Band Display.png
X-Band Display.png [ 30.83 KiB | Viewed 2898 times ]


The frequency is then read on the meter.
Attachment:
Measured Freq.JPG
Measured Freq.JPG [ 87.64 KiB | Viewed 2898 times ]


Conclusions

1. As suspected, the otuput frequency of the microwave source was not 10.525 GHz, but was instead 10.553 GHz - which is off by 28 MHZ or 0.27%.

2. Utilite again shows its usefullness in specialized applications.


Fri May 09, 2014 1:42 am
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