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I am interested in measuring the changing dielectric constant of a liquid as it undergoes a process. My background research of scientific articles shows that the 2-5GHz range provides the best contrast between the before and after dielectric constants of this liquid. In those papers they use a vector network analyser which is not suitable for me due to cost and the large size. Hence I want to develop a system to measure the dielectric constant at a fixed microwave frequency somewhere in the 2-5GHz range.

From what I understand I need a signal generator and a matched receiver. The output of the analyser will be connected to a coaxial cable with an open ended probe suspended in the liquid. What are recommended or suggested designs for such a device? It needs to be stable and reasonably cheap to make. Are there ICs designed for this? I note Analog Devices has a range of direct digital synthesis chips that look suitable but the prices become insane at the gigahertz range.

EDIT: Just to clarify, the design does not need to sweep. It can be a fixed frequency, it just has to be within 2-5GHz.

ljbade
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  • There are several designs of VCO that cover the 2.45GHz band - if you were limited to just that range would this be a problem? – Andy aka May 02 '13 at 12:32
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    Leith, I believe you have summarized the issue perfectly: *the prices become insane at the gigahertz range*. Not just for the core DDS or other signal generator elements, but even more so for supporting components and skills in signal design / board design at those frequencies. If the solution were as simple as putting together a DIY generator and analyzer, the price commanded by such devices would be significantly lower. Perhaps you may want to revisit the requirements, and examine practical options within your budget? – Anindo Ghosh May 02 '13 at 12:40
  • 2.45GHz would be fine. Why would all designs in the GHz range be very expensive? There are lots of cheap phones and wifi routers operating in the GHz range. – ljbade May 02 '13 at 14:52

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This is a very big question to try to answer here, but I'll try to get you started.

Since you are only doing a one-port (reflection) measurement, you are quite correct that you don't need a full vector network analyzer. You could do this with either a vector voltmeter or an even simpler VSWR meter and a swept-frequency source (as your question suggests you intend).

The source should not require a DDS. A basic VCO should do, although of course the lower you can keep the harmonic distortion, the more accurate your measurements will be. VCOs covering more than an octave (as you need) are somewhat more difficult to find than narrowband ones covering frequency bands with important commercial uses. But you barely need more than an octave and you should be able to find one. Normally you would operate the VCO in a phase-locked loop, locked to a suitable reference frequency (say 10 MHz), to tune the the desired frequency in your 2 - 5 GHz band. This is the essence of an rf synthesizer.

On the receiver side, what you really need is a directional coupler to pick off the reflected signal coming back from your sample, and an rf detector to measure the reflection amplitude. A pair of matched couplers and detectors would let you measure the ratio between the transmitted signal and reflection for more accurate measurements.

Finally, the probe. I am not familiar with best practice for designing probes for liquid dielectric constant measurements, but I can give you some ideas about it. Details probably depend on physical factors like the viscosity of your sample, whether its a corrosive material, how easily residue from one sample could contaminate the next sample, the volume of the available samples, etc.

If you think the dielectric constant is basically constant over the 2-5 GHz range, then I'd suggest to build a resonant chamber and flood it with the sample, for example a roughly 1/2-wavelength segment of air-filled coaxial line, where you replace the air with your sample. If the dielectric constant changes, so will the resonant frequency of the chamber. Sweep the source frequency and find the reflection minimum or maximum to determine the sample dielectric constant. This type measurement could be done with just a VSWR meter as the sensor.

If the dielectric "constant" is changing in the 2-5 GHz range (due to material resonances, this is reasonably likely), then you will need a vector-voltmeter type detector. Again, though, I'd consider making a near-resonant chamber to hold the sample. With some math, the rf reflection from the chamber (amplitude and phase) will tell you the sample dielectric constant at each frequency.

The Photon
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  • I should probably clarify that I only need a fixed frequency somewhere in the 2-5GHz, it does not need to sweep. – ljbade May 02 '13 at 14:50
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    @Leith, with a fixed-frequency source you will need to know both the amplitude and phase of the reflection from the probe...it will simplify your source but complicate the receiver. Either way is doable. – The Photon May 02 '13 at 14:54
  • OK, I think I understand it now. If I use a PLL the reference frequency can be low (10MHz) which is a lot cheaper than the $60-80 2-5GHz VCOs I found. – ljbade May 02 '13 at 14:59
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    You still need a VCO (which you control by a PLL), but a narrow band VCO will probably be lower price than a very wide band (> 1 octave) one. DDS is expensive because it's effectively a VCO plus a very high frequency DAC plus additional logic. – The Photon May 02 '13 at 15:38
  • Actually I just came across some chips from Analog Devices (and a few others) that integrate the VCO inside the PLL reducing the cost significantly. – ljbade May 03 '13 at 06:01