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AD8302 and AD9834 for phase and magnitude measurement

Question asked by baban79 on Apr 3, 2014
Latest reply on Aug 27, 2014 by BobC

Hi! I'm new this experience but, felt interesting to see the queries and associated answers given.

My Question:

 

I'm trying to use AD8302 EVAl board and AD9834 EVal boards. Intend to measure relative magnitude and phase variations between ref.signal (INPB) and measurand (ref.signal travelled back from load) using AD8302.  I'm also referring to following concept in my application, an article written by EVA Murphy. Below is the link that i've referred to, and intend to implement in actual using said IC's .

 

Article web link:      http://www.eetimes.com/document.asp?doc_id=1156511

 

From this article, the excerpts are copied below for further discussion on this topic.

 

Copied Excerpts:


A DDS chip can control the frequency and phase of the stimulus with very tight resolution and can replace the whole discrete circuit. No external components are required for frequency control. The DDS also has the flexibility to control the output phase with 10-bit resolution (0.35 degrees ).

The system works by forcing a known amplitude, frequency and phase at V1, as shown in the figure. The signal at V2 will exhibit amplitude and phase shifts that depend on the network characteristics. Time and frequency domain data, and therefore the network characteristics, can be calculated based on those shifts.

In operation, the AD9834 DDS chip, driven with a 50-MHz crystal oscillator, provides the stimulus to the system. Its frequency resolution is 28 bits, or about 0.2 Hz, and its amplitude is controlled by varying an external resistor to ground. The RC low-pass filter reduces clock feed-through, images and higher frequencies. A gain stage drives the network, which is represented by an LRC circuit. The reference signal for the network (V1) is connected to channel 1 of a simultaneously sampling A/D converter; the response signal (V2) is connected to channel 2. The DSP is used as both system controller and data-processing engine.

 

 

My queries are as follows:

Query 1:

1. Am I following right scheme for the said purpose? Ofcourse! before A/D, I will insert AD8302 in my approach, and o/p of the AD8302 will go to A/D, that's the addition I want to bring to your attention.

2.What LPF1 (low pass filter 1) should I be using??, what specs I should order to manufacturer for that if I intend to sweep frequencies in load from 100KHz- 50 MHz??. Can you suggest some manufacturer who can readily do this job for me without asking too many electrical calculations. Do you recommend particular make to source LPF1 component.

2. Similarly, as shown, I need variable gain ampr (VGA1) specs, in general, to tune gain as desired by load (At present I don't know swing values of load except that the load will bring phase and magnitude shift in 'impedance' due to change in permittivity values of load conditions). This VGA1 is added before V1. Once the power requirement of the load is tested/known, we can fixed gain of VGA1 constant. Same is the case for VGA2 that is required to manipulate signal after V2.

3.Post VGA2, I plan to add LPF2 as an antialiasing component. And the signal after LPF2 will go to ADC (A/D).

4. I intend to add power splitter exactly after VGA1 i.e. between VGA1 and load. This will split original input signal generated by DDS in to two outputs without changing phase and magnitude, one will go as a ref (INPB of AD8302) and the other will enter through load (post V1 point). Past travelling 'load', the original signal will shift phase and magnitude. This measurand, signal to be measured past load (V2-point) will be fed to  VGA2 (to adjust signal levels to match LPF2), followed by LPF2,  and will reach INPA (2nd end of AD8302). In this way, the relative variations of phase and magnitude in the unknown signal (travelled through load) due to permittivity changes in load conditions will be measured using AD8302 IC.

 

Is above scheme seems workable?? (I have also attached the block diagrammatic representation of what is explained above, which resembles block diagram of the article stated above except minor additional blocks such as AD8302, LPF and VGA.

 

Can you support and provide knowledge selecting intermittent blocks (Figure1, attached) that are there other than AD9834 and AD8302.

 

Query 2:

I was wondering if I could characterize phase variation performance as stated in the data sheet (figure 16) of AD8302 using two function generators, CRO and DMM.

Expected VPHS o/p swing:

VPHS min = 30mVDC when phase diff betn.INPA and INPB is 180 deg

VPHS max = 1.8VDC when phase diff betn.INPA and INPB is 0 deg.

I was keeping INPB = -30dBm, freq=1.10MHz (Normal freq mode) and varied INPA: freq=1.0MHz, amplitude:-30dBm, phase=0-180 & 0 to - 180 deg using 'Burst' function of 33250A (Agilents  arbitray waveform generator). Because I didn't find any direct phase variation function in 33250A. It has frequency, amplitude but not phase variation function. Hope I'm right in saying that!. However, I couldn't notice VPHS variation using DMM connected on it.

 

Attachment: Block diagram, Figure 1 as above

 

Looking forward to your efficient favorable reply at earliest.

Dr.Babankumar

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