project technical specifications, a list of measurables. not sure if i get this

Discussion in 'Homework Help' started by ninjaman, Oct 25, 2014.

May 18, 2013
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hello

im doing an HNC in electronics at college and I have to do a project as one of the units. the project I have chosen is an antenna analyser built using arduino, AD9850 DDS module, 16x2 LCD, wheat stone bridge and op amp circuit. there is a rotary encoder with push button that will be used to choose frequencies. I hope to be able to choose a starting frequency, press a button and the analyser will sweep through 1MHz and record the swr below 2:1. this recorded swr will be shown as the bandwidth of the antenna.
I have to write a report on what I intend to do and as part of the report I have to write a technical statement that shows a list of measurables. things that the circuit should do that I can prove has been accomplished with the final circuit. that is how I understand it. so I have written my technical spec as I understand it and would really really really appreciate the input from people who know what they are doing.

so this is my tech spec.....so far

Technical specification
The meter will aim to measure complex impedance and SWR and display the results on a 16x2 LCD display
DDS module will generate a frequency chosen by the user
The control device is a single rotary encoder with push button
The frequency will increment in steps chosen using the rotary encoder
The frequency will increase in 100Hz, 1kHz, 10kHz, 100kHz, 1Mhz
The DDS module will output a 2v pk-pk signal
The chosen frequency will have a start point and sweep through 1MHz
Any value below 2:1 SWR will be recorded and shown as the bandwidth of the unknown impedance
A wheat stone bridge will measure the unknown impedance
An Arduino mini pro will control the DDS and the display and perform math operations on measured voltages

any help would be great

thanks
simon

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2. Alec_t AAC Fanatic!

Sep 17, 2013
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I see a list of features, but not a list of measurables, which is what you are asked for. Perhaps have two separate lists?
Would you consider angular position to be something measurable (with the encoder)?

3. WBahn Moderator

Mar 31, 2012
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Think in terms of pass/fail terms for someone testing your system. It's not enough to say that it will output 2Vpp. What if they measure it and it is 2.01Vpp? Based on what you gave them, this is a fail because you said 2Vpp and they have no basis to judge what is "close enough" since it is YOUR specifications that they must follow. So your measureables need to give them the tolerances on everything. When you say the frequency will sweep, at what rate will it sweep? Is that rate adjustable? If so, what are the limits on the adjustment?

May 18, 2013
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hello Alec t

the only things im required to measure are the things that the analyser does, like produce a sine wave with the DDS from 1-30MHz. so that should be one of the things that I can measure after building the circuit.
the DDS will put out 2Vpeak that should be something I can measure as well.
this is what my tutor explained. I have looked at technical specs for commercial antennas and they look different to what I have to do.
what would be considered a measurable?

5. Alec_t AAC Fanatic!

Sep 17, 2013
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Any variable/parameter which is involved in the system and which you could reasonably be expected to have to verify, with equipment available to you, to ensure the system meets your specification.

6. crutschow Expert

Mar 14, 2008
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It isn't just measuring the parameters, it's specifying their value and limits. Any parameter needs to have a specified value, precision, and accuracy (accuracy and precision are related but are not the same). Thus every parameter you listed (frequency range, frequency steps, SWR, voltage, etc.) needs to have these limits stated so that they meet the requirements of the task, but are not too difficult to reasonably meet within the time and budget constraints. Often deriving these specifications can be one of the more difficult parts of the project.

7. WBahn Moderator

Mar 31, 2012
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And the specification needs to include tolerances.

If you just say that your machine is going to produce a sine wave at 1 MHz that is 2 Vpp, then you are saying that your machine is going to produce a PURE sine wave that is EXACTLY 2 Vpp at EXACTLY 1 MHz. Any deviation at all from that is grounds for failure or, more likely, grounds for endless debate (and potential lawsuits in the real world) over whether your machine does or does not meet spec.

But if you say that it will be 2 Vpp +/- 5% at a frequency of 1 MHz +/- 50 ppm with no more than 2% THD, then things are clear cut and there will be very little, if any, debate over whether you do or do not meet spec. The debate occurs beforehand when you present the specs to your client (the instructor) and they determine whether they are acceptable to the client for their needs. Perhaps the tolerances need to be changed or perhaps additional measurable specs need to be added. But once agreed to, you can then design the system to meet those specs and, if it does, then it is good enough.

Similarly, before the test, you agree whether a test result that is shown as being out of spec but by an amount that is within the tolerance of the measuring equipment will be considered a pass or a fail.

Note that the specs are seldom engraved in stone -- if it turns out that you can't meet the specs for some reason, then you can negotiate with the customer to change the specs or other parameters. Perhaps the customer will be willing to do so, or perhaps they will be willing to spend more money to achieve the specs, or perhaps they will abandon the project as infeasible.

May 18, 2013
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WBahn was saying "if it turns out that you can't meet the specs for some reason, then you can negotiate with the customer to change the specs or other parameters",
I am not exactly that switched on with this stuff. I wanted to build something that I thought may be useful to other beginners, people who are interested in amateur radio and building stuff. in particular, Arduino projects. if it didn't meet the spec but did what it said to some degree, all I want it to do is show swr and impedance preferably with the reactance value. if the reactance value is there is should show either more capacitive or inductive. this is what I believe is more helpful when testing an antenna.
I did think about getting information about what current and voltage the arduino and related modules used but i don't think that is an issue. the idea is that it works, to some precision. (i got a funny feeling your going to say "to what precision?") im not sure.
i think that maximum swr should be set, most people seem to say that 3:1 is maximum. take this as the maximum allowable swr (i think the impedance is 150 ohms) and use this as a cut off point. the sark 100 will show a 9:1 and record anything below 2:1.
i have no clue how to figure this out. im trying to think it through but nothing is coming to mind. any web sites that could explain this process would be great!

thanks
simon

May 18, 2013
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these are some specs i got from a manual. i used this layout for a technical spec buy my tutor didn't agree with it. are these the sort of things that i could measure. he did want some description about what i was going to measure so i think that may mean a bit of explanation instead of a laundry list.

10. WBahn Moderator

Mar 31, 2012
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This is for a grade, right? Wouldn't it be nice to know up front what will constitute adequate performance to achieve the desired grade? Because you're new and because it's a student project, there's generally a lot more flexibility, both in terms of setting loose specs to begin with and for being acceptable even it if doesn't meet them. But you can start by asking yourself, given what you've already described as being your vision for who might use this and how they might do so, what YOU would consider acceptable if you were looking for something like this that someone else built. If they said that it output a 2 Vpp waveform, would you consider it acceptable for the intended purpose if it turnout out to only be 1 Vpp or if it was actually 5 Vpp? Decide what you think might be barely acceptable (even if it's more of a guess than a decision) and then tighten it up some (maybe 25% closer to the nominal) and use that as your initial spec. Present that to your teacher and see if they agree that it is reasonable, as in not so loose that a monkey could meet it but not so tight that a student at your level probably can't.

You also need to consider things like whether it needs to indicate what frequency it is outputting or whether that is expected to be done using a separate piece of test equipment. If it needs to indicate the frequency, then how close does the indicated frequency need to match the actual frequency. Again, ask yourself what YOU would find barely acceptable and use that as your starting point.

May 18, 2013
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thanks WBahn!

that brought up a few things.
1. the accuracy of the wave form. the DDS puts out 2vpk at 7MHz ( did a little research on DDS) so I imagine that it rolls off after this. the op amp has a gain of nine and the microcontroller controls the DDS frequency and amplitude. so I imagine that the controller specifies an amplitude that will remain constant at the output. I think it is lower than 2vpk as the unknown impedance should not radiate as this could cause interference. so the output power has to be low. I think the op amp is there to compensate for the roll off and keep the output constant. so I have to find the output power and check that this remains constant. although I have read that as frequency increases the required power decreases? I don't know why this is. I think a wattmeter would be required for this.
2. the frequency will be displayed on the LCD, it will be a starting frequency that will sweep through 1MHz. this can be viewed on the display. the frequency will have to be as close as possible. im not to sure how to measure that. I know I can measure frequency on an oscilloscope then compare this to the readout. then do some percentage error maths. off the top of my head with out really thinking about it, output frequency over displayed frequency x 100 to give a percentage. I don't know what kind of percentage it should be. as close as possible to the antenna would work! I suppose with a 1:3/1 swr. though I don't know how to figure this out.

so things to research are power output and comparison method for displayed frequency.

Apr 5, 2008
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May 18, 2013
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ive seen a lot of that stuff already, thanks though Bertus!

what im now thinking is the output power/voltage at the bnc connector. I have looked at the output of other analyzers some say +13dbm some say -10dbm. 20mW is the +13dbm but I cant find -10dbm. I don't know what I would get out of the DDS. I know its 2vpk at 7MHz but I don't know what that is at the output. I know the op amp has a gain of nine but that is DC to be measured by the microcontroller.

I suppose if it was from a user point of view I would only want the output frequency to be the same as the displayed frequency and the results to be as accurate as possible. the bandwidth of the antenna should be measured and recorded up to 2:1 swr, I have read that at this swr the transceiver starts to cut back power. so the output frequency should be measured and compared to the diplay frequency along with the increments used (100Hz, 1kHz, 10kHz so on)
then the accuracy of the device should be checked. I could do this by making some mock antennas. components connected together to imitate an antenna. then the quality of the sine wave over the entire frequency. its to go from 1 - 30 MHz, after 20 MHz its supposed to get distorted and drop off. I think I could measure this on a scope and plot the results on a spread sheet.

thanks

simon

Apr 5, 2008
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15. WBahn Moderator

Mar 31, 2012
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This is the kind of phrasing that you need to get out of your vocabulary as an engineer, at least when talking about specifications and it is at the heart of the whole notion of "measurable". What does it mean for the output frequency to be the same as the displayed frequency? What does it mean for the results to be as accurate as possible? You will NEVER meet specs like this because they require infinite precision and accuracy. You need to start talking and thinking in terms like, "The output frequency needs to be within 1% of the displayed frequency and the results need to be accurate within 5%." Those are measureable specs.

16. crutschow Expert

Mar 14, 2008
12,585
3,087
The user always wants the device to be perfect. Since that's not possible, it's your job as an engineer to determine what can be supplied within the constraints of the project time and money available.
We are not talking about how to measure the accuracy, we are talking about what that accuracy should reasonably be.
That's where the engineering begins in the project. You don't just randomly design the device and then measure it's accuracy to see what it might be. You need to understand that to be a good engineer.

May 18, 2013
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well, it comes down to the output frequency accuracy. what do I compare this to or how do I compare this. I don't know what the output is. the output of the DDS is 2vpk. this is at 7MHz. but I think the output would drop from around the 20MHz- 30MHz. and I don't know how the frequency would affect the outcome.
this is what I am thinking about it. the antenna has resistance capacitance and inductance.
the frequency affects the capacitance and inductance.
so for a 30MHz resonant antenna the resistance of a half wave dipole should be around 73 ohms (+45 imaginary) this should give a 1.5:1 swr. if the frequency increases past resonance the impedance will increase. if the frequency decreases past resonance the impedance will increase. (if I understand what I just read).
i don't know how to find the frequency variation or whatever its called. does anyone know of any site that explains this stuff. i have looked around and cant find anything
the maths is over my head.
if the frequency moves either way too much the swr will increase because the impedance will increase. there is a limit to the swr (2:1) though this is the bandwidth limit. i would like a limit that means a low swr of about 1.5:1 with 1.3:1 being good and acceptable. (the dipole above would be cut to 70 ohms and no imaginary and this would provide a lower swr)
so the frequency would move either way and provide a maximum of 1.5:1 problem is i don't know how to work this out. i did try to find impedance of an antenna at that frequency and think that changing the frequency would provide new impedance values that i could somehow find a new swr for. this was the plan but i cant figure it out.

any help!!! PLEASE!!! AAAAAAAARRRRRRGGGGGGGHHHHHHHH!!!! (pulls out remaining hair from head and falls over unconscious)

cheers!

simon

18. WBahn Moderator

Mar 31, 2012
17,461
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One way to get a handle on it would be to take some actual test equipment and make some measurements and see what kind of numbers you need to achieve what you want from the unit you are designing.

Apr 5, 2008
15,540
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May 18, 2013
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im not sure exactly the best route. there is some test equipment at college. they have function generators and oscilloscopes.
if i use multisim and rlc circuits, with 30MHz i want 50 ohms. then adjust the values until the 50 ohms goes too high and produce the unwanted swr of 2:1?

i will give this ago, if this fails i shall buy some fireworks and shoot myself into space.
know that i love you all!!!

good bye

simon

quick return to the drawing board as i have no clue what im doing.
originally i was going to have an AC source with frequency of 30MHz going into resistor, inductor and capacitor in series. but then where do i go. i thought using a bode plotter would do something but i don't think that would work. so i think that some calculations using RLC circuits. might do something. i would have to find the reactance of the components.... thinking about it i would be using imaginary numbers instead. so RLC is still required?
im off to do some reading, any hints would be great!!
thanks
simon

Last edited: Oct 26, 2014