Yes, I am going to work on the PCB layout design right after all the circuits have been confirmed since there are only two months left before my hardware presentation. I will use the fiber glass PCB for PCB etching, is it alright to use this type of PCB in etching?

By the way, I am planning of doing ranging measurement for ~100 meter (About 0.35 lamda. Note: the measurement can reach 0.45 lamda using this system theoretically). Is the transmitter’s power high enough for the purpose? How we can determine the antenna gain if there is no datasheet for the antennas? I cant determine the gain for each stage as I don’t know the antenna gain… Any suggestions on the way to determine the gain for each stage?

Regarding the bandpass filter between the amplifier and the phase detector, is it alright to implement a crystal filter there? I have no idea of any filter with better cutoff/ rolloff…

I am thinking of putting a 2nd Order Butterworth LP Filter before the ADC input of the MicroController, is it alright for me to do so?

Do you have any clues on the op-amp that I should use (I mean the spec of the op-amp)? Preferably the op-amp from TI (Since i can get the sample from there ), I have looked through the op-amp in Ti website, most of the op-amps provide very low unity gain with 3MHz bandwidth...

I have enclosed a new diagram of the system, as well as the schematic of the Colpitts Oscillator used in the transmitter... Any comments on the oscillator?

Attached Images

block.jpg (141.7 KB, 7 views)

Attached Files

Colpitts Oscillator.pdf (41.3 KB, 6 views)

 

Yes, standard FR4 fibreglass is fine. In fact, just about any kind of pcb is ok for this.

So many difficult questions! I'm not sure if the transmit power is high enough without doing some analysis. What is the legal maximum for such radiation in your country? Do you care about complying with rules, since this is only for demonstration? You diagram shows 75 mW which is a good level, perhaps enough to do the job. Again, some analyis is needed and I will show you how.

The rod antenna is very easy to simulate and I can do that quickly and tell you the gain. But the loop antenna is difficult and I cannot use my simulator for that one. We will have to find out the antenna gain by finding similar antennas that others have built and who have measured their gain figure and reported this in a report or website. This will take some time. Another way is to measure the gain of the antenna. This is possible if we are not too fussy about our accuracy. Can you provide any details about the tx antenna including a picture?

You could use a crystal filter and in fact the input impedance of the phase detector is suited to that. But you will have to find a crystal at the 1.05 MHz frequency. Since it is ok for the bandwidth to be quite narrow, you can get away with a crystal oscillator resonator instead of a filter. But we might have to cascade two of these and warp them a bit to get a few hz of bandwidth. If I were you, I would not use crystal to start with as it is an extra complication. I would build a cascaded tank circuit filter, same as recommended for the input filter.
That is alright.

Yes I have many clues. If I were designing this, I would not use an op amp. Instead, I would use an RF amplifier IC (not sure which one, this would require some research), or I would build a discrete tuned amplifier using a FET. At 1.05 MHz it is not hard to get a FET amplifier to work well for small signals.
Yes, it looks very strange. This circuit is not appropriate. You must use a crystal in the resonant circuit to stabilize the signal. Also, I would never use an opamp as a 1 MHz oscillator when it is so much simpler with a FET or bipolar transistor as an oscillator followed by a simple tuned buffer amplifier again made out of a FET or bipolar transistor. It seems that you are afraid of, or not knowedgeable of, design of amplifiers with discrete transistors. Such amplifiers are much more commonly used than opamps at frequencies like 1 MHz, especially in a narrowband application like this one where the tuned amplifier works well. But this is up to you. In any case, you must use a crystal.

Do you know how to make a simple crystal oscillator for use as a logic clock? This is usually done with a simple CMOS logic inverter. This would be the ideal oscillator and transmitter for you. Use a HEX Buffer IC (eg 74HCU04) and configure one buffer as a crystal oscillator, then feed the other buffers all in parallel and combine their outputs and then pass the signal through a 1 MHz chebyshev lowpass filter to remove harmonics. This is far simpler and is easy to do. I have seen links in another thread and will look for the example.

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RadioRon

 

Thank you for your advices. I am now working on redesigning the oscillator with crystal oscillator using 2N2222, but i cant find the oscillator for 1.07 MHz... Do I need to change my operating frequency for the system?

 

I presume that you mean that you cannot find the crystal for 1.07 MHz. If you can't find 1.07 MHz, then you have the choice of changing your frequency to a common crystal frequency, like 1.0 MHz (which might not be a good choice as there are many devices running at this frequency and they may interfere), or you can order a custom crystal for 1.07. Such custom crystals are not expensive, but they take quite a few weeks to manufacture and may not fit your schedule. Consider 1.0 MHz as there are many suppliers at this frequency and you will need several crystals, for each tx and perhaps four for the rx.

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RadioRon

 

Do you mean Pierce Oscillator for the oscillator built using HEX Buffer IC and crystal? I have built the circuit of it, but the output suffers from severe distortion... I have built the oscillator using crystal as well... But it is not working...

Again, for the filters, is the enclosed filter is the one in your mind?

Attached Files

	Crystal Oscilator.pdf (39.6 KB, 4 views)
	Pierce Oscilltor.pdf (37.8 KB, 4 views)
	Preselect Filter.pdf (36.4 KB, 3 views)

 

Yes, the second link, Pierce Oscillator using 74HC04 buffer IC is very close to what I was suggesting. It is necessary to put additional buffer amplifiers between the oscillator and the low pass filter to provide enough drive for the low impedance looking into the filter. Connect the remaining five gates in parallel to be this driver. I have built similar to this before and the distortion is low. In any case, distortion is not a problem because your output filter should eliminate all harmonics, which will clean up the waveform to be a sine wave. Your output filter must have at least 20 dB attenuation at 2xfc.

If there is not enough power from this kind of circuit, it is easy to add one additional discrete amplifier using an NPN transistor.

As for the output filter, your design is ok, but I was thinking of a simpler configuration where L2 and L4 are omitted and C2 and C4 are small value, high impedance. In my simplified configuration, it is necessary to tap into the first resonator made up of L1 and C1 by either tapping into the coil, or using two capacitors to couple in. In this way, the low source impedance does not load the Q of this tank circuit. C2 would be a small value, sufficient to couple energy to L3, C3 which would be resonant. C4 same as C2. The output would also have to tap into the L5,C5 resonator by tapping the coil or by splitting C5 into two values and connecting between them. This type of configuration is simple and easy to tune. You then have three tank circuits that all have high impedance at their tops, and high Q for good selectivity. The loss through such a circuit is reasonably about 2 dB or less.

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RadioRon

 

Whats the difference between crystal and crystal oscillator? I can hardly purchase a 1MHz crystal at my place, but the 1MHz crystal oscillator is easy to get here...

 

A crystal is a small passive component with two leads. Inside the component is a small piece of quartz rock connected to the two wires. It is a piezoelectric device used as one of many components to make up an oscillator.

A crystal oscillator is an active circuit containing several components configured to oscillate when DC power is applied. A crystal is one of the internal components and is used to determine the frequency of the oscillator.

Your previous posting had a circuit containing a 2N2222 and a crystal amongst other things. That is an oscillator. The single component shown as a kind of capacitor with a rectangle between the plates, that is a crystal.

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RadioRon

 

I cant simulate the circuit by connecting the gates in parallel... Why this happens? Any suggestion that I can do to make the simulation runs? I am using MultiSim to perform the simulation.

 

I don't know. Hopefully someone else can answer this question.

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RadioRon

 

The way to estimate your range is as follows:

calculate path loss (PL) = RxSens (dBm)-Ptx(dBm)-Gtx-Grx

Since we are in near field, we cannot use typical farfield path loss calculations and so must find the correct forumula to translate PL into distance.

In the meantime, let's estimate that your RXSense = -70 dBm, Ptx=17 dBm, Gtx=-16 dBi, and Grx=-16 dBi.

This means we can afford a path loss of -55 dB.

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RadioRon

 

What simulation software that you used to simulate the Pierce Oscillator?

 

Again, can we just buy a crystal oscillator to provide the sine wave and do amplification to the output of the crystal oscillator?

http://sg.farnell.com/jsp/search/bro...equestid=36149

 

Yes, that will work fine.

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RadioRon

 

I have not simulated this oscillator.

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RadioRon