# Questions about a design for transmitting and receiving a pulse over 100 meters or less (Measure distance to within 1 mm)

#### gitel1987

Joined Dec 9, 2020
3
Note: I've dabbled in electronics for many years, but I'm a newb when it comes to radio circuitry, so please forgive me for anything dumb I may say/ask.

I'm trying to design a system where I can send a single radio pulse (no complex information, just a single pulse) periodically. This pulse needs to be detectable up to 100 meters away. At the receiver, I need to be able to determine the distance from the transmitter with a high degree of accuracy (within 1mm). To do this, I'm thinking I could measure the voltage of the pulse picked up and calculate the distance based on the falloff of the signal, given that the transmitter will always produce a pulse of the same strength.

For the transmitter, I need to be able to have the signal emit from as small of a point as possible, so large antennae won't work. The antenna can be up to a few mm in size. The transmitter itself needs to be as small as possible in size as well., with a footprint maybe around 10 cm x 8 cm x 2cm. The transmitter will need to be able to send the pulse at a rate of about 100,000 pulses per second (with equally-timed pauses between each pulse).

At the receiver, I'll be sending the signal strength as a value to a computer so that I can do distance calculations in software. I'm thinking I can use multiple op-amps to amplify the signal, convert each into a digital value, then send these values over to the PC to analyze them.

So here are the questions I have:
1. Is this even possible? (Specifically, measuring a radio signal's strength to such an accuracy to be able to determine the travel distance down to the mm, given a known source strength)
2. What is a simple circuit which I could use to generate this pulse?
3. What kind of antennae can I use that would meet these requirements? I've seen some tiny wifi antennae that are a few mms in length. Would that work for this application?
4. What kind of power requirements would I need in order to produce a detectable signal up to 100 meters away and determine the strength accurately?
5. If the signal is very weak at 100 meters away, I'm assuming noise could become a problem with determining a precise signal strength. I need these measurements to be as accurate as possible. How do I deal with this?
6. Is there a better way to do this?

If I've left any crucial information out, please let me know.

#### gitel1987

Joined Dec 9, 2020
3
To add one other detail, I need to determine the average distance during each duration of about 1/100th of a second. I figure if I send 1000 pulses every 1/100th of a second, and then I average the signal strength out from all of these 1000 measurements, maybe that will cancel out most of the noise and give me a good value for what the distance was during that timer period. That's the only reason for the 100,000 pulses per second. If I could have accurate measurements with a single pulse, I would be fine with only 100 pulses per second.

#### jpanhalt

Joined Jan 18, 2008
11,087
Welcome to AAC.

In brief, your plan will not work at 1 mm precision. Your requirement for a very short antenna pretty much indicates you plan to use a high frequency, perhaps microwave, carrier for the pulses. In any event, propagation from the transmitter and receiver response (relative intensity) are affected by many factors, even if you could control the output intensity that closely. Here's an introduction: https://www.eetimes.com/rf-basics-radio-propagation/#

#### Deleted member 115935

Joined Dec 31, 1969
0
To re enforce,
using amplitude is going to be troublesome. The transmit path is way to variable.
today its damp round here, fog, and the attenuation is many dB more than it was last night which was clear and frosty so little moisture.

Distance measurement,
tends to be done by time of flight.

This sort of thing is also done on robots,
which tend to use optical pulses ( IR leds ) , or occasionally ultrasonic.

https://www.sparkfun.com/distance_sensor_comparison_guide

#### gitel1987

Joined Dec 9, 2020
3
To re enforce,
using amplitude is going to be troublesome. The transmit path is way to variable.
today its damp round here, fog, and the attenuation is many dB more than it was last night which was clear and frosty so little moisture.

Distance measurement,
tends to be done by time of flight.

This sort of thing is also done on robots,
which tend to use optical pulses ( IR leds ) , or occasionally ultrasonic.

https://www.sparkfun.com/distance_sensor_comparison_guide
I thought this might be the case. I have been thinking about an alternative using ToF for a while, the only trouble with that is I've been unable to find a way to measure time a signal is received to the degree of accuracy needed using simple/inexpensive electronics (ICs). I would need to have timer accuracy down to an interval of about 30 picoseconds...I can't actually use IR or ultrasonic due to other requirements and environmental situations. Looks like I'll have to think about more clever ways to do this.

Thank you.

#### jpanhalt

Joined Jan 18, 2008
11,087
IR has the same problem as radio. That is, it travels at the same speed. Ultrasonic over that distance would be even more affected by environmental changes than radio.

As for alternatives, 1 mm in 100 m is 1 part in 100,000 or 10 ppm. Can you measure 1 m with an accuracy of 10 microns?

#### Deleted member 115935

Joined Dec 31, 1969
0
If you want to stay with Rf,

You can either transmit a pulse,
or a continuous wave,

A pulse, can be useful, but is relatively less accurate . Radars do things like pulse compression, chirp's etc to gain netter resolution, but that involves a fair amount of processing power. You give impression that you want lower cost.

Constant, CW, is an alternative,
you could send a constantly sweeping frequency range, and mix the received wave with what your transmitting,
the difference frequency generated is proportional to the distance. It can be done in pure analog, but not very stable.

You then have to deal with the range ambiguity ,
two object at a multiple of sweep rate , will be received as the same difference frequency..
if you transmit a fast sweeping CW , to get maximum resolution, then the distance between object that can not be resolved is short
conversely, if you transmit a slow sweep, then you remove the ambiguity, but have a great error in the range estimation.

Over the distance you have, and the resolution you have, you seem doomed.

The "answer" to this is to transmit multiple different signals.
Classically the CW sweep speed could be varied sweep to sweep, and you have a bit of software to distinguish.

Or you could transmit a pulse , then a sweep, pulse to remove range ambiguity, sweep to achieve better resolution.

All starting to get complicated is it not.

If you don't want to go the analog route, then its down to FFT's,
but the simple Arduino type board would have difficulty with that ,
you would need one of the 600 Mhz Arduinos for that

https://www.pjrc.com/teensy/

You can may be see why RF range sencing is not used in small products .

The "best" IMHO current RF range sensors like this are used in self drive cars,
They operate up at the 60 GHz region, but do not have that good a resolution.,

#### AlbertHall

Joined Jun 4, 2014
12,269
I used to work on cable fault locators which sent a simple but fast edged pulse down the cable. The pulse reflects back from the end of the cable or any discontinuity along the way (e.g. the cable being pinched somewhere). This was done with a patented method which used two crystals around 2MHz and one TTL logic gate. The rest was CMOS chips. I don't remember the accuracy specification but I very much doubt it was anywhere near 1mm over 100m. It was used so they could dig a hole in the right place to find a fault on power or telephone cables.

#### jpanhalt

Joined Jan 18, 2008
11,087
Laser interferometry* can be very precise, meaning it can measure very small changes in distance. Heck , just build another LIGO in your backyard. That should suffice., and you might just discover a collision of a couple of black holes in the process.

The original plan won't work. It can be done, but at a cost.

*https://www.ligo.caltech.edu/

#### Phil-S

Joined Dec 4, 2015
238
Yes, laser range finders are very accurate and not too expensive.
Coupled with a suitable reflector, multifaceted type like a bike reflector, 100-metres is easily achievable.
I would pitch my Leica Disto against any other method over those sort of distances.

#### jpanhalt

Joined Jan 18, 2008
11,087
The D2 looks like it meets or is very close to what the TS needs. Accuracy is claimed to be about ±1.5 mm up to 100 m. That's better than my much older Stanley does.

#### Phil-S

Joined Dec 4, 2015
238
The D2 looks like it meets or is very close to what the TS needs. Accuracy is claimed to be about ±1.5 mm up to 100 m. That's better than my much older Stanley does.
Yes, it's a terrific bit of kit and even made in Austria.
I'm amazed just how good it is.
It paid for itself many times over when fitting kitchens, bathrooms etc, building dimensions, floor to ceiling heights etc.
When trying to measure openings for doors and windows it's so quick and 100 times better than a tape, especially working single handed, trying to measure lengths of gutters etc. Not too good on measuring cloud base, but a good long range pointer as well.
The companion self-levelling cross hair level is equally good (forgot the type name, but also Leica)

#### jpanhalt

Joined Jan 18, 2008
11,087
Not to distract from this thread, I have the cross-line level and like it. Only problem was the latch on the battery compartment failed, and I had to jerry rig something for it. Will definitely look at the range finder.

#### Tron Jockey

Joined May 3, 2020
19
Note: I've dabbled in electronics for many years, but I'm a newb when it comes to radio circuitry, so please forgive me for anything dumb I may say/ask.

I'm trying to design a system where I can send a single radio pulse (no complex information, just a single pulse) periodically. This pulse needs to be detectable up to 100 meters away. At the receiver, I need to be able to determine the distance from the transmitter with a high degree of accuracy (within 1mm). To do this, I'm thinking I could measure the voltage of the pulse picked up and calculate the distance based on the falloff of the signal, given that the transmitter will always produce a pulse of the same strength.

For the transmitter, I need to be able to have the signal emit from as small of a point as possible, so large antennae won't work. The antenna can be up to a few mm in size. The transmitter itself needs to be as small as possible in size as well., with a footprint maybe around 10 cm x 8 cm x 2cm. The transmitter will need to be able to send the pulse at a rate of about 100,000 pulses per second (with equally-timed pauses between each pulse).

At the receiver, I'll be sending the signal strength as a value to a computer so that I can do distance calculations in software. I'm thinking I can use multiple op-amps to amplify the signal, convert each into a digital value, then send these values over to the PC to analyze them.

So here are the questions I have:
1. Is this even possible? (Specifically, measuring a radio signal's strength to such an accuracy to be able to determine the travel distance down to the mm, given a known source strength)
2. What is a simple circuit which I could use to generate this pulse?
3. What kind of antennae can I use that would meet these requirements? I've seen some tiny wifi antennae that are a few mms in length. Would that work for this application?
4. What kind of power requirements would I need in order to produce a detectable signal up to 100 meters away and determine the strength accurately?
5. If the signal is very weak at 100 meters away, I'm assuming noise could become a problem with determining a precise signal strength. I need these measurements to be as accurate as possible. How do I deal with this?
6. Is there a better way to do this?

If I've left any crucial information out, please let me know.
I think to measure within 1mm with high accuracy is going to be tough with RF or microwave. If you're determined to make something yourself and are open to non-traditional approaches you might try triangulation using two high quality lasers, some optics and very sensitive photo sensors. Or if you have the funding just invest in Light-Wave Surveying Instruments. These instruments rely on the propagation of modulated light waves. Their accuracy varies from 0.5mm to 5mm/km, and they have a daytime range of 3km and night range of 2.5km.

#### MIS42N

Joined Jan 25, 2013
22
Using pulses down a cable won't work. The signal doesn't propagate at light speed, quite a bit slower. Changes in temperature will be enough to throw the reading out by many mm over 100m. Same applies to signal loss, changes with the environment. If we knew why you wanted to do this maybe we could offer alternates.

#### Deleted member 115935

Joined Dec 31, 1969
0
We seem to be getting lots of replies to this, but I can not see any recent replies from the OP.
is this still active ?