27MHz receiver

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
9
I want measure the distance between a small mobile 27MHz transmitter and a fixed receiver, the maximum distance is 10meters. I'm only interested in have far the transmitter moves relative to the receiver. I should be able to use the phase difference between the received signal and a local oscillator to calculate the change in distance.

Since I only need a maximum of 10m, a 27MHz rf signal would be ideal.

Building a simple 27Mhz transmitter is simple but I'm struggling with the receiver.

Initially I thought a super regenerative receiver would work. I have tried to build a couple but failed, ( I did make a nice 27Mhz oscillator though). On reading more about this type of receiver, I'm not convinced it would work anyway as I think the quenching will destroy the carrier that I want to keep.

So what I want to do is receive the RF signal and amplify it so that I keep the original signal and maintain the phase. Once I have the receiver working I will add the local oscillator and phase detection. At this point I'm not interested in any modulation.

I have search the internet but I think my terminology is wrong as I cannot find any info.

Can anyone point me to the type of receiver I need for this. With simple circuits based around the 2n2222 if possible

Thanks

Garry
 

Papabravo

Joined Feb 24, 2006
12,943
I'd be surprised if you can get this to work since the wavelength is on the order of 11 meters and you have no reference at the receiver to measure the phase against. The total time frame is 30 nanoseconds and the signal is traveling about 1 foot per nanosecond.
 

jpanhalt

Joined Jan 18, 2008
8,765
Long range LADAR (laser time of flight) distance rangers have been around for quite awhile. I got mine several years (Stanley Tools) for about $50. ST makes shorter range devices (up to 4 m) as single chips that are quite inexpensive, e.g., VL53LnX, n=0 or 1 (DigiKey and other distributors carry them.)

Here's one link to another type: https://www.terabee.com/shop/lidar-tof-range-finders/teraranger-evo-60m/?gclid=EAIaIQobChMIopmg85_P5wIVDdbACh0j8AveEAAYAyAAEgLEsPD_BwE

With a laser you could point it at your receiver and find its distance. Since the ranging laser is near IR, the devices I have seen also have a red spotter laser so you know what you are measuring.
 

Janis59

Joined Aug 21, 2017
1,057
Dstance of 10 meters makes a time shift of 10 m / 300 000 000 m/sec=1/30E-6 sec=0,03 microsecond. To measure sth like that You need at least a 30 MHz (but if distance is shorter then accordingly higher) and the quartz stability as well the phase stability must be as well 1E-8 to 1E9 or even higher - what I try to say is the quartz must have an active thermostat regime otherhow all the measurement values goes beyound of noise background. Am sure solution of both problems maks Ypour work to the State of the Art product. Advice - find a better way to measure the distance. Laser, inductance, capacitance, ultrasound - whatever may want but not a radio.
 

Beau Schwabe

Joined Nov 7, 2019
37
I don't know if this is applicable, but it might give you an idea ... We have a cottage that looks out over lake Michigan 140 feet above the water. From the shore line, you can't see the cottage. So one year, what I did was to create two transmitters that were at a slightly different frequency from one another and placed one at our cottage, and one at our neighbors cottage. The difference in frequency was just enough to create an audible beat frequency. I used a portable grundig radio receiver to "listen" for the beat frequency. In the exact Middle of the two antennas, the audible frequency was the loudest, where closer to either transmitter, the audible frequency was the softest. I was able to walk the shoreline and pinpoint exactly where our cottage was using this technique.
 

Janis59

Joined Aug 21, 2017
1,057
Yes, sure, so minor frequency beatnotes may happen be observable. So, the aim is to measure the distance. Well, where is the problem then? For me it looks the automated beatnotes will be received if to apply the certain LF filter after antenna and both transmitters are rather well in the same frequency. First idea is Vačkar circuit, another is Pierce circuit. For receiver - LF selector, diode, and amplification - LNA with cascode cascade having sth about 40-60 dB. That is only way to Miller Killer, without the maximum K(u) will wonder if over 12 dB. However, Ultrasound will make this work thousand-fold more accurate.
 
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