Author Topic: Microwave Black Magic Guru Needed  (Read 1549 times)

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Offline tecman

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Re: Microwave Black Magic Guru Needed
« Reply #25 on: March 28, 2018, 02:24:15 am »


How fast does the object need to be moving, in order for it to detect it?  Seems like over ranges, about 10-30 cm/sec

How far away does it detect the object? Up to 4 meters


paul
 

Offline Howardlong

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Re: Microwave Black Magic Guru Needed
« Reply #26 on: March 28, 2018, 03:16:00 am »
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I am wondering what the purpose is, if any, of it FMing at ~20MHz, or is that just a feature of the design, possibly noise from the power supply?

Anyway, I ordered some to keep me occupied.

I am running mine off a 9 v battery, so PSU noise is not a factor.  On the spectrum analyzer I saw a much narrower spread of frequency compared to the plot in the attached documentation.

paul

I was thinking more about locally induced noise on the exciter’s supply, rather than the source of power, perhaps from the chip at 20MHz, but it seems that the display on some of the documentation isn’t necessarily representative.
 

Online rfeecs

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Re: Microwave Black Magic Guru Needed
« Reply #27 on: March 28, 2018, 04:00:27 am »

A Doppler radar would measure the speed of the target by measuring the frequency shift of the reflected signal.  This doesn't appear to measure the frequency shift, but just measures the amplitude of the combined incident and reflected signals and compares that to some thresholds.

I just doubt that this approach could work especially in a circuit without any trimming and produced at the lowest possible cost due to its criticity. I'm sticking with doppler.

Best,
0xfede

I realize i didn't really word that right.  I didn't mean combine the two signals and then detect.  I meant mix the two signals, then detect.  So it is detecting the downconverted signal which has a frequency that is at the difference frequency.

I think we are all saying the same thing here.  If people are using the term doppler loosely to refer to mixing the two signals together to get the difference frequency signal.
 

Offline 0xfede

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Re: Microwave Black Magic Guru Needed
« Reply #28 on: March 28, 2018, 05:28:19 am »

A Doppler radar would measure the speed of the target by measuring the frequency shift of the reflected signal.  This doesn't appear to measure the frequency shift, but just measures the amplitude of the combined incident and reflected signals and compares that to some thresholds.

I just doubt that this approach could work especially in a circuit without any trimming and produced at the lowest possible cost due to its criticity. I'm sticking with doppler.

Best,
0xfede

I realize i didn't really word that right.  I didn't mean combine the two signals and then detect.  I meant mix the two signals, then detect.  So it is detecting the downconverted signal which has a frequency that is at the difference frequency.

I think we are all saying the same thing here.  If people are using the term doppler loosely to refer to mixing the two signals together to get the difference frequency signal.


Now I understand what you were saying and yes I believe we are at the same point.
By the way I finished the schematic entry in Genesys and I'm at 70% of the layout. It is pretty crude and imperfect since I'm extracting the data from PDF but it should be a starting point.
May I ask if someone measure the PCB thickness please?

Best,
0xfede
Semel in anno licet insanire.
 

Offline Hero999

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Re: Microwave Black Magic Guru Needed
« Reply #29 on: March 28, 2018, 10:01:30 am »
How stable is the oscillator?

How much phase noise does it have?

How fast does the object need to be moving, in order for it to detect it?

How far away does it detect the object?

A stable frequency is required to detect the speed of slowly moving objects.


The Gunn diode cavities in Doppler radar are also notoriously unstable, but as the mixer uses exactly the same unstable but coherent reference, it is of negligible consequence.

While I agree this isn't technically Doppler radar, many of the underlying phenomena that make Doppler radar work are the same with this device. You can indeed do very much the same thing with a Gunnplexer. This device is measuring phase differences, whereas Doppler traditionally measures frequency differences, but keep in mind that frequency is simply the derivative of phase wrt time.

In addition to detecting phase, this device may also be detecting amplitude, a not uncommon side effect both in detectors, and as a result of the constructive and destructive interference.

I am wondering what the purpose is, if any, of it FMing at ~20MHz, or is that just a feature of the design, possibly noise from the power supply?

Anyway, I ordered some to keep me occupied.
You're right. The oscillator doesn't need to be super-stable, since the source is mixed with the reflection, but I would have thought the phase noise would still need to be reasonable in order for it not to change by more than a fraction of a Hz, in the 26.7ns it takes for the signal to travel the 8m round trip from the antenna to object and back.

I don't think the fact there's FM at about 20MHz and the object detection distance being around 4m, is a coincidence.
 

Offline Howardlong

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Re: Microwave Black Magic Guru Needed
« Reply #30 on: March 29, 2018, 09:26:16 am »
I received some of these today, £6 for five on Amazon next day delivery.

The first one I tried didn't work, its exciter works but nothing came out of the opamps.

Looking at it with a nearfield probe shows a frequency of 3.15GHz but with strong sidebands.

As you can imagine, probing a few mm away detunes it a MHz or two.

Dusted off the old 8565A: 3GHz is as high as I go in new fangled equipment, this 1977 relic goes to 22GHz, 40GHz with external mixers.

Looking at the second opamp output on a scope while jiggling your hand around tells you all you need to know.

 
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Offline tecman

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Re: Microwave Black Magic Guru Needed
« Reply #31 on: March 29, 2018, 11:51:10 pm »
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Looking at the second opamp output on a scope while jiggling your hand around tells you all you need to know.

I did not hook up a scope yet, so what do you know (that we don't) ?

paul


 

Offline Howardlong

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Re: Microwave Black Magic Guru Needed
« Reply #32 on: March 30, 2018, 12:22:11 am »
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Looking at the second opamp output on a scope while jiggling your hand around tells you all you need to know.

I did not hook up a scope yet, so what do you know (that we don't) ?

paul

When I get a moment I’ll do a short vid.
 

Online rfeecs

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Re: Microwave Black Magic Guru Needed
« Reply #33 on: March 31, 2018, 10:27:07 am »
I received some of these, too.  Board thickness is about 1.2mm.

The spectrum has already been reported, the carrier is around 3.14GHz, and pulsing at about 20MHz (ignore the 3GHz signal, that is a 6W transistor being tested on another bench.)



The emitter voltage waveform shows this, too:



The oscillator is doing this all by itself, so it is squegging(https://en.wikipedia.org/wiki/Squegging) at 20MHz.
It looks like this frequency shifts slightly when there is movement.

I tried simulating the oscillator.  It looks like it can oscillate pretty readily at around 3GHz.  With a lot of fiddling I could even getting it to squeg at about 20MHz as well:




« Last Edit: March 31, 2018, 10:28:53 am by rfeecs »
 
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Offline 0xfede

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Re: Microwave Black Magic Guru Needed
« Reply #34 on: March 31, 2018, 09:08:06 pm »
Thank you rfeecs for your simulation. Just to show the topology of the analyzed circuit I attached to the post the schematic of a typical super regenerative circuit. The description, for who is interested can be found here (it is fig.3):
http://www.eix.co.uk/Articles/Radio/Welcome.htm

As you can see we are dealing exactly with this circuit just with different values.
In the meantime I've ordered a couple of pieces of this device.

Best,
0xfede
Semel in anno licet insanire.
 


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