Doppler radar experiment at 44 kHz, small dual gate MOSFET mixer mixes transmitted and received signals, the difference frequency is in the audible range, and depends on the speed of the object the beam is reflected from, listen to the howling when I move the object back and forward. There is only a simple one pole RC lowpass, so on the scope, apart from the difference signal, you also see quite a bit 44 kHz breakthrough.
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I wound the coil for the Rx side 8.9 mH, 90 turns on a 1200 uH Al E core, right in resomance... 5 minutes...
I never heard of a dual gate FET mixer. I made this 'silly' audio mixer with an audio center tap transformer and a ring of diodes. Is the dual gate FET better or worse than the diode double balanced mixer?
On a sunny day (Wed, 27 Nov 2013 07:12:52 -0800 (PST)) it happened George Herold wrote in :
Probably worse, but it is so easy to make.... You find that in every TV tuner here, works up to GHz, high input impedance, easy adjust. Here is an old one:
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Dual gates make great variable gain small signal RF amps too.
It's worse, much worse, than a diode ring mixer. It has basically no RF-IF or LO-IF isolation, poor dynamic range and lots of spurious mixing products. LO-RF leakage should be OK though. However, it's simple and sometimes good enough.
How easy was it to set (and keep!) a 1 Hz difference between your oscillators? I'd expect them to be either widely apart or locked together in a circuit like that!
On a sunny day (Wed, 27 Nov 2013 16:46:52 +0100) it happened Jeroen Belleman wrote in :
There is only ONE oscillator, it transmits 44 kHz, the other transducer is a resonance ciruit only receiveing. Both signals, the main and the reflected, are mixed. So zero = DC = no moton. And as soon as the reflecting object moves towards or away from the transmitter / receiver pair, then the difference frequency tells you about its speed. Not about the direction. I guess one could chirp it... The next part of this experiment is to make a wind tunnel, and see what the detected audio spectrum tells us abut the wind (air) speed).
Many years ago I did that experiment, and turbulent air causes high frequencies too. I just want to know if that is usable for an air speed meter.
A dual-gate MOSFET is more or less a single-balanced mixer, because ideally the LO doesn't come out the RF port. (This is because the G1-G2 capacitance is small, not because it's actually a balanced circuit, because it isn't.
You make a regular single-ended common-source amp, and connect the LO to one gate and the RF to the other. Basically the RF only makes it to the output when the LO is high.
Dual-gate MOSFETs actually make amazingly good sampling gates--drive the source, connect G2 to S (bias may be needed) and put the sampling gate on G1.
Cheers
Phil Hobbs-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203 Briarcliff Manor NY 10510
Ahhh I don't think that works. (But I'm not sure.) If Tx and Rx are stationary (or moving at same speed) Then the sound is (say) upshifted on the Tx side and down shifted on the Rx side.. but no net change in frequency.
This certainly is more entertaining the the R Pi stuff.
To "see" the wind, I presume you need a change in air density to cause an impedance shift, hence the reflected engergy. That is, the wind would have to be at 90 degree to the beam from the sensor. Correct?
So I think you could detect the presence of air turbulence, but I don't see how this detects speed.
this is close to a product I would like to make for bike riders, an acousti cal radar device that sounds an alert when cars approach from behind. If the radar is aimed back, then as the rider is moving, most object are reced ing. The radar would alert only when it detects an object approaching, so you would need to to make the detector a bit more complex to differentiate between a return above the carrier or below the carrier. an object approac hing of course will create a return above the carrier.
This is easy to do. I built such a thing in the 1970s or 1980s to track the motion of rats (for an instrument used in pharmacology research). The prototype used 41 KHz burglar alarm transducers, which rats can hear, so had the project ever gotten to a marketable product, the 41 KHz carrier would have had to be in the megahertz.
The transmitter was self tuned to oscillate at the resonant frequency of the 41 KHz TX transducer. This was the carrier used as the LO signal to drive the mixers.
The RX transducer was the same make and model, and required no tuning. The RX signal was amplified and fed to the mixers, which were ordinary bipolar transistors used as switches that periodically shorted the signal to ground. I used a NPN transistor upside down (collector to ground, emitter to signal line, base driven from zero to a positive voltage via a resistor), to reduce offset when the transistor was on. A FET would have worked as well, but they were too rare and expensive then.
But anyway, the trick was to use two mixers in parallel driven with the carrier at 0 and 90 degrees, being In-phase and Quadrature respectively, and detect the sense of rotation of the resulting I and Q baseband signals. In this case, the antennas were not moving, so returns from the rat cage et al were down converted to DC, and blocked by a coupling capacitor.
If the antennas were moving, as on a bicycle, there would be a very large return from ground clutter that will swamp the desired return. The simplest solution is to digitize the I+Q signals and take the fourier transform of blocks of I&Q samples, and use the difference in doppler shift to tell clutter from overtaking car.
The clutter will be a negative frequency, while the car will be a positive frequency.
Leakage between TX and RX will still land at DC, and be eliminated by a blocking capacitor, and by ignoring the DC bin of the FFT.
The main difficulty with this kind of approach is ensuring sufficient dynamic range so clutter residue doesn't make the sensitivity to overtaking cars too low. This is a classic issue in radar design, discussed at length in such textbooks as Peebles: .
By the way, 44 KHz doesn't travel that far in the open air, and 26 KHz may be a better choice.
If you generate the 44KHz with a DSP scheme, i.e. something you can lock to a crystal, then you can make a DSP demodulator in two ways, presuming it is locked to the same crystal.
If you sample at the 44KHz signal frequency, that beats the received signal down to baseband. A common trick in the DSP based modems for POTS use. You can also just sample at a higher rate and decimate.
If you sample at a higher clock rate, you can make the mixer by periodically flipping the polarity of the digital stream. If there is a factor of 4 involved, you can do a combinations of flipping the polarity and decimation to make a IQ demo. Typical in many comm systems. Also a basis of lots of patents. Kind of like the coordic, when you do this you don't talk about it.
On a sunny day (Wed, 27 Nov 2013 13:56:06 -0800) it happened miso wrote in :
So far I used 2 transistors, one NPN and one dual gate MOSFET, and the signal level is huge, To get rid of the 44 kHz in the output, and avoid a many pole low pass, I will try a suck LC series to ground at 44 kHz at the output. Output level is high enough for the line input of a sound card, the spectrum can then be analyzed on the PC.
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