Science and Technology

WaveRider wrote ...

It's been a while since I have done a radio project. So, I thought I would experiment with data transmission using BPSK (Binary Phase Shift Keying) on the 433 MHz European ISM band.

What's so special about BPSK?
Most commercial systems seem to use frequency shift keying (FSK) or some form of on-off keying (OOK). I wanted to explore the possibilities of constructing a simple system for binary phase shift keying (BPSK). Binary phase shift keying is a form of modulation that is very "power efficient." This means that to achieve a certain bit error rate, the required signal to noise ratio is lower than that required by, e.g., frequency shift keying or on-off keying. BPSK (or QPSK) is often the modulation mode of choice for deep space probes where power consumption must be minimised and signal path losses are enormous. See graphic (taken from ) for comparison of error rates....




Why 433.92 MHz?
Besides sitting in the middle of the 433MHz European ISM (industrial, scientific and medical) band, it is also in the amateur 70cm band, so parts were easy to get and it can be modified to operate under amateur radio rules. For those in the US, I think this band also falls under Part 15 of FCC regulations for low-power communications devices. Also, the local oscillators use of-the-shelf SAW resonators instead of a PLL synthesizer. This simplifies the design greatly (at the expense of having to operate on a fixed frequency).

What will the transmitter power be?
Operating below 10mW in this band is permitted without a license. I plan to limit the emissions to somewhat less than this. Many garage door openers and automotive key fobs seem to operate in this band (altho' at much below this power level). Hopefully this won't mean intolerable interference in my experiments!

First step: The receiver design
Since the receiver design is more challenging than the transmitter, I started off there. Here is a block diagram of my proposed receiver.





I am basing the system around the Philips SA639 RF/IF subsystem chip, which is really designed for FSK systems, using a 10.7MHz IF. I managed to find a 500kHz bandwidth ceramic IF filter which should allow upwards of 384kbit/sec communications. BPSK demodulation is not as easy as FSK. Basically, we need to add a carrier regeneration system (a PLL) for demodulating the BPSK signal coherently. I do this with a simple "squaring loop" which doubles the IF carrier frequency. A NE564 PLL/VCO system is the heart of this.

The system
The received signal enters the system (as you would expect ) at the antenna input. A low noise amplifier, based on the BFR540 NPN transistor amplifies the antenna signal. I was a bit torn about whether to put the microstrip-based band/image-reject filter before the low noise amplifier. If I put the filter before the amplifier, the filter insertion loss will kill the noise figure of the system. However, putting the filter after the amplifier increases the chances of amplifier overload by strong out-of-band signals. I opted for putting the filter after the amplifier. I will rely on the antenna system to reduce out-of-band signals. Plus, the BFR540 amplifier as I have designed it should be able to withstand -20dBm on the input terminals without saturating.

The next stage is the downconverting mixer. I found a SAW resonator for 423.22MHz which is exactly 10.7MHz away from 433.92MHz. Hence, the local oscillator (based on a BFR92 transistor) fed to the mixer (part of the SA639 chip) downconverts perfectly to 10.7MHz. The ceramic IF filter passes a sharply defined 500kHz channel to the
first IF amp and limiter.

At this point, we split off the signal to the second mixer as well as a push-push active frequency doubler. The output of the doubler is passed through a 21.4MHz (second harmonic) bandpass filter (with a band-stop notch at 10.7MHz) to the PLL carrier regeneration circuit. The 21.4MHz signal is divided by 2 using a flip-flop. After a phase correction and level adjusting circuit, the regenerated carrier is fed to the final mixer, where it is mixed with the original amplitude limited IF signal. The output will be the original BPSK digital data signal (a 250kHz low-pass filter on the output remives any residual RF). A schmitt trigger squares up the output and produces a TTL compatible signal.

I am hoping for a BER at 384kbit/sec of under 10e-6 with -106dBm input at the antenna terminals (in the absence of other interfering transmitters).

Dr. Spark wrote ...

Good day great coilers,

I am now madly in love with solid state Bi_Polar coils. How little power they take and how fast they can respond is incredible ! Demo Lady Red for DC Cox this weekend and he was grinning big time.

I have 1.2 months left before the W.W.T. so thought I could squeeze one more project in for demo at the Thon. A small SSTC Bi-Polar with a surprise feature. So pics tonight are the starting project, coil will be all plexy with UV LED’s. Will update with pics but this will be the fastest built coil I have done in many, many years….. Have a goal to meet, so off the computer and back in the garage…

Cheers,
Ch_r

Uzzors wrote ...

It's about time I tried using a Coolidge X-ray tube I bought on Ebay last summer. I was given no information on it other than it's a dentist tube, so everything will need to be determined empirically. The x-ray tube is 30cm long and 6cm in diameter. The filament has a resistance of about 0.2 - 0.3 Ohms, and is wound "Benson" style, or as a helical coil. So where do I start? I'll need to know an approximate power rating of the tube, and maximum voltage. Since the x-ray intensity is proportional to the filament temperature, I thought I could measure the anode current at very low filament temps, and assuming the anode current rises linearly with filament current plot a graph. A suitable filament current range can then be determined.



I don't even have ballpark figures for this tube, so your guess is as good as mine! Tube power and tube voltage.

Cesiumsponge wrote ...

Gather around and witness the fruits of Tesla's lifelong work! Expeditiously, I shall be unveiling an unrealized dream of his, a masterful thermionic valve apparatus of superlative wonder and whimsy after many a sleepless night of scheming and laborious toil.

A mechanism of excessively high potential and very considerable frequency.
A contraption to capture the wild possibilities of a dawning age of industrialism.
A device to astound and mystify the youth,
A machine to bewilder and confound the old!
Fascination and fear,
Stupor and surprise!

More lively than an electrotherapy session and twice as delicious as a cool radium tonic, guaranteed!

Before the grand unveiling, verification, and tuning takes place scant weeks from now with all notes, I will tantalize this kind audience with some previews.