Monday, 16 December 2013

Cooling

Why cooling is important


Electronic chips generate noise and heat during use: your digital camera or smart phone gets progressively hotter the more you use it. Sensitive applications, such as astrophotography MUST eliminate this heat, as heat is noise, black sky will be grainy black sky.
This heat is also present with the RTL stick, appears as frequency drift and internal noise - radio will not stay on a frequency.
Gain applied in Configuration panel increases heat, hunt weak signals and need to increase gain = increases internal noise generated by the stick.
Improve cooling - reduce noise.
Recently introduced temperature-controlled oscillator promises to solve this issue, around 50 dollars. Regular RTL stick sells for 8 USD. Hmm.

Do you need to worry about it?

No: Leave the stick warm up for 15 minutes - half an hour, let it reach thermal equilibrium, after stick is warmed up set ppm correction. My particular stick drifts 4 ppm from cold to warmed up, then stays on frequency.
Yes: You are a perfectionist, or you want less internal noise with high gain settings.
Performance increase with modifications below will be really, really small, less noise on shortwave, maybe one or two less noisy lines on WXSat images.

Air cooling

The brain of the stick lives under the plastic cover, open up the stick and drill holes, especially above the black squares, or leave the board bare without its plastic cover.
Stick will run cooler, frequency drift will be reduced somewhat.
Heat-sinks are a possible solution, reliable connection is hard to achieve due to small surface area, edge of the chip can break off.
Additional airlow with PC fans, run on 5V, telephone chargers can be used. Two PC fans installed outside a PC power supply case, with stick in the middle provides a cheap air-cooled solution.

Using fans have the following drawbacks: 1) Needs external power source, 2) magnets in close proximity to the stick, 3) the awful sssssssh noise fans generate.

Water cooling

Electricity plus water equals disaster, insulator (hot glue or epoxy) must be used to seal electrical components from water. Tried several methods back in the Pentium 4 days, messy, expensive, hot glue won't dry evenly, epoxy eats components: water cooling is unsuitable.

Oil cooling

Place stick into metal can, with antenna and USB cable connected and led through lid of can, position stick middle of can, slightly above bottom.
Pour sunflower or vegetable oil into can with the RTL stick inside, completely fill up can.
Repeat: fill up can as much as you can, any interior surface will pick up condensation.
Cover with lid, plug in USB and connect antenna, fire up SDRSharp.
Oil will provide cooling, it is not conductive, see stick blue light on image below.
RTL-SDR, Software Defined Radio

Place can in the fridge or freezer: sunflower oil theoretically has a very low freezing point, practically after one night at -20 degrees Celsius can will be very cold to touch.
Active heating is unnecessary, oil volume and metal can surface will be enough to dissipate heat.

Cherry on the cake: FM Noise Reduction

Metal can also acts as a Faraday cage, FM broadcast pickup will be greatly reduced.

Traditionalists love to place stick in metal enclosure for the same reason, not realizing that heat buildup will effect performance - a chip running at a very low temperature will have lower internal noise and greater performance.

Cleaning stick after oil cleaning

Stick should not be connected to computer or any other electricity.
Lukewarm water in sink, spoonful washing powder or cup of liquid detergent, the more the better, dissolve until glassy reflection and feel of glove on hand. Pour liquid detergent on stick/  place stick and USB cable in washing powder, wash off oil and detergent with lots of water.
Repeat steps above 2-3 times, wash hands, clean with towel / toilet paper.
Leave stick to dry for a day. small heat, not close to radiator, never on top of a heater.

3 comments:

  1. Mineral oil has better long term stability. Why use sunflower/vegetable oil which breaks down after a few years in oxygen. Although in saying that sunflower/vegetable oil is biodegradable, which when you think about it, is an advantage when the SDR dongles eventually break.

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    Replies
    1. Thanks for your comment.
      "which breaks down after a few years in oxygen"
      1. Fill up can to the very top, no space for condensation or oxygen.
      2. Sunflower oil is cheap, readily available, freezes at - 17 Celsius.
      3. If smell or any other consideration comes up, why not change the oil? 2 litres is 4 euros / 5 dollars, change every month if really bothers you. It won't, receptacle is sealed.

      RTL SDR dongles are really hard to break, despite 6+ months usage of my Nooelec stick minimum 12 hours daily, it's still kicking, no issues. Terratec dongle at third the price (identical in all aspects) subjected to torture not normally encountered by electronics but necessitated by testing for the book, worst was heavy rain, dongle ended up in mud, soaking wet - still works after drying.
      Using mineral oil never crossed my mind, I'm a great fan of recycling so would never use stuff I can't utilise later - sunflower oil from the deep fat fryer is just as good coolant as new one, filtered I used to run my diesel car on it.

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  2. I do not understand how reducing the FM signal is a "cherry on the cake" ? We are looking for the strongest signal and low QRM, no ? Your sentence is only valid if you look for weak signal out of BC and ham bands. Then, if you do not plunge the antenna in the oil, it remains out of the influence of the can.

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