EEVblog #970 - Mailbag

[trophosphere]

Did the version you sent to Dave include the 50 Hz or 60 Hz notch filter?

It included a 50Hz notch filter. I made sure I sent a 50Hz version as I had made a blue mark on the module with a sharpie after I had set the notch frequency. In fact, I used Dave's sample to derive the frequency response graph that is present in the module's datasheet.



The actual trough of the of the notch filter is around 49.7Hz (because of component tolerances) but I didn't want to specifically trim it as I won't be doing that to other units and I wanted a fair representation of what others will be getting. In essence, there is no cherry-picking as I have another 50Hz version sitting on my desk that has a deeper notch depth around -34dB as its component values are more closer to ideal.

[TheAmmoniacal]

Alright, a starter kit has been ordered Thanks.

[SeanB]

Those radiosonde batteries are a special design, an 18V battery using a stack of dry electrodes, zinc sheet anode, manganese dioxide and ammonium perchlorate with sawdust powder blend electrolyte and a graphite cathode on a zinc sheet, which is also the anode of the next cell in the series pack. These are in a waxed paper housing with a small vent on top, and a hole at the base that allows you to activate them. Activation is simple, you take it out of the sealed plastic pouch it comes in, in the base of the sonde, and dunk it in water for 5 minutes to wet the electodes, which then wet the electrolyte and activate the battery.

Then you put it back in, plug in the power lead and close the case, and take the small strip of teletype tape ( or for some the calibration data printed on a strip of paper, for the particular device from the manufacturer, but the tape is machine readable) and put it in the recorder to get the calibration data for the particular unit, as they can have a large unit to unit variation, and there is no cal before flight. Then you attach the yellow handle to the inflated balloon, which also seals the rubber fill section, unwind the cord and let it go.

[ornea]

Incase it has not been mentioned,  i believe the batteries are water activated.  As the ballon is readied for deployment,  the battery is sprayed with water.

[Regulus]

Just a few more facts about the radiosonde.

The round metal thingy shown separately (with the text "humidity sensor") is actually a barometer aneroid (i.e. a sensor for atmospheric pressure). There is vacuum inside the flexible metal capsule. Atmospheric pressure pushes the sides closer together and the distance is measured. In the case of this particular sensor the distance is measured by sensing the capacitance of two plates inside the capsule, attached to the two sides. Atmospheric pressure decreases with altitude, sonde altitude is calculated based on that measurement.

Judging by the antenna this radiosonde might have been a 1.6 GHz band version. I am not an RF guy so don't take my word for it... Flight of the sonde can be tracked with a radiotheodolite or a tracking radar. Wind speed and direction at different altitudes can be calculated from the radiosonde track.

This sonde has lost it's temperature sensor, which has been at the end of the flexible "arm", the chip remaining there is the humidity sensor. This version of a radiosonde has used only capacitive sensors (including the temperature sensor), the mysterious IC is an ASIC used to measure the capacitance values.

Most of the real magic in this device is in the sensors and how they are measured, not really in the radio transmitter (although it appeared to have rather unique construction). A radiosonde is a disposable precision instrument with e.g. temperature measurement range from +60 degC down to -90 degC with accuracy of 0.2 degC, quite a special device.