[DSP-10] Receiver system temperature

Wed Oct 4 01:11:03 EDT 2006

Perry, thanks for the tips.

They are carbon resistors, 1/8 watt, so they don't have much thermal 
mass except for the plastic (or whatever it is) packaging.

I didn't really insure even heating of the resistors, and knew that 
this would be a source of error.  I just put the soldering iron close 
to one of them, put the thermocouple right in between and kind of 
waved my hands and said, "Well, if one of them is 80C and the other 
is 120C, the meter ought to read 100C and the actual temperature 
should be close +/- 5C, right?"

Maybe not.  It depends on more stuff than I have good visibility into.

I was just wondering if I was doing the right calculation. 
Currently, I'm adding up the power (from an 8-bit spectrum, CW mode) 
in the 255 frequency bins over about 95 samples, about one minute's 
worth, and dividing by the N (about 25,000).  Since this gives an 
answer like 4 with a standard deviation of 2, I know it's not very 
precise.

I know there's ripple in the passband (a big fraction of a dB in 
places) so was thinking about calculating a temperature for each 
frequency bin then doing mean and standard deviation on all 255 
temperatures.  If I'm being hurt by ripple, that might help, but I 
doubt if it will help a lot.  Something more like 5-10 minutes of 
data instead of one minute at each temperature would probably help 
more.

Also I could use LTI mode and get 16 bit spectra.  I doubt that would 
help a whole lot either, but I should probably be doing both these 
things anyway, for whatever improvement they're worth, since both are 
easy.

If it looks like the limiting factor is knowing the precise 
temperature of the bulk resistor material, I can repeat the 
experiment in the kitchen (I'm just getting an old E-Bay laptop 
hooked up for this so it would be possible), in the freezer, the 
refrigerator, in air, and in warm or hot oil.  My meter only reads 
accurate to one degree C though, so I'd ultimately be limited by that.

I'll probably end up extending the software to do a least squares fit 
on all this data too.  An extension I need to do anyway.

I should say that the goal here is to be able to make some sort of 
rough kitchen-style measurement that verifies performance of the 
radio, or some preamp that I might buy or build, independently from 
the spec sheets, to get a warm fuzzy that it's all working as it 
should.  If I could get a resolution of 25K in the measurement, that 
would be good enough for this.  Looks like this first outing was more 
like 250K!

It wouldn't take much to modify this to do a sun-noise measurement, a 
QRPp one at that.

You know, this experiment isn't high tech by today's standards, but 
one of the great things about DSP-10 is that you can just go off on a 
whim and measure stuff like this.  Maybe if I did a brief piece for 
QEX or something it would get other guys drooling about the 
possibilities....

(Of course I'd have to have presentable results.)

Thanks again,

Courtney, n5bf/6

>What type resistors did you use?  I suspect you may have a 
>non-linear noise factor in the resistors coupled with accumulated 
>measurement errors.  Did you heat the resistors in a "bath"?  If you 
>used hot air, what did you do to insure even heating of the 
>resistors.
>You might want to take your samples at 0C and at 20C and see how it 
>measures up.
>
>----- Original Message ----- From: "Courtney Duncan" <cbduncan at earthlink.net>
>To: "DSP-10" <dsp-10 at mailman.qth.net>
>Sent: Sunday, October 01, 2006 1:44 AM
>Subject: [DSP-10] Receiver system temperature
>
>>I've just measured the receiver temperature of my DSP-10 in the 
>>following way:
>>
>>Solder a pair of 100 ohm resistors in parallel at the end of a 
>>piece of coax with a BNC on the other end.
>>
>>Connect this to the DSP-10 antenna connector in receive mode.
>>
>>Measure the bulk temperature of the pair of resistors by inserting 
>>the thermocouple from a BK Test Bench 390 in between them.
>>
>>Captured a minute of data in CW mode with this terminator at 20C (293.16K).
>>
>>Warmed up the resistor pair to 101C (374.16K) and took another 
>>minute of data.
>>
>>Did two other such data captures, one at 105C (plus or minus 5) and 
>>another at 20C.
>>
>>Sum up all 255 bins of noise over each file and calculate the 
>>implied receive noise temperature using the formula:
>>
>>P2m / P1m = R = ( Tr + T2 ) / ( Tr + T1 )
>>
>>Where R is the power ratio between the warmer ( T2 ) and cooler ( 
>>T1 ) measurements and Tr is the unknown receiver temperature.
>>
>>I'm assuming that I can just add the temperature of the receiver to 
>>the temperature of the load like this.
>>
>>This reduces to
>>
>>Tr = ( T2 - R*T1 ) / ( R - 1 )
>>
>>My four measurement files are:
>>
>>Temp.   Power average
>>20C     3.87855
>>20C-2   4.01719
>>101C    4.17844
>>105C    4.25063
>>
>>Using 20C and 101C, I get Tr of 754K.
>>Using 20C and 105C, I get Tr of 593K.
>>Using 20C-2 and 105C, I get Tr of 1170K
>>Using 101C and 105C, I get nonsense.
>>
>>This is admittedly this is a "noisy" measurement, but the results 
>>are not inconsistent with either 600K or 1000K.  Based on this, I 
>>think I'll keep using 600K for calculations for now.
>>
>>Does anyone recall offhand what the spec sheet for the first RF 
>>amplifier says about its noise figure?
>>
>>Courtney, n5bf/6
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>
>
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