Kitz Forum
Chat => Tech Chat => Topic started by: burakkucat on July 06, 2018, 09:43:36 PM
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Earlier today I was looking at the label on the back of some gubbins. It read --
Frequency: 1600 Hz
Level: -10 dB
Impeadance: 600 Ohms
My initial thought was "O.k. Perfectly normal" but then my eye returned to the second line. Minus ten decibels . . . relative to what? ??? High tide level at Newlyn? One volt? One milliwatt? One watt?
An experiment was duly performed. The gubbins was connected to a Rheostat No. 1A, set to 600 Ohms. The gubbins was turned on and the AC voltage across the load was measured with a Fluke 8022B DMM. A value of 0.242 V was observed.
The power = (0.242)2 x 103 / 600 mW
= 0.0976 mW
That power is equivalent to 10 x log10 0.0976 dBm
= -10.1 dBm
Finally, to convince myself that the arithmetic is correct, the gubbins was connected to a Level Measuring Set No. 35A configured to terminate the circuit with a load of 600 Ohms. A value of -9.9 dBm was observed.
I suspect that seeing "Level: -10 dB" and automatically knowing that dBm is the relevant unit would be second nature to those of our members who are escapees from the Newgate Street empire. :D
[Edited to insert the correct values. :-[ ]
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I suspect that seeing "Level: -10 dB" and automatically knowing that dBm is the relevant unit would be second nature to those of our members who are escapees from the Newgate Street empire. :D
Indeed so!
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I suspect that seeing "Level: -10 dB" and automatically knowing that dBm is the relevant unit would be second nature to those of our members who are escapees from the Newgate Street empire. :D
You took the words out of my intended reply. ;D
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When I typed the word "escapees" I had two particular members in mind. Two members who, over the past years, have helped me understand the finer points of various topics . . .
b*cat makes one of his Japanese-style bows. :)
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Reciprocated <in ungainly British way> :)
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Likewise
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Having edited my initial post to insert the correct values, I decided to perform another experiment.
The gubbins and the Fluke 8022B DMM were connected in parallel to the terminals of the Level Measuring Set No. 35A, which was configured to terminate the circuit with a load of 600 Ohms.. All three items were powered on. The DMM indicated a value of 0.250 V, whilst the LMS indicated a value of -9.9 dBm.
The power = (0.250)2 x 103 / 600 mW
= 0.1042 mW
That power is equivalent to 10 x log10 0.1042 dBm
= -9.8 dBm
I am satisfied that all the observed values are essentially consistent with one another.
The only deduction that I can make is that the Rheostat No. 1A and the Level Measuring Set No. 35A differ slightly in their presentation of 600 Ohms. The former being somewhat smaller and the latter being somewhat greater than the nominal 600 Ohms.
Looking closely at both sets of results, we can see that it is possible to calculate the deviation from the nominal 600 Ohms. We make two assumptions --
- The gubbins was providing the same power in both experiments.
- The arithmetic mean of the two resistances is 600 Ohms.
Let RR be the resistance of the Rheostat No. 1A when it is set to 600 Ohms.
Let RL be the resistance of the Level Measuring Set No. 35A when it is set to terminate the circuit with 600 Ohms.
We now substitute RR & RL into the respective power calculations and then equate them (assumption 1., above) --
(0.242)2 x 103 / RR = (0.250)2 x 103 / RL
Rearranging, we can see --
RR = RL x (0.242)2 x 103 / ((0.250)2 x 103)
= RL x (0.242)2 / (0.250)2
= RL x 0.937024 (equation 1)
From assumption 2., above, we write --
(RR + RL) / 2 = 600
Rearranging, we can see --
RR = 1200 - RL (equation 2)
Substituting equation 2 into equation 1 gives --
1200 - RL = RL x 0.937024
Rearranging, we can see --
(RL x 0.937024) + RL = 1200
Thus --
RL = 1200 / (1 + 0.937024)
= 619.5 Ohms (equation 3)
Substituting equation 3 into equation 2 gives --
RR = 1200 - 619.5
= 580.5 Ohms
A rather surprising conclusion. But one that fits the facts. :o
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Are these readings and calculations not somewhat compromised by the fact that the resistances in a Rheostat 1A comprise wound coils, not carbon, metal film etc resistors?
At least I presume they are. My two examples are actually Rheostats No. 2A, but look the same externally.
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When I first acquired the Rheostat No. 1A I removed it from its mu-metal case to check upon its overall condition. I noted that each resistive spool was wound by folding the relevant length of resistance wire in half . . . thus self-nullifying any inductive effect.
Back in the day, when such items as the LMS No. 35A and the Rheostat No. 1A were built, they were built to a design and not to the cheapest possible price. Are you aware, from your days using such equipment, of any inductive problems when using those rheostats?
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Are you aware, from your days using such equipment, of any inductive problems when using those rheostats?
It was all a long time ago, but now I think about it, no. The same goes for experiments at home since escaping.
As you rightly say, the equipment in those days was of exceptional quality.
Talking of which, this evening I connected my W&G Pegelsender to the Pegelmesser, sending 20kHz at 0dBm and recording a 'through' reading of +6dBm. Connecting the Rheostat No 2A at 600 ohms across the pair gave precisely 0dBm, so NO inductive effect at all.
Incidentally, the legend on the back of the 2As reads "P.O. C.P. 1/1 74", indicating a Weston Electrical Instruments product of 1974. I popped it on the scales and it's about 2·25 kg - they don't make them like that any more!
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Close scrutiny of the Rheostat No. 1A shows a very degraded "GPO Batch Sampled" stamping on its rear. The four digits that followed are too indistinct to determine.