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Volts to dBm - sanity check?

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elroy:
I'm comparing notes with a friend in another part of the country, where each of us is running tests feeding signals to an SDR from a signal generator.

His HP8640B signal generator apparently has output calibrated in dBm, while my DDS Signal Generator/Counter (brand name Koolertron) seems to be set up for output level to be specified only in volts.

I assume I can convert volts to dBm first by calculating power in watts:

P(w) = V^2 / R

where R is the SDR's 50-ohm load

Then to convert the power from watts to dBm:

P(dBm) = 10 * log10(1000 * P(w))

So for our 50-ohm system, the simplified equation would be:

P(dBm) = 10 * log10(20 * V^2)

And plugging this into my TI-89, I get a few values:

Volts                  dBm

1                       13.0103
0.1                    -6.9897
0.01                  -26.9897
0.001                -46.9897
0.0001              -66.9897
0.00001            -86.9897
0.000001          -106.99

So the value is 20 dBm less every time the decimal of the voltage moves over one place.

Am I doing this right? When my friend talks about his SDR detecting a -147 dBm signal, that would be equivalent to 0.01 microvolt from my signal generator?

awallin:
try this chart:
http://wera.cen.uni-hamburg.de/DBM.shtml

your "volt" corresponds to "volt rms" in the chart..

not sure about our notation in the dBm formula...

elroy:
Thanks for bringing up the distinction between RMS, peak and peak-to-peak voltage.

I see from my signal generator's manual that its voltage output is stated as peak-to-peak. The minimum it is capable of at any frequency is 2 mVpp, which from your chart would be -50 dBm. So it sounds as though if my friend and I are comparing SDR sensitivity at levels in the -100 to -140 dBm range, I'm going to have to insert some attenuators of reliable value.

Incidentally, since my signal generator's manual states that its output impedance is 50 ohms (+/- 10%), I assume I can just use a coax cable to attach it to the 50-ohm antenna port on my SDR and not worry about terminators, as would need to be used with, say, an oscilloscope?

radiolistener:
First of all you're need to make sure that your receiver has 50 Ohm impedance at specified frequency.
Very often it is not 50 Ohm. You can check it with vector antenna anlyzer.

The second, there are different volts: Vrms, Vpk, Vpk-pk.

So, if your receiver input has Z = 50 + j0 [Ohm], and you're talking about Vrms, then your calculations are valid.

For the reference, with Z = 50 + j0 Ohm:

0 dBm = 0.001 W = 0.2236 Vrms = 0.3162 Vpk = 0.6325 Vpk-pk = 106.9897 dBμV PD = 113.0103 dBμV EMF = S9 + 73 dB HF = S9 + 93 VHF


If your generator is marked for peak-to-peak at 50 Ohm load, then power can be calculated with

P = (Vpk-pk/2)^2 / (2 * R)

For Vpk (peak amplitude):

P = Vpk^2 / (2 * R)

Please note that Chinese DDS such as FY6600 shows invalid voltage. As I know it shows Vpk for open output (with no load at all). It means that you don't know what voltage will be with the load, you're needs to measure it. But if you know output impedance of your DDS is 50 Ohm, then you can just divide the value which is shown by two.

As I know FY6600 has 50 Ohm output, but you're need to check it. In order check, do the following:

1) Select 50 Hz 1 Vpk (1 Vpk = 2 Vpk-pk) sine on your generator, but do not connect anything on the output, leave output connector open

2) Measure RMS voltage on the output connector with TrueRMS multimeter.

3) Connect 50 Ohm resistor on the output connector and measure RMS voltage with TrueRMS multimeter.

If your gen has 50 Ohm output, the value measured on the step 2 will be exact 2 times higher that the value measured on the step 3.

If your multimeter shows 0.7071 Vrms on the step 3, then your generator shows correct voltage.

If your multimeter shows 0.3536 Vrms on the step 3, then your generator shows incorrect voltage (this is Chinese volts). It shows voltage for open output. So, you're need to measure actual value with connected load. If your gen has 50 Ohm output, you can just setup twice higher value on the gen in order to get required voltage.

radiolistener:

--- Quote from: elroy on April 03, 2019, 06:31:02 pm ---Incidentally, since my signal generator's manual states that its output impedance is 50 ohms (+/- 10%), I assume I can just use a coax cable to attach it to the 50-ohm antenna port on my SDR and not worry about terminators, as would need to be used with, say, an oscilloscope?

--- End quote ---

if your SDR has 50 Ohm input, it is true, there is no need matching circuit. But very often SDR input is just marked 50 Ohm, because it is about that value in average for all frequency band. But it's actual impedance may very very significantly depend on the frequency.

You're need to measure your SDR input impedance at specified frequency, I'm sure it will be at least slightly different from 50 Ohm even for very precise SDR.

For example RTLSDRv3 doesn't have 50 Ohm on the input. It's impedance vary in wide range depends on the frequency.

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