Spectrum Analyzer measured levels of DTMF tones too low

Thread Starter

Bobby Briem

Joined Feb 1, 2024
3
Hello,

I've about exhausted my limited abilities trying to understand why I cannot get the same measurements that a test lab seems to be getting for a device of mine. I am using a MDO3034 Mixed Domain Oscilloscope to measure the levels of the dual frequencies generated by an analog telephones DTMF keypad. I should be getting two frequencies of 697hz at -4.28dbm and 1209hz at -2.12dbm when pressing the "1" key for example. And while my spectrum analysis is indeed showing those frequencies, my measured levels are actually -51dbm and -39dbm respectively. My attenuation is set to 1x but I've about run out of ideas as to why my measurements are off by such a wide margin.

Would another option be for me to use a bandpass filter and isolate the individual tones on the time domain rather and calculate the dbm via the measured Vp-p?

Thank you in advance for any ideas!
 

Papabravo

Joined Feb 24, 2006
21,218
I would say that the measurement is being done incorrectly. How much of the original telephone mechanism have you preserved. If you are not using the original telephone mechanism, how do you know that you have duplicated the original setup. You do know that the POTS network had a power supply of nearly 50 volts DC - right?
 

crutschow

Joined Mar 14, 2008
34,386
Dbm is power measured at a specific load resistance (usually 600Ω for a POTS line).
If you have some other impedance, than the measurement value will also be different.
 

Thread Starter

Bobby Briem

Joined Feb 1, 2024
3
I would say that the measurement is being done incorrectly. How much of the original telephone mechanism have you preserved. If you are not using the original telephone mechanism, how do you know that you have duplicated the original setup. You do know that the POTS network had a power supply of nearly 50 volts DC - right?
The telephone in question is my device, so it is an exact version of what the lab has for their testing. It is a complete analog unit I had sent one of the devices to a lab for certification testing and am using another one that I have handy for trying to compare to the results they are getting. As far as recreating their testing environment - I only know that they used a spectrum analyzer as well to perform the test but I do not have further details past that as of now.

And yes I am also quite familiar on the telephony side of things - I am using a network simulator and providing 48Vdc and 40mA to the device. I am measuring the output from the telephone to the "telephone line". But even if I measure the output directly from the IC responsible for my tone generation my levels are quite far under the -2dbm and -4dbm they seem to be reading which also leads me to believe I am measuring incorrectly. I just am unsure as to what it could possibly be at this point.
 

Thread Starter

Bobby Briem

Joined Feb 1, 2024
3
Dbm is power measured at a specific load resistance (usually 600Ω for a POTS line).
If you have some other impedance, than the measurement value will also be different.
Dbm is power measured at a specific load resistance (usually 600Ω for a POTS line).
If you have some other impedance, than the measurement value will also be different.
Is there a way to calculate this? My analyzer I know has an input impedance of 50Ω so this sounds promising. Or would putting a 550 ohm resistor in series to match the impedances work as well without risking any damage to the machine?

I'm very new to operating this device so there has been somewhat of a learning curve!
 

Papabravo

Joined Feb 24, 2006
21,218
Is there a way to calculate this? My analyzer I know has an input impedance of 50Ω so this sounds promising. Or would putting a 550 ohm resistor in series to match the impedances work as well without risking any damage to the machine?

I'm very new to operating this device so there has been somewhat of a learning curve!
A resistor in series will create a voltage divider and not help very much. I think what you want is a 600Ω match and a conversion from 600Ω to 50Ω. Such a thing might not be easy. Quick research suggests that going from 50 to 600 is easier than going from 600 to 50. You might need to use a transformer.
 
Last edited:

crutschow

Joined Mar 14, 2008
34,386
My analyzer I know has an input impedance of 50Ω
The MDO3034 should have a selectable input impedance of 1MΩ.
If your system already has a standard POTS impedance, then use that.
Otherwise put an external 600Ω resistor in parallel (not series) with that for your measurement.

But you still will likely have to calculate the equivalent dBm from the actual voltages your are measuring.
Remember that all dB values are relative, not absolute.
It appears the MDO3034 measures in dBv (relative to 1Vrms at the input).
(dBm is relative to 1mW at some stated impedance, such as 50Ω or 600Ω)

Look up the definition of dB measurements if you are not familiar with them.
They can be a little confusing when going between dB voltages (20log(V2/V1) and dB power (10log(P2/P1).
 
Last edited:

Papabravo

Joined Feb 24, 2006
21,218
The MDO3034 should have a selectable input impedance of 1MΩ.
If you system already has a standard POTS impedance, then use that.
Otherwise put an external 600Ω resistor in parallel (not series) with that for your measurement.

But you still will likely have to calculate the equivalent dBm from the actual voltages your are measuring.
Remember that all dB values are relative, not absolute.
It appears the MDO3034 measures in dBv (relative to 1Vrms at the input).
(dBm is relative to 1mW at some stated impedance, such as 50Ω or 600Ω)

Look up the definition of dB measurements if you are not familiar with them.
They can be a little confusing when going between dB voltages (20log(V2/V1) and dB power (10log(P2/P1).
Isn't 600Ω || 50Ω < 50Ω? How does the parallel combination help you.
 

Ya’akov

Joined Jan 27, 2019
9,100
Welcome to AAC.

Alternatively, you could use a second instrument as the termination so the circuit is hybrid to hybrid and measure the levels at the high impedance 1MΩ of the analyzer.

In any case, it doesn’t make sense to put a 50Ω load on the DUT since it would never see that in practice. A realistic test would certainly be on a 600Ω circuit however you manage it.
 
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