Small signal value for characterization

[STOIKOV]

what is the dbm value of an 18-milliwatt signal

If using a VNA or a RF signal generator for characterization, what is the minumum or a typical amplitude of a test signal to be applied ?

 

[xxargs]

what is the dbm value of an 18-milliwatt signal?

If using a VNA or a RF signal generator for characterization, what is the minumum or a typical amplitude of a test signal to be applied ?

Is very depend of equipment you test.

it's not fun put in 0 dBm on reciver input if recivers design working normally around -120 dBm - -60 dBm, if you drive 20 dB power amplifier you want +15 dBm to feed amplifier for 35 dBm out in test...


Mostly RF-signal generator have very wide dynamic range attenuator on output (-143dBm to +15 dBm in some case) to fit level to equiment from near termic noise only for noise factor measure to high level and testing overdrive capability, protect or AGC-circurit function in reciver or amplifier.

Few SA trace generator (TG) have acceptable adjusting dynamic (hp8594 have range between +2.5 dBm to -66 dBm in 0.1 dB resolution and only need 1-2 30 dB external attenuator for signal test range down to -126 dBm. 68 dB adjust range on TG in SA is a minimum IMHO) and mostly other SA from R&S, Anritsu etc. have terrible range on TG (only 20 - 50 dB) and near complete useless working with high gain block testing sensitivity and AGC or ALC testing 60 dB higher level in same measure moment in field.... I not want crewing on/off externa attenuator whole time (and forget adjusting referensce offset level in moment time to time and read wrong measure value) in measure moment...

Many small field SA is userless depend on worthless tracing generator and this poor dynamic range + stupid software is not allow TG going same time is using SA is normaly way and only 'package' to very specific measure problem ex. filter measure only... and TG going off in other measure mode GHAAAAAAAAAAA!!!!!



/Xxargs

 

[STOIKOV]

"...hp8594 have range between +2.5 dBm to -66 dBm in 0.1 dB resolution ..."

it means that the range is between 1e-6mV and 1e-3mV, in 1.01V resolution ?
I did the conversion V=10 (db/20)

 

[xxargs]

"...hp8594 have range between +2.5 dBm to -66 dBm in 0.1 dB resolution ..."

it means that the range is between 1e-6mV and 1e-3mV, in 1.01V resolution ?
I did the conversion V=10 (db/20)

In RF-world uses always power, not Voltage as reference depend of voltage can vary very wide for same power transport in circurits depend of transforming impedances up and down between parts of components or standing wave can cancel voltage just on measure point and few centimeters away on line have large voltage amplitude (measure with high impedive probe).

SA and VNA measure power flow on know 50 Ohm loading impedances in port.

Only situation using Volt, Volt/m, dBuV/m as reference is field strengh measure in air (know air have 377 Ohm impedances) and other near antenna work. Old mobile radio people and HAM-people refer very often to uV or dBuV on reciver input sensivity -> coupled to field strengt antenna picked up and easy measure with primitive instruments before modern SA etc.


dBm mean always power relation to 1 mW over 50 Ohm impedances

(broadcast and TV-equipment using 1 mW over 75 Ohm as reference)




1 mW = 0 dBm = 107 dBuV = 0.2236 Volt over 50 Ohm



+2.5 dBm = 1.77mW = 0.298 Volt @ 50 ohm
0 dBm = 1 mW = 0.2236 Volt @ 50 Ohm
-10 dBm = 0.1 mW = 0.0707 Volt @ 50 ohm
-20 dBm = 0.01 mW = 0.0224 Volt @ 50 Ohm
-30 dBm = 0.001 mW =0.00707 Volt @ 50 Ohm
-40 dBm = 0.0001 mW = 0.00224 Volt @ 50 Ohm
-50 dBm = 0.00001 mW =0.000707 Volt @ 50 Ohm
-60 dBm = 0.000001 mW = 0.000224 Volt @ 50 Ohm
-66 dBm = 0.000000252 mW = 0.0001121 Volt @ 50 Ohm

ie. voltage range over 50 Ohm load = 298 mV - 0.112 mV for attenuate range from +2.5 dBm down to -66 dBm in hp8594

0.1 dB resolution is not same as 0.01 volt resolution of 1 Volt, dB work always
fraction/quote of exist power, (-)3 dB change is aways (half) or double of actual power, 0.1 dB change around 2.28% of exist power level (or 1.14% change of actual voltage).

ex.

0.1 dB change between 2.5 dBm and 2.4 dBm give 3.4 mV change (over 50 Ohm).
0.1 dB change between -65.9 dBm and -66 dBm give 1.3 uV change (over 50 Ohm)

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good reciver have Squelch level around -120 dBm

ie -120 dBm = 10e-12 mW = 0.224 uV

termic noise over 50 Ohm at 17 degree celsius

-174 dBm/1Hz = 3.98e-18 mW = 4.46e-10 Volt noise per 1 Hz band width.

for HiFi-stereo range frequency (20-20000 Hz) give -130 dBm in noise power from 50 Ohm or ~63 nV RMS noise level.

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if leave generator output open and measure voltage very high impedance probe, voltage is doubled (6 dB higher) on output.

Oscillocope probe have around 1 or 10 MOhm DC impedance, but for 10 MHz and above, oscilloscope probe have impedances arounde 70 -80 Ohm and heavy load on high impedance RF-circurit.

 

[STOIKOV]

thanks,
just to confirm (did this question before), a DC component can be added to VNA output RF port ? Need to apply a AC+DC signal to the input of an amplifier.

 

[flyhigh]

Hi,

just to add something. Considering power comming out from NA port you should be very careful because it is usually not what power source menu option is telling you, it is much less (10-20dB !!), especially for old HP NAs. You should use spectrum analizer or power meter to confirm the output level.

flyhigh[/b]