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How to calculate the attenuator pad needed for 30W radio?

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Full Member level 3
Nov 24, 2003
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I have notice thy fit 30dB pad on on mod analyzer when a radio is mesuared. Assume im mesuaring 30W radio and my modulation analyzer can take max 1W input. Can i knw hw many 30 db attunater i need so i won damage my mod analyzer. And can any one tech me hw to calculate tht?

-30dB pad

10*log(30W/1W) = 14.77 dB -> 15 dB
So you need only one 30 dB attenuator.
With a 30 dB attenuation you get 30mW=14.7 dBm
in the mod analyzer.

Re: -30dB pad

Not so easily answered. Yeah your analyzer might be able to take "1 watt", but what does that mean? Peak power, average power? Most communications modulations contain a lot of peaking, so the peak power will be as much as 12 dB higher than the average power.

So if I were going to use a modulation analyzer that published a "1 watt max" input power, I would set up to have a 15 dB margin between that and my actual transmit power. You should call the analyzer manufacturer to be sure!

1 watt max input = 10 Log (1000 milliwatt) = +30 dBm

Your source has
30 watts = 10 Log (30,000 mw) = 44.8 dBm

44.8 dBm + 15 dB safety margin - 30 dBm max input = 29.8 dB attenuator

So you want a combination of attenuators that add up to at least 30 dB.

The attenuator (s) that you connect directly to your 30 watt source had better be rated to handle that power (they will have big heat sinks attached to them). Standard attenuators lying around the lab only handle around 1/2 watt.

Depending on your analyzer, the above 30 dB attenuator may allow it to safely handle the power, but it may still overload the front end. You may have to add further attenuation, either external or internal to the analyzer, to keep the waveforms linear.

Re: -30dB pad

maybe some data from INTERNET will help you,
the following data are from the R&S, Boonton and Yokogawa.

Max RF input =
R&S Model FMA/FMB= 30 dBm
Boonton Model 8201 = 1W (7V RMS, 30 dBm)
Yokogawa Model VN7100 = 30 dBm(Full Scale Range)

Overload protection=
R&S FMA/FMB = 5W(15V RMS) = 37 dBm
Yokogawa Model VN7100 = (ADC overload) Full scale range + 10dB (typical) ~ 40 dBm

Max.peak Voltage=
R&S Model FMA/FMB = 25V (including DC)
Boonton Model 8201 = 40 VDC, 35VAC, 25 W for a source SWR<4

so, The spec say that the max input is 30 dBm, that usually there is protection
for overload at about 37-40 dBm and the max peak voltage is about 25 V- 40 V(AC or

the peak power will be as much as 12 dB higher than the average power
is plain BS, BS....BS.....US (New England) BS.

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