Attenuating the input to my Signal Hound SA44B

[djsb]

My Signal Hound SA44B (arrives next week) has a maximum AC amplitude limit of -20dBm. According to my handy chart this a maximum of 0.063 Volts peak to peak. I am trying to understand how to practically use the -10,-20,-30dBm (or dB attenuators) that can be inserted inline to a test cable to reduce the signal amplitude. For instance if my signal source was outputting a 1 volt peak to peak signal how would I calculate the required attenuation? Are there any tricks, rules of thumb that I could use to help me get the level correct? Also, is there such a thing as a variable RF attenuator that I could buy or build myself? Thanks.



PS I have some calculations on the chart I'm using (from MarkiMicrowave) but not sure how to put equations in this post. Also, what happens when fixed attenuators are connected in series? For instance what is the total attenuation when 2 x 20db attenuators are put in series?

[djsb]

Hi,
Just double checked and it's +20dBm (which is 6.325v Pk to Pk) according to this forum post

https://signalhound.com/support/forums/topic/max-rf-pwr-into-sa44b/

It recommends a maximum of +10dBm on the input (2 volts Pk to Pk). Anyway I'm still a bit confused about dBm, dB etc.

[Mechatrommer]

ps: re-re-re post i hate "last edited"

My Signal Hound SA44B (arrives next week) has a maximum AC amplitude limit of -20dBm.

i think its +10dBm https://signalhound.com/download/usb-sa44b-datasheet/ it will be irrational for a $1000 device to only accept -20dBm. about your attenuator question. if output dB is not given, you can workout from output power. say a device outputting 100mW. use formula 10 x log(100) = 20dBm. since your SA44B only accept 10dBm, you need to attenuate output signal by -10dB (20 - 10), so you need -10dB attenuator, ymmv.

PS I have some calculations on the chart I'm using (from MarkiMicrowave) but not sure how to put equations in this post. Also, what happens when fixed attenuators are connected in series? For instance what is the total attenuation when 2 x 20db attenuators are put in series?

you add them all up. 2 x -20dB in series will be -(20 + 20) = -40dB. you can buy few attenuators and experiment. you are going to need GHz generator to play with.

[German_EE]

The nice thing about using dBm for RF measurements is that all you need to do is add and subtract. So, if you have a 100W RF transmitter (which is 50 dBm) and the maximum you want to feed into your Signal Hound is 10 dBm then you need a 40 dB attenuator. Note that a 100W 40dB attenuator will be a very large and heavy device so most engineers use something called a power tap that takes a sample of the signal, with a 50 dBm input 40 dBm will go to output 1 (then on to an antenna or a dummy load) and 10 dBm will go to your test equipment.

Attenuators in series add, so in your example 2 x 20 dB attenuators in series will give 40 dB of attenuation.

A figure quoted in dBm means that it is relative to 1mW of RF. So:

0 dBm     1mW
3 dBm     2mW
6 dBm     4mW
10 dBm   10mW
20 dBm   100mW
 So, using this chart you can say that there is 90 mW of gain between a 10 mW input and a 100 mW output OR 10 dB.

If you send me a PM with an email address I can send you a chart that compares dBm, Volts P-P (for a scope), Volts RMS and Power in Watts over a very wide range.

[bson]

Yup, get a power tap and a dummy load.