5600 attenuation

[Alann]

I have generated a signal with following specs

Frequency: 100MHz

Level: -10dBm

In RFSA configration, the power of the received signal is around -40dBm. Why is the signal getting attenuated?  What is the source of this attenuation?

[Corby_B]

Hello  Alann

With a -10dBm signal coming into RF In you should see -30 dBm at IF Out when attenuation is left to the default values. There are 2 attenuation stages present, RF attenuation and IF attenuation. You can set the attenuation settings in software. Here is a link to the 5600 Help that will can help you with the functions that you can change to adjust this setting.  

NI RF Signal Analyzers Help

NI 5660 Theory of Operation

The RF attenuation level is automatically set by the NI-RFSA drive based on the reference level and mixer level using the following equation:

RF Attenuation [dB] = Reference Level [dBm] - Mixer Level [dBm]

So you can either set the reference level or there is a property node to specifically set the attenuation level.

Let us know if this helps!

Cheers!

Corby_B

http://www.ni.com/support

[Alann]

Hello Corby_B,

I understand that there are two set of attenuators, RF attenuators (20,20 & 10 dBm) and IF attenuators (20 & 10dBm). The maximum overall attenuation is 80dB.

RFSA help says that "You can modify attenuation settings using the Attenuation property or the NIRFSA_ATTR_ATTENUATION attribute". The attribute referred has range 0-50dBm. Is it just for controlling the RF attenuators? Even if I set that to zero, attenuation up to 30dB persists. This can possibly be the IF attenuation. Can I set IF attenuation to zero?

Message Edited by Alann on 02-24-2009 10:00 PM

[Alann]

I am uploading some screenshots of RFSA demo panel to clarify my problem.

in the first screenshot, i generated a signal of 22MHz and -10dBm level. The received signal power is -34dBm.

in the second screenshot, i generated a siganl of level -30dBm. The received signal power is -57dBm.

whereas in both cases the attenuation was set to zero. why is the signal getting attenuated??

[dansch]

Hi Alann,

I have seen this type of problem occur most often when using bad cables. Is there a chance that your cables may be bent improperly (such as if using a rigid SMA cable) or something similar? Also, what are you using to generate your signal, and can you confirm for sure that it is generating at the output you expect? One last thing I want to confirm is that you are using the PXI-5661 device (5600 downconverter + 5142 digitizer).

When generating a signal using a PXI-5671 and connecting it directly to a PXI-5661, I have not had any problems with the signal showing up correctly, as the software will account for whatever attenuation is being applied by the hardware. However, I once used a rigid SMA cable that had been bent too much and was broken at the midpoint, and I found that my signal was always showing up as being attenuated by just under 30 dB. Hope this helps,

[Alann]

yes I am using 5661..

and there is no problem with the cable.I gave a -10 dBm signal directly to digitizer (configured through NI-SCOPE) and it showed correct received power. (by the same cable and generator)

[dansch]

Hi Alann,

I am still not sure what may be causing the issue, short of something being wrong with the connection or just something broken with the downconverter. To connect directly to the scope, you would need a BNC connector, whereas the 5600 downconverter's input is an SMA connector. Thus, I am wondering what difference must have been used between the two (maybe a BNC-to-SMA converter). Some cables and connectors have attenuation built-in, so this is also something to look for. Finally, the scope has a 1 MOhm input impedance by default. I am interested in knowing if you can probe the end of your SMA cable from the source with the scope to see if it returns the signal correctly. The signal read by the scope should be at least 3 dB higher than normally expected due to the high impedance (since you are no longer halving the signal with the 50 Ohm impedances). Please let me know what details you find. Thanks,