MiniCircuits Attenuator : BW-40N100W+ : Test Procedure - Page 1
Forum section: electronics / testgear

author: killingtime
author_rank: Regular Contributor
timestamp: May 21, 2023, 12:27:33 am
content: Hi,I picked up a used MiniCircuits Attenuator 40dB (BW-40N100W+). The data sheet can be found below.https://www.minicircuits.com/pdfs/BW-40N100W+.pdfIt shows typical performance to 4GHz of 40dB +/- 0.5dB, with guaranteed attenuation accuracy at +/- 1.6dB. So that's 38.4 - 41.6 dB.I'm testing the attenuator on a SAA3021X+. Overall amplitude spec for this instrument is 0.7dB. The results I'm getting are attached and look very different to the spec sheet. An average of -37.5 dB with a dip to -35.75 dB and a peak to -44.1 dB. That's over 8.35 dB of variation and well outside spec.Either the attenuator is damaged, or I'm not testing it correctly.I've set the RBW to 10KHz to lower the noise floor. BW step is 1MHz. Trace average is 100. Tracking gen is on and I connect both N cables together via a barrel connector. Let the unit complete one full sweep, then activate 'Normalization' to null out any cable loss. I get a flat trace after this. I then insert the attenuator between both cables keeping the barrel connector in the signal path. RG142 cables an N connectors are spec'd to 3GHz.Can anyone see anything wrong with this test procedure?Thanks.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 21, 2023, 01:28:19 am
content: Looks like your SAA3021X+ was spec'ed for 9 kHz ~ 2.1 GHz but you show it sweeping to 3GHz:https://siglentna.com/spectrum-analyzers/ssa3000x-plus/Regardless if you modified the Siglent or not, how did you verify your setup prior to trying to measure the Mini-Circuits device?

author: killingtime
author_rank: Regular Contributor
timestamp: May 21, 2023, 03:35:00 am
content: Quote from: joeqsmith on May 21, 2023, 01:28:19 amYes, you can extend the frequency range of this device by entering a valid licence key. No modification required. This instrument shares the same hardware as the 3.2GHz version SAA3032X, which is why the key works.Quote from: joeqsmith on May 21, 2023, 01:28:19 amVia the Normalization feature. The analyzer sweeps to 3GHz with the cables in the signal path and you get a flat trace back. Without Normalization on I see a gradual attenuation up to 3GHz, which is what you'd expect for RG142 and some connectors. Also put a different 20dB attenuator in using the same cables. Same settings. Got 20dB with 0.7 dB variation. Not too bad. TekBox20dB.jpg attached.After some more trial and error I think I've figured out why I got the original attenuation plot.1. I noticed one of the N connectors was a bit lose. It was already reasonably tight. Tightened it up by no more than 1/8 a turn and the huge dip at 2GHz disappeared.2. Reduced the internal attenuator to 0dB (from -20) and the plot smooths out and gets closer to -40dB. 0Atten.jpg attached. The attenuator is now (just) within the 1.6dB spec given by MiniCircuits.With the cables tightened up, I put the -20dB internal attenuator back on, and got the 20dBAtten.jpg plot. Much more variation and the whole plot shifts +1dB.The internal attenuation is having an effect but I'm not sure at this stage why. Could be that with the TG set at -20dBm, and with -20dB of internal attenuation and -40dB of external attenuation I'm pushing the signal close to the noise floor of the analyzer (-90dBm). The internal pre-amp is off.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 21, 2023, 05:37:41 pm
content: Member Xrunner was testing some low cost 40dB attenuators with a Siglent as well. I posted some similar data using one of the low cost VNAs.https://www.eevblog.com/forum/rf-microwave/cb-and-ham-radio-techs-love-their-bird-wattmeters/msg4423153/#msg4423153

author: Weston
author_rank: Regular Contributor
timestamp: May 21, 2023, 06:17:33 pm
content: The cable and input port of the VNA are not a perfect match to 50 ohms, which can cause a change in S22 with frequency.By changing the internal attenuator measurements you might be changing the input match of the VNA itself.Its pretty common to take S21 measurements with a ~10dB or so attenuator at the DUT end of the return coax cable (side connecting to the input port of the VNA). Try calibrating the through measurement with the 20dB attenuator in place and see if that makes a difference. If the noise floor is an issue you can enable averaging.

author: tautech
author_rank: Super Contributor
timestamp: May 21, 2023, 09:42:36 pm
content: Quote from: Weston on May 21, 2023, 06:17:33 pmFYISSA3000X Plus models cannot provide S measurements unless they'reconvincedto become SVA1000X models.Even so, SVA1000X models are single port VNA in that only the TG (Port 1) can provide a stimulus to provide S11 and S21 measurements so for S12 and S22 measurements the DUT needs be reversed.

author: killingtime
author_rank: Regular Contributor
timestamp: June 02, 2023, 12:10:51 pm
content: After about a week of testing I found out why the results were off.The RBW and VBW settings have a big impact on displayed attenuation when using the tracking generator.I couldn't get the analyzer to show the correct attenuation value with default VBW settings.I had to manually override the VBW down to 10 KHz, and leave the RBW at default (1 MHz) for a 3 GHz span. Narrowing the RBW to less than default also shifted the readings. Plot averaging was set to 100, but 10 would have worked just as well.Siglent have a AppNote on making attenuator readings, but it says nothing about changing RBW or VBW settings. Screenshots just show them at default.https://siglentna.com/application-note/attenuator-verification-spectrum-analyzer/Here are 3 plots of a 40db attenuator showing different RBW and VBW settings. The most accurate results are found with a 1MHz RBW (default) and a 10KHz VBW (although 30KHz wasn't much different).rbw30k_vbw30k.jpg - Reducing the RBW down to (non-default) 30K has a huge impact. Almost 4.5dB off.rbw1m_vbw1m.jpg - Default RBW and VBW settings (1 MHz). Off by about 0.5dB.rbw1m_vbw10k.jpg - Default RBW but a manual VBW of 10K. This is the most accurate plot.If you're wondering how or why I think the last plot is the most accurate, I contacted minicircuits w/serial number and they very helpfully handed over the test data for the sample of attenuators tested in the same batch that my device came from.Top marks to MiniCircuits for customer service!I even got to speak with an engineer.I also ran some tests using Zero Span, and T-Power for spot readings. T-Power was the most accurate, as it didn't seem affected by changes in any RBW or VBW settings. T-Power results agreed with the last plot (rbw1m_vbw10k), and were also within the test data range provided by minicircuits.If you do use T-Power to make spot readings, I found that there's a technique for the most accurate results. Normalize the analyzer with a pass through on the cables - take a reading, then connect the attenuator and take another reading. Don't change the frequency between readings. Just moving the frequency off target and back again results in a small error. This technique does mean that you have to fiddle about with cables for every different frequency reading though.

author: tautech
author_rank: Super Contributor
timestamp: June 02, 2023, 07:40:35 pm
content: Quote from: killingtime on June 02, 2023, 12:10:51 pmYep, standard SA operational procedure !One of your realier screenshots displays the UNCAL warning indicating you changed settings after Normalize.When chasing those last few dB everything needs be done methodically.