DC Block response - Page 1
Forum section: electronics / testgear

author: Jester
author_rank: Frequent Contributor
timestamp: September 30, 2016, 10:35:41 pm
content: I picked up one of these on eBayhttp://www.ebay.com/itm/Attenuator-DC-Block-N-male-to-Female-DC-6-0GHZ-50ohms-RF-coaxial-Power-2W-200V-/361279079925?hash=item541de945f5:g:XAQAAOSwPgxVOetNInitial test was with a signal generator and scope, it blocks DC as advertisedNext test I swept on the SA and it looks like this, what am I doing wrong?

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2016, 10:42:23 pm
content: Your screendump doesn't make any sense. The centre frequency is supposed to be 15MHz but your marker shows 89kHz. Perhaps you should reset the spectrum analyser and test again.

author: hendorog
author_rank: Super Contributor
timestamp: September 30, 2016, 10:48:34 pm
content: Quote from: nctnico on September 30, 2016, 10:42:23 pmIt's a log frequency scale - the 'centre' value is 1.5 divisions in from the Right edge of the screen!

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2016, 10:54:19 pm
content: Quote from: hendorog on September 30, 2016, 10:48:34 pmI missed that! But appearantly so did to OP because it would mean the left part of the screen is in the Hz range which is where (AFAIK) all spectrum analysers show a zero level.

author: Jester
author_rank: Frequent Contributor
timestamp: September 30, 2016, 10:56:56 pm
content: Ooops and not to mention the TG minimum frequency is 100kHz

author: hendorog
author_rank: Super Contributor
timestamp: September 30, 2016, 11:07:13 pm
content: Quote from: nctnico on September 30, 2016, 10:54:19 pmYep, you passed the test - you are humanIt's still within spec for the SA at 90kHz at the marker (spec says 9kHz) so I'm thinking the LO feedthrough shouldn't take over at that point.However google says that that a Siglent TG starts at 100kHz, so just out of spec for that.https://mediacdn.eu/m/media/wysiwyg/siglent/Downloads/DataSheet/SSA3000X_DataSheet.pdfThe 'stair-step' looks a bit odd too. Like it is operating at the limit of the ADC, perhaps the atten/gain settings are not sensible for this measurement? Hit preset and try again?

author: TheSteve
author_rank: Unknown
timestamp: September 30, 2016, 11:26:29 pm
content: I love that the listing says it is good from DC-6 GHz - yet it blocks DC.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 30, 2016, 11:52:30 pm
content: Hi,The frequency response is given by:F = 1/2 x PI x RCIf R= 50 Ohms (Spectrum analyzer input)And C = 100pFThe F (-3dB point) = 31.8 MHzThis is consistent with your measurements.If you use the same DC block on a scope with a 1M Ohm inputThe -3dB frequency is 1.59kHzI assume that in this context DC-6GHz means low frequencies to 6Ghz.But for some applications, like EMC measurements, a higher value capacitor is needed.22nF will give you a -3dB point at 150kHz with a 50 Ohm SA.Regards,Jay_Diddy_B

author: Jester
author_rank: Frequent Contributor
timestamp: October 01, 2016, 01:41:38 am
content: Quote from: Jay_Diddy_B on September 30, 2016, 11:52:30 pmJ-D,Your analysis is correct, looks like I need to find a DC block with a larger capacitor for conducted emissions testing.Updated graph with 50scope termination as well as TG/SA from 100k to 100MHz

author: Performa01
author_rank: Super Contributor
timestamp: October 01, 2016, 10:58:24 pm
content: Quote from: Jester on October 01, 2016, 01:41:38 amI use this one from Mini Circuits and can highly recommend it:https://www.minicircuits.com/pdfs/BLK-89+.pdfEven though it’s rated from 100kHz to 8GHz, for most practical purposes it actually starts much earlier:This means -0.5dB error at 15kHz.It has SMA connectors though.Mini Circuits also have an N-type DC-Block, but that also is 10MHz to 6GHz, hence very similar to the one you have.

author: Jester
author_rank: Frequent Contributor
timestamp: October 02, 2016, 11:45:38 pm
content: Quote from: Performa01 on October 01, 2016, 10:58:24 pmThanks for that, I found that exact one when I searched after J-D's response.