Spectrum Analyzer DC Block Question - Page 1
Forum section: electronics / beginners

author: mainman
author_rank: Regular Contributor
timestamp: February 04, 2024, 05:20:23 am
content: Don't want to blow out the front end of my Siglent SSA3021X. My question is I don't have a DC Block on hand, but I do have attenuators. My question is do attenuators pull double duty and block DC also.

author: rteodor
author_rank: Frequent Contributor
timestamp: February 04, 2024, 06:48:06 am
content: No they doNOT.They show 50 ohm (or whatever) ACand DCto any circuit. Without DC block the probed circuit could also be also in danger.Measure them with a DMM.You could at least use a capacitor (where DC is present) but do not count on proper results.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 04, 2024, 12:51:08 pm
content: Generally spectrum analysers are not used in the same manner as an oscilloscope, you would normally connect them to 50 Ohm sources, such as the output of a power attenuator, or where you know the expected output level, for example the feed to a diode ring mixer.However, you can make a high impedance probe that will protect the spectrum analyser, there is a design by Marek in the files section of the TinySA group,https://groups.io/g/tinysa/filesYou will probably have to join the group in order to view the files. The attached image shows my version of the probe, the PDF is the article by Marek. The probe is powered by 5 Volts from USB, takes a maximum input of around 20 Volts peak to peak with a saturated output level of +6 dBm, or with the alternative isolated probe an input up to 100 Volts peak to peak.SJ

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 04, 2024, 01:51:14 pm
content: What are you intending to measure? The output of a transmitter? Or the freq of an oscillator? Or something else? Let us know. For lots of things you can use a loop antenna on a Spectrum Analyzer by just using a Scope probe and attaching the Gnd lead to the tip and putting it near the source you are attempting to analyze. This works surprisingly well.This will give you frequency but magnitude will be hard to quantity.Scope probes are not DC blocks either.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 04, 2024, 05:58:06 pm
content: Quote from: mainman on February 04, 2024, 05:20:23 amThey attenuate DC and AC alike (most are 50 ohms both ends) - but note that many circuits won't tolerate a DC 50 ohm load on an output, so you would need a block capacitor too. Normally you'd use a bias-T for this (and not connect the bias).For general probing a higher impedance probe is very useful, for instance a 100:1 5k:50 ohms attenuator can simply be a 5k resistor on the end of 50ohm cable. But of course you lose 40dB sensitivity...

author: mainman
author_rank: Regular Contributor
timestamp: February 05, 2024, 01:43:45 am
content: Quote from: MarkT on February 04, 2024, 05:58:06 pmThanks for the help all you guys.I want to measure output power of my hand radios. Also wanted to hook the analyzer to my wave form generator. I am aware of using attenuation, I am just afraid of feeding the front end any DC by mistake

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: February 05, 2024, 09:41:08 am
content: If you look at your spectrum analyzer's input port, you'll see it can handle 50V DC (it has an internal DC block).https://siglentna.com/wp-content/uploads/dlm_uploads/2024/02/SSA3000X_DataSheet_DS0703X_E05B.pdfOn page 5 of the manual, you can see it says +/- 50VDC is allowed.I'd worry more about the 1 watt power rating. Don't put more than 30dBm or 1W RF in. If your radio is 10 watts, get a 20dB attenuator or something. If your radio is at 27MHz or some other low enough frequency, you can get away with making a DC block by soldering two BNCs with a capacitor in the middle.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 05, 2024, 09:49:25 am
content: For transmitters up to 100 Watt and up to around 700 MHz, the RF tap in this article will work well. It is easy to make, one I built is within +/- 1dB from 9 KHz to 500 MHz (measured using calibrated test gear).http://on4khg.be/wordpress/wp-content/uploads/2015/02/Simple-RF-Power-measurement-W7ZOI-W7PUA.pdfYou should add a 20 dB attenuator on the output if using more than a Watt or two.SJ

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 05, 2024, 03:47:53 pm
content: I have never had any problems with DC on the output of hundreds of radios. Besides, the attenuator will not only decrease RF but DC also. It is a bunch of resistors. It will not eliminate either.Make sure your attenuator can handle the wattage and will decrease signal to level that the SA can handle.I do not think SA in general is a good way to measure power output.Here is another thread:https://www.eevblog.com/forum/rf-microwave/performing-rf-power-measurement-with-ssa3021x/

author: pdenisowski
author_rank: Super Contributor
timestamp: February 05, 2024, 05:44:36 pm
content: Quote from: EggertEnjoyer123 on February 05, 2024, 09:41:08 amThis. Most handheld radios have a Tx power in excess of 1 watt (+30 dBm), which is usually the upper limit on spectrum analyzers.Even a fairly massive attenuator (like this one) is a lot cheaper than a spec an front end

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 05, 2024, 08:24:10 pm
content: And if you get such a nice attenuator, likepdenisowskishows. you could put a little load on the output and have a nicehigh wattage 50 ohmload.The bigger the load the better.I try to pick up such stuff at Hamfests.

author: mainman
author_rank: Regular Contributor
timestamp: February 05, 2024, 10:59:13 pm
content: Quote from: EggertEnjoyer123 on February 05, 2024, 09:41:08 amYeah I know the spec on my front end has a DC block but I aint going to have blind faith in it, this thing is getting a external DC Block. I have a wide range of attenuators on hand up to 50db.Thanks for the feed back guys