Build a low freq DC-Block with transient protection - Page 1
Forum section: electronics / rf-microwave

author: apetersw
author_rank: Newbie
timestamp: January 31, 2025, 03:36:31 pm
content: I was out to build a simple low frequency DC-Block, just using a large capacitor in order to probe power rails or to measure S11 on power rails. Then I had to realize this task no being so super simple.- 1 Point: a simple X7R MLCC will show a DC-Bias effect. So the effective capacitance when calibrating the VNA will be different from the value when probing. This might only effect the low frequency side, but that is where I want to look at.- 2. Point: Port protection: With a big capacitor in series, connecting and disconnecting to a hot power rail will produce charge and discharge pulses, transients to run into the connected 50 Ohm port. That might kill my instrument.So I started some research and found- Steve Sandlers "Port Saver™ Instrument Protection DC Block"https://www.picotest.com/Port-Savers.html.and an open source solution, using a 450µF 450µF Bipolar Elektrolyt together with two RF PIN diodes (CLA4607-085LF)-https://www.limpkin.fr/index.php?post%2F2023%2F03%2F28%2FThe-DC-Block-For-Low-FrequenciesMr. Sandler writes about "Tantalum construction materials" how is that working? Can u use a non polarized cap in such an application?What do u think of using a 10µF foil capacitor followed by a resistive 1dB T-Type attenuator. Behind the fist 2.3Ohm resistor shunting with a SMAJ5.0 to GND and behind the second resistor shunting with a BAV99 or a TVS.I dont have a SMAJ5.0 at hand to measure its capacitance. The diode might show to mutch capacitance to be used at higer frequencies. However I think 50Mhz will be sufficient.

author: TimFox
author_rank: Super Contributor
timestamp: January 31, 2025, 04:23:12 pm
content: I have used 10 uF or 1 uF polypropylene capacitors loaded by 10 kor 100 kto check for 60 and 120 Hz ripple on DC rails, but the self-resonance of those capacitors is far below 50 MHz.

author: Marco
author_rank: Super Contributor
timestamp: January 31, 2025, 04:33:52 pm
content: Rubycon has SMD 10uf film capacitor, combine with USB 3 TVS?

author: TimFox
author_rank: Super Contributor
timestamp: January 31, 2025, 04:41:09 pm
content: Those capacitors look good, but the self-resonant frequency for 10 uF is only 500 kHz (which is quite respectable for 10 uF).

author: apetersw
author_rank: Newbie
timestamp: February 03, 2025, 01:43:16 pm
content: Quote from: TimFox on January 31, 2025, 04:41:09 pmI took a measurement of a 10µF Rubycon thin film acrylic dielectric 35MU106KD35750I would say this looks pretty good. Using this cap in a DC Block what will be the problem beyond its resonant frequency or beyond 1Meg Hz? It will still block the DC and it will be low impedance.

author: TimFox
author_rank: Super Contributor
timestamp: February 03, 2025, 02:02:07 pm
content: Above self-resonance, the impedance becomes inductive and rises with frequency.

author: jonpaul
author_rank: Super Contributor
timestamp: February 03, 2025, 03:18:26 pm
content: Most SA Zo are 50 (or 75 Ohms)For RF analyzers, the LF corner is 10...100 kHz.Thus the DC block is usually in the 30-200 nF range. Lytics are bad for HF use.For microwave use > 100 MHz, we use 30 pF works fine.We use a A constant Zo enclosure with bulkhead mt BNC/Type N connectors.MiniCircuits has excellet DC blocks art very reasonable cost.Transient protection devces cant be trusted to protect SA mizersZeners, TAZ, MOV, are ineffective and add shunt C.Just use an ext attn or BE CAREFUL, dont blow your SA mixerEnjoy, 'JonEE68

author: Marco
author_rank: Super Contributor
timestamp: February 03, 2025, 04:58:36 pm
content: 10 uF on a side path, with 100 nF straight through, then USB 3 TVS connected between channels to make it bidirectional, then 9 dB attenuator?

author: joeqsmith
author_rank: Super Contributor
timestamp: February 03, 2025, 05:00:03 pm
content: Quote from: apetersw on January 31, 2025, 03:36:31 pmHow low of a frequency are you needing? What sort of voltage are these power rails you plan to couple to?You could certainly damage your equipment doing this. You never state what the instrument is. If you want something generic, I would hunt down a TVS that doesn't perturb the signal you are after. 50MHz shouldn't be a problem. Depending on the cap size/voltage... you may need an attenuator to limit the current to the TVS.

author: apetersw
author_rank: Newbie
timestamp: February 04, 2025, 12:16:30 pm
content: My plan is to probe power rails from 1V through 3.3V, 5V, 12V up to 24V.Shunt through impedance measurements using two of these DC-Block with VNAs like Bode100 or even nanoVNA should work.The frequency range of interest is in between 100Hz to 50MHz.Protection should work for input ports specified at max +20dBm.Here is my current schematic.R4 serves as a discharge R.C1 and C2 will provide a 3dB conner as low as app. 200Hz (a compromise).R1, R2 and R3 will form a 0.75dB attenuator.R1 and V1 together with R2 and V2+V3 should do the protection, even when hookong on directly to a 24V rail.