DC Block for spectrum analyzer: minimum safe low cutoff frequency - Page 1
Forum section: electronics / testgear

author: matthuszagh
author_rank: Frequent Contributor
timestamp: May 31, 2022, 05:28:42 pm
content: Most DC blocks for RF signals seem to have a low cutoff frequency of 10 MHz (corresponding to a series capacitance of 318 pF for 50 ohm Z0). Presumably, this is generally considered safe for an SA input to protect against a transient from a reasonable DC voltage. I have 2 questions: (1) how do we quantify what an acceptable level of input power or energy to the SA input is? and (2) how low can we set this cutoff frequency before the transients start to get dangerous.1: for instance, do we set thresholds for max power and average power through a 50 ohm resistor? What would reasonable values for these thresholds be?2: it's possible to find DC blocks with much lower cutoff frequencies (for example this one (https://www.centricrf.com/content/pdf/C0927.pdf), which goes down to 9 kHz).Obviously, if it's possible to answer (1), that will also answer (2).

author: bob91343
author_rank: Super Contributor
timestamp: May 31, 2022, 05:42:32 pm
content: The governing issue here is the thermal time constant of the input circuitry. I would imagine something on the order of 50 or 100 milliseconds, corresponding perhaps to 10 or 20 Hz. And down at least 10 dB at this frequency.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: May 31, 2022, 06:41:15 pm
content: Quote from: bob91343 on May 31, 2022, 05:42:32 pmThis is helpful, thanks. Out of curiosity, how did you arrive at that thermal time constant? Is there somewhere I can read more about this?

author: bob91343
author_rank: Super Contributor
timestamp: May 31, 2022, 11:04:46 pm
content: It was just an educated guess. I figured one second was much too long - the parts are tiny.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 31, 2022, 11:15:28 pm
content: You can get them with a much lower cutoff like the P2130A. I think you are confused about input protection. Fast transients like ESD will blow right through...I would consider the min / max freq, voltage, how tight of tolerance you need (effects of the voltage)...Maybe invest in a humidifier and not poke the equipment with things you shouldn't? Maybe look at finding some sort of limiter? Maybe make a limiter?

author: matthuszagh
author_rank: Frequent Contributor
timestamp: May 31, 2022, 11:51:39 pm
content: Quote from: joeqsmith on May 31, 2022, 11:15:28 pmThere are two different considerations here: (1) is protecting against fast transients, like you mention, and (2) is protecting against low-frequency - particularly DC - voltages of sufficient power.A DC block is not the right tool for protecting against fast transients and I didn't ask about that sort of protection. I specifically asked about protection against applied DC voltages (or low frequency signals). Now it may be that the low-frequency cutoff of just about any DC block you buy provides ample attenuation for the spectral content of reasonable DC voltages, but that was precisely the point of the question. What is good enough and how do we know? I want some way of quantifying this.Quote from: joeqsmith on May 31, 2022, 11:15:28 pmI'm exceedingly careful with the input of my SA and even with the DC block I leave at the input I never apply a DC voltage to it. I also use it at an ESD-safe workbench. I'm also not asking this question so that I can do stupid things to my SA. There are low frequency tools including high-impedance SAs that work here. The DC block is simply a form of insurance in case I accidentally do something stupid. But more importantly, the point is to understand the underlying principles so that I can answer these sorts of questions for myself.My SA goes up to 22 GHz and I doubt I'd be able to construct a limiter that reaches up that high. You can buy these but the ones I've seen would cost me the same or more than I spent on the SA - doesn't make a whole lot of sense to spend more on insurance than the cost of the device it's insuring.And, while limiters are great devices, they have their own limitations. They're non-linear devices and create distortion even before the limiting action is triggered. For many applications that distortion is sufficiently low to not matter, but they're not some magical device you apply that solves all your problems.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 01, 2022, 12:19:14 am
content: Quote from: matthuszagh on May 31, 2022, 11:51:39 pmAttaching to a DC source can certainly induce a fast transient. Same for removing it.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 01, 2022, 12:32:18 am
content: Quote from: joeqsmith on June 01, 2022, 12:19:14 amI know, that's why I asked the question. A DC block with a higher cutoff frequency will attenuate that signal / shorten its duration more than a DC block with a lower cutoff frequency. Again, my question is how do I quantify this in terms of damage to the input? What sort of peak or average power dissipation is too much, and how is this modeled? Just looking at the max input power of the SA isn't very useful here, since the initial power of an applied and sufficient DC signal through a block will exceed this, but that doesn't mean it will damage the SA.

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2022, 12:59:38 am
content: A DC block with an RF limiter sounds like a very good idea to me.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 01, 2022, 01:02:44 am
content: QuoteYou could characterize your worse case transient and test some number of SAs to failure to get some idea. An expensive but maybe interesting study.

author: bdunham7
author_rank: Super Contributor
timestamp: June 01, 2022, 01:31:29 am
content: Quote from: joeqsmith on June 01, 2022, 12:19:14 amTrue, but a series capacitor won't do much about that. The DC block in front of an SA or other instrument will prevent thermal damage from theslowtransient only if it limits the total energy per event to less than the damage threshold. To calculate that you need the capacitance of the DC block, the maximum DC voltage you expect the system to survive, the SA or device input impedance and the minimum Joules to explode it.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 01, 2022, 02:23:53 am
content: Quote from: bdunham7 on June 01, 2022, 01:31:29 amPlease correct me if I've misunderstood, but it sounds like this is saying that the event of a DC voltage applied to the input through the block (RC highpass filter) is short enough to assume the input circuitry approximately doesn't dissipate any energy as heat to its surroundings, so we only need to consider the total energy of the signal applied to it. If that's true, is it temperature rise that ultimately kills the input? If so, how would one determine the temperature threshold? If not, how do we calculate the minimum damage energy?

author: bdunham7
author_rank: Super Contributor
timestamp: June 01, 2022, 03:03:39 am
content: Quote from: matthuszagh on June 01, 2022, 02:23:53 amI suppose, in the same way that blowing any overloaded component usually involves temperature rise at some point, perhaps with overvoltage as an occasional exception. But knowing the amount of energy it takes to fry the inputs of an SA would require knowing all of the details of the input circuitry. For a 50-ohm oscilloscope input, it is just a 50R resistor that can probably take some surge and the input as a whole doesn't mind DC at all. That's easy. An SA with a 0VDC input spec probably has something close to the input like a diode mixer that can't tolerate any DC, or at least not much, from a low-impedance source.So to answer your original questions 1) you can't model anything as a 50R resistor here unless you know that is where the issue with DC is, like in an oscilloscope. and 2) obviously you found one, but as to whether a 9kHz DC block is 'safe' for an SA with a 10MHz lower bound is unknown andwould depend on the DC voltage applied in any case. I would assume that a given block would be adequate for an SA with a minimum frequency in the same ballpark--say a 5MHz block for a <10MHz SA--and a reasonable applied DC bias. What's reasonable? No idea. Probably at least the maximum peak AC voltage. And in many cases, probably a lot more.Edit: I just realized that the DC block you posted is SMA not Type N, so that's pretty amazing that they can fit a ~350nF capacitor in there.

author: bob91343
author_rank: Super Contributor
timestamp: June 01, 2022, 04:40:28 am
content: It's almost always temperature rise that destroys input circuitry. Exception would be if you are working with voltages high enough to cause arcing.Even a zener diode can handle a large transient as long as its duration doesn't cause local overheating. When a device fails from a transient, it's because local heating melted something.I have some old analog power supplies that I use on the bench. When powering circuitry with them, I disconnect the supply output before shutting it off because many of them have outputs that rise during turn off. A bit off topic, but if you have a feel for what's going on, you can guard against some things.

author: jonpaul
author_rank: Super Contributor
timestamp: June 01, 2022, 05:47:54 am
content: Decades ago a Solar LISN with dual L1/L2 capability via switch was coupled to a TEK SA 50 Ohm.The SA front end mixer blew instantly as the switch applied a 1/2 Vline transient, with a large series cap perahps 470 nF.Both a 1..10 MHz HP and transient limited are needed.Check the SA specs for max V and max power, nomally 500 mWJon

author: bob91343
author_rank: Super Contributor
timestamp: June 02, 2022, 04:29:24 am
content: Are you trying to say that you need to think about this? The OP opened a can of worms. Yes, it's always heat that destroys electronic components (assuming no physical damage). Even an arc has a hot spot and the temperature could easily exceed a safe value.Connecting dc via a dc block doesn't protect anything unless the rate of change of the signal is slow enough. Otherwise the leading edge will pass through and perhaps last long enough to raise the local temperature excessively.I remember hearing on the radio about fire damage. The lady, who knew what she was talking about, said that everything will burn if the temperature is high enough.

author: jonpaul
author_rank: Super Contributor
timestamp: June 05, 2022, 10:15:56 am
content: Rebonjour: Usie of SA for EMI, esp conducted with LISN can result in DC, high level AC transients, ~ 1/2 peak of mains, etc.We use:1/ External 20 db 50 Ohm atten2/ DC block3/ Set SA on LEAST sensitive RF/IF range4/ Dual diode transient clamp (low capacitance)After you see the levels, you can gradually relax the protection to get the required sensitivity.Believe me a dead preamp or mixer is NOT a simple or cheap repair on most SA.Jon

author: Feynman
author_rank: Regular Contributor
timestamp: June 05, 2022, 11:12:56 am
content: Quote from: jonpaul on June 05, 2022, 10:15:56 am5/ Never switch something on/off or plug something in/out, when the SA is attached