Build this Spectrum Analyzer Protection Circuit 

Outstanding stuff. Love the fact that you're an obvious expert, fastidious maker, precise technician and have an artist's eye for color, layout, aesthetics and efficient form. Add to that your vids are clear and you take the time to make sure the subject is well lit and focused and the audio is always right up front. AND you have great diction!! I'm an old retired two-way radio tech and now I just fool around with amateur radio for fun (for 50 years) but I'm always delighted to learn something new, or see something visually confirmed that I always knew. Thanks so much, Mr. C. and many 73 JEFF K4UIE

Once again, thank you for your time and sharing your expert knowledge for the benefit of us viewers. Much appreciated.

Once again, another great instructional video. The limitation for many is crafting superior printed circuit boards that even approach the apparent quality of yours. Here again is a marketable product for those of us who may prefer to buy than build. Thanks again! Fred

If you ever shot a analyser front end you know how important this little box is! Great video Paul!

I'm new to electronics. Thank you for sharing your vast knowledge with us all on RU-vid. 100 year's from now, people will be watching this for sure!

Thank you for such a great quality videos! : )

Genius is simplicity. Spot on.

I always enjoy your videos. Thanks.

I did enjoy the video, nice design .

You're one of the best video blogger ever!!! Fantastic :)

Thanks for the lightning fast production in response to our queries! Well done! 73 - Dino KL0S

More enjoyable learning, many thanks for sharing your lab tools, appreciated.

Another excellent video and thanks again for sharing your design. I work with spectrum analyzers at work, but don't own one. Hopefully one day I can add one to my collection and incorporate this circuit. Best, Don

Thank you for the great video. Very well explained. All the best. Bobby

Great project Mr Carlson. I will be using 1n914's as I have tons of them. I thank you, and my 8566B thank you.

Very nice, you have given me some ideas about how to protect some of my own equipment.

Amen on the being careful what you put into your spectrum-analyzer. I have to REALLY have a compelling reason to hook mine up directly to any circuit. I have an HP-8557A which has a 30VDC but ONLY 2.2VACRMS input capability! I've had to repair this unit and it is not for the faint-of-heart (just look up a manual for one of these and you'll see!) as it is full of specialized and one-of-a-kind parts that are nearly impossible to find. In the past, I used a very small-value capacitor and a 120dB step attenuator, then "work my way up" to getting a usable/displayable signal. So, anyway great video on both the value of "light" coupling and protection of the SA's front end. Also, nice construction project and showing the value of placing a "service-loop" on the BNC center-pin to prevent breakage/damage to the PCB. Finally, another project idea might be to make a high-gain broadband RF amplifier with a detector that would allow your viewers (without spectrum analyzers/scopes) to be able to both very lightly-couple and display the amplitude on a analog multi-meter for alignment purposes...

Another great video Paul. Very useful flat, wide frequency ranged coupler. I've never built a PCB using the etching method and laid out on the computer. It would be great if you showed us how you do it. I know, I said to myself; "What, that's it?" LOL I am so use to your long drawn out sagas. I "LOVE" long drag out sagas! I see approx 3,350 watches on your DeForest Crosley Empress video as of this moment. Congratulations are in order here! Great work my friend, Tom

Yes I believe another name for it is a "DC block". I used to use one on the input of the spectrum analyzer when I was probing around an RF circuit sniffing for frequencies I was looking to tune with voltage controlled oscillators. The DC block provided protection for the spectrum analyzer in case you inadvertently hit a voltage test point which could blow out the face plate of the spectrum analyzer.

Very helpful, thank you.

I was wondering if you can do any videos on VNAs or using one do do an alignment? I'd like to see that. Great video!

Nice little circuit :-)

Excellent video congratulations, as all your videos are! I would like to see a version to be able to make these measurements with my 50homs spectrum analyzer :(

Thanks for the great video. Very informative, as always. My HP 8590B only has a 50Ω input. This may seem like a dumb question, but…would an inductive coupling scheme be a better way to provide attenuation, isolation and impedance matching for 50Ω input S.A.s, or is it best to stick with capacitive coupling?

always great when something you slap together works even better than expected?

Very interesting video. Thank you. I would welcome your thoughts about building an isolator for the audio jack for smartphones that would allow various apps such as signal generators and spectrum analysers to be used in a safe manner whereby the smartphone is electrically isolated from the circuit under test. Ideally this should be both way (output and input).

Hi Paul, I am a novice in electronics, can you make a video on how to use a spectrum analyzer? I know there are videos on this on RU-vid but you have the gift of teaching and your level of detail is perfect. Thanks

Very nice video. I am adding my request for a impedance matching for 50 Ohm input on my spectrum analyzer. I am wondering if you really need to worry about impedance matching as long as you get your flat line when using the tracking generator.

Nice and clean projecto Tnx for share another piece of art 73

Great video and advice, as always. Thanks a lot!

HP used to sell a DC block to connect to the front end of your spectrum Analyzer, Also they had reverse power protection for some of their Signal Generators. The DC block was nothing more than a cap. Try changing your volts/div to a smaller value and see what your curve looks like? Great video keep them comming.

Thanks Paul, great video! Exactly what I was looking for! Do you think there will be any issue using perf board and through hole components? Thanks again. 73 Chris

This is a great thing and video! Do you know where you can get something like this on the market already assembled? My days of working with tiny parts are in the past, unfortunately. Thanks again for a great video.

Thanks, enjoyed the video. Would something like this also work with a Scope input to reduce capacitance, so you don't detune an IF stage? Would it still be useful if built with non-SMD components? Thanks again,

This guy is the BEST in the U.S.

I built one snd it works quite well. I used 10 1N4004 diodes insted of 12. Thats all I had. Thanks

I reproduced this circuit using Circuit Maker and etching it to a single-sided copper PCB - using 1206 caps. I only added a 50 ohm resistor to ground before the 10nf cap at the output. I made a pair of them and so far, so good. I’ll try them on the IF alignment of a SX-99.

Beautiful. You are a crack Mr ..!

Thanks Paul for your videos. I am restoring a Hammarlund HQ-129-X. I do not have a spectrum analyzer but I do have a late model Tektronix 2 channel oscilloscope and I just bought a Siglent SDG1025 function/sweep generator. The scope has a 1 Meg input impedance and the function generator can be selected for a 50 ohm or Hi Z output. The back of the function generator has a trigger output that I will feed to the oscilloscope trigger input. Can you tell me what type of protection circuit should be on the output of the function generator and what should be on the input of the oscilloscope to align the IF? I think I need to build a demodulator box to rectify my signal coming into the oscilloscope. Thanks, 73 Mark

Thank you! I stumbled upon your channel a few days back and guess what i watched nearly every video even thought i have little to nothing to do with RF or HAM radio (got a little RTL SDR decoding Pocsag,FSM(german) and sometimes i tune in for voice) It's just stunning how you can ban (mesmerize?) me and i absolutely love the 20+ min videos you just take your time to explain everything in detail! Great! Maybe the Dave Jones of radio technology?! Keep up the good work SUBSCRIBED

I'm interested in learning how you're calculating how many pF of coupling you have there through the capacitors.

What mod is required for a 50 ohm input I am in the process of getting one of the RF explorer spectrum analyzers and I want to protect the front end of that they also recommend using a 30db attenuator on the inputs even when stored to protect the input the front end front from large RF voltages which I already ordered 2 of them to protect the unit. I would build one or of the units you have shown in the video.

This simple little video has generated a great number of comments. This is certainly an interesting device. Now for my question. . . . Would it be possible to show how you make your circuit boards? They look professionally done. I dabbled in this about 15 years ago making attenuators at work. My boards worked fine, but the sure didn't look as good as yours do. Thanks again. Nice video!

Great video, as always! It's also very timely for me since I just picked up a HP 3588A analyzer. One quick question: Is there a design consideration for putting 8 capacitors in the circuit? I won't be using surface mount components, so could I just use a single 12 pf high voltage (6 KV) ceramic cap as an equivalent? Thanks again, and keep up the good work ... so much to learn.

I'm currently building my own small spectrum analyser, quite a basic one, however I'll add something like this to the front end, really good video as always Paul. Paul M0BSW

just a question what software for your video did you use if you dont mind me asking?

Great video as usual. Paul I need the 50 ohm impedance I think for my old HP 141T. Can I use a 10x Scope probe and a 2w dc block like you find on Amazon to keep my Analyser safe??

Another Great video Paul! Very kind of you to act so quickly to all of us who wanted to know what was inside the magic white box. I like so many others have a Rigol with a 50 ohm input I might try to build up the combination of the two circuits and see what happens. It would be an awesome bit of kit if we had one with select-able output impedance. That last sentence was so Dave Jones! Do you not get a bit concerned with the alligator clips penetrating the wire you are lightly coupling too? I was thinking it might be better to use a different type of clip more like a scope probe clip.

What if you built a cheap spectrum analyzer using one of those RTL Dongles, it wont be accurate to a t. but your just snooping?

Hi John, looking at that very small and neatly made PCB I wondered what you use to solder all the "surface mount" components. Is it a conventional solder iron or what?

Sweet! Thank you a bunch!

were can i find a grid square plastic or glass for an old scope, which has a 6inch viewer round

Paul I wish you would make a video same but using a Rigol 850 Spectrum if you have one. Mine is a bit complex due to the menu and not great manual.

Thanks Paul. Maybe you could build one with a switch to change between 1M, 75 & 50 Ohms.

Fantastic work as always. I don't have a spectrum analyzer but I know that I've killed 2 MP3 players and 2 channels of my PC sound card with "high voltage". Sound card took about 200V spike from a Vacuum Tube Vu Meter project, the first MP3 player took a few KV from an ignition coil (I was trying to make a singing arc with a 555 timer) and the Philips MP3 player took about 7.5V reversed polarity and wen't kaput in the sense that the audio stage is dead. So after so much grief with them, I've considered making myself a protection box with some diodes, resistors and capacitors. Cheers :)

This is a handy little device! I don't have a spectrum analyzer at this time, but I'm using an old RCA WR-50B analog RF signal generator which has an output range of 85 kc to 40 mHz. What I've been doing is putting a capacitor in series with the leads from the signal generator for blocking DC. I have various rating caps that I keep on hand for different receivers since alignment procedures vary somewhat with make & model receivers. The clamping circuit is valuable. This may be worth me building one! With the BNC connector on the end of the box, it can pose an issue of shorting against other components & terminals in a chassis. What could be done is, install a hard-wired pigtail coax out of the box with a BNC connector at the end of the lead far enough away from the chassis. What's your take on that? Frank Ferraro--Audio Craft Electronics.

Hy, i have recently got a spectrum analyser, i will use it for old tube radio, but it hase a 50ohm impul, +10dbm and 0zero volts dc imput. Can u make a vid about how to use it and how to adapt this little boxes to protect my spectrum analyser?

Oh I haven’t seen this video yet ! Yay.

I would love to build this, any chance on a board sketch already made up?

Do you have the circuit board pattern file for this ? I would like to iron it on some copper and make a couple of these. I’m a member of your Patreon channel as well, but did not find it there. Thank you. Jamie WB4YDL

Hi and thanks for your infos. What about that B+ line. What means B+? Is there a B-? I saw it in a diagram of a VHS machine and I didn't understand the notation.

hi.....i really enjoy your videos..they are very informative can you please make a video on spectrum analyzer.. we or at least me would be very grateful....thanks again much obliged

Such a great idea to build this thing.. thanks mr Paul. I really enjoyed this video.. maybe some episode you can tell us too where we can buy those parts in a legit store or site that selling cheapes.. you know our money value here is very big difference than yours.. $1 is equivalent of php52 now..

What model is the HP spectrum analyzer?

Blow out the front end,--- which circuit board is the front end,---- I have a Hp 3580A.?

Any chance you could share the artwork for the spectrum analyzer protection interface so that I could make a couple of them for my new Rigol SA? Craig W9CLA

Why is one lead shorter than the other? Also, wouldn't the input capacitance be 12.5 pF in series with 0.01 uF, not 12.5 pF?

paul have you ever pushed its limits to see what frequency it drops out on'''' like trying it up to 30mhz and see if its still flat'''Nice video paul''''

Very good How much you charge for one you say you have extra parts I could never make one as nice as yours thanks Paul

Hi Paul !I was wondering how you would change this circuit to work with a Spectrum analyzer with a 50 ohm input.My analyzer has an input MAX RATING of + 20 DBM and +/- 25volt. ---- Very good videos many thanks, John

Berni Kendrick...................Great video Mr Carlson but like others I would appreciate a video on how you make your PCB boards they look really professional certainly better than mine.

I would like to buy the unpopulated circuit boards you made for them.

this video.... it went so fast! had me laughing there :)

Super video! Would you be willing to make the PCB drawings available so I can try my hand at making them (using your other excellent video as a guide, of course!)

After some research, I found that this will be a real problem on a 50 ohm input SA, basically everyone you find today.

Great video Paul - what am I going to do for the rest of the evening! I am currently scouting EBay for a Spectrum Analyser for IF alignments on tube radios (450KHz - 10.7MHz typically). Can you tell me what to look for bandwidth and resolution wise?

I have an question: if I have tube radio from 1962 do I have to change anything? or repair?

Hello ! I have seen the datasheet of the BAV99, and you say that limiter circuit limits 10Vpp, how do you know, if the current is low enough, that the diodes won't be "on" at 4,2V and not 5 and limiting to maximum of 8,4V and not 10V ? Maybe you never even get close to this values but...

Do you have any of the Spectrum Analyzer Protection Circuit boxes? Mr Carlson's Lab

Hello Paul I have a big supply of Attenuators I used for catv installs not sure but maybe 75 ohm being cable tv could these be used in 50 ohm lines ,would that be a mismatch,,wanting to use them for spectrum analyzer use

Where can I find some of that high voltage silicone test lead cable?

I have a question, have you ever used that conglomeration of parts and tested to see if it worked. I have my doubts that it would save the front end of anything. I worked for a cell phone company manufacture of test equipment and how we eliminated the possibility of overload the front end of the receiver part of the communications moniator was to sample the input power before it was connected. We used a diode as a peak detector driving an IC Compactor which operated a relay. If the power was sensed to be to high, it would NOT connect and would light a LED as an indicator of the power being to high.

after the bnc is it a fuse ?

15:00 "This is not intended for high frequency use." Tho those leads are probably the least of your problems in the high frequency domain, the row of capacitors are going to be a nice hairpin filter, and considering the vast amount of turns in the design it's going to have a lot of inductance. So yes, I see what you mean with not intended for high frequency use. Tho that box looks a little to small for a better high frequency design. Tho still a rather nice piece of equipment.

I just finished building two of these boxes! I created the pcb layout in Driptrace. I etched the two boards in a bath of muriatic acid and hydrogen peroxide. I used liquid tin to cover the copper traces. I soldered all the surface mount components on, which is my first time soldering surface mount. The test on my HP 3588A spectrum analyzer looks really good, so I'm looking forward to trying these out on my Hammarlund HQ-120X when it comes time to do an alignment. I posted a photo of the boxes: www.flickr.com/photos/87241965@N00/33516373892/in/dateposted-public/

Could you please mention the specs of this Protection circuit in text

Sometimes the most handy tools are the ones we make.

What's the chances to purchase some of the PCB's?

dear mr carlson

Fine job OM 73s de Sv2bnr

Hi Paul any ideas my instek S/A gsp-810 only displays from center to right not all across, if I connect it to computer it displays fine in monitor using the remote software

There is little capacitance at each of diodes.

man you really like to do things the professional way lol.

Another nice video. Thought you would have been tired from the DeForest Crosley rebuild. :-) Keep an eye out for an email I just sent using the address I found from About.

With all those caps in series and the shunt caps across the diodes you have limited the lower frequency response of the SA. I have used a SA for over 40 years and have never blown the front end of and SA. This circuit is NOT fast enough to protect the input of the SA. The only way to protect the front end of the SA is to use a fuse In doing so the circuit is more complicated.

I built the 1M unit for my tracking generator .,but my S. A. is 50 ohm input .How about showing the 50 ohm design. You sure know your stuff. Thanks PWS.

Nice Circuit, here is how to improve it: Instead of capacitors use resistors, they have flat frequency response, as you said this circuit has high pass frequency response. The value of the resistors depends on the input voltage, so you might use a switch to select the voltage range. Normally though, as you know, an attenuator is used to achieve what you are trying to do. You can also use inductors to ground after each capacitor, to make it like transmission line. The value of inductor and capacitor depends on what impedance you want your transmission line to be (75 Ohm or 50 Ohm).

paul'''''''i dont know what ''ohm in'' on my S.A.'''' its a avcom msa-90a'''''i called the company avcom and they are bewildered'''''they never ever heard of this model msa-90a''''''''all i know is i know it came from ''NASA'''kenedy space center'''''''if i cant get any info on this S.A.''''well i dont know what to do'''''do you have any idea's'''''''i want to build one of ur little cool item's''''

r