How to **Safely** and properly align a Transmitter with a spectrum analyzer  - Page 1
Forum section: electronics / beginners

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 13, 2019, 10:29:33 pm
content: Dear All ,I would like to thank everyone in advance that will answer my question/s and for their time ,So I have a Spectrum analyzer a "Siglent SSA3021X"so I learned how to tune filters by doing and failing and doing some light reading , and finally succeedingalthough I know that the front end of Spectrums are very sensitive into input power ,so as per recommendation of a friend I bought this :"100W N Attenuator 50db male to female DC-3GHZ 50ohm RF"https://www.ebay.com/itm/100W-N-Attenuator-50db-male-to-female-DC-3GHZ-50ohm-RF/272899586791Although still I am very skeptical if I can plug a transmitter at its lowest power output at 2W with this attenuator in line and still be safe from the maximum power input the Spectrum can handle ,Is there any other way ( mathematically to be able to calculate if I fall within the maximum acceptable range of my analyser and how to do this safely and properly )I was thinking to get a signal sampler although they are too expensive and almost nowhere to be found in a price range around 100$ 200$ maxSo the question is how is the most proper and safe way to align and check a transmitter (health) and generally see the bandwidth etcIf this question was answered elsewhere I am sorry in advancethank you in advanceThe_Spectrum.A_idiot

author: radiolistener
author_rank: Super Contributor
timestamp: August 13, 2019, 10:53:15 pm
content: yes, you can connect it.2 W = 33 dBm, after 50 dB attenuator it will be 33 - 50 = -17 dBm, it should be safe for spectrum analyzer.Before connect I suggest you to check if your attenuator works properly.And note! These Chinese attenuators have just one side which is intended to apply high power. If you connect transmitter to the wrong side, your attenuator will burn out.This is not trivial to understand which side is intended for high power. I found that Chinese manufacturers probably marking it with a small "SU" marking on connector, but this is not 100% information, just my finding. I burned out several attenuators in such way, because these attenuators don't have any instruction or manual or any kind of note about where is input and where is output and there is even no any mention that they are single way. And the worse thing is that sellers also don't know itSo it's just your luck to find where is proper inputI think that your attenuator probably has high power input on the bottom connector (see first photo on the product description). Check if it has "SU" mark. If it is present, then this connector probably high power. But I cannot guarantee...May be someone can suggest more reliable way to check which connector is intended for high power input.

author: hfleming
author_rank: Regular Contributor
timestamp: August 13, 2019, 11:26:56 pm
content: Another trick that we used used years ago to protect spectrum analyzers, is to use a «protection box» at the input of the analyzer. All it was was a 375mA fast acting picofuse in line with the RF input. (In series with the fuse, you can also add a 100nF 1kV cap just to make sure no DC can get in, as well as a high value resistor to GND to drain static, if you forget to disconnect you analyzer from your external antnnana). You can also use a PIN-diode limiter, that you can get on e-Bay. (Frequency respons might roll off, but you can easily compensate for it).But in anycase, a 50dB pad will definitely be good enough, even if you forget to switch your radio to 2W. (I assume you don’t have a 1kW transmitter).

author: fourfathom
author_rank: Super Contributor
timestamp: August 14, 2019, 01:25:02 am
content: I bought a 50dB power attenuator (Chinese), and measured the actual attenuation at 39dB. It was reasonably flat from DC to 1 GHz, so I just put a "39dB" label on it and still use it. I sometimes use it for testing my ham transmitters.Another test device I have is a -40dB tap. This has two BNC connectors with a direct "through" connection: one goes to the device under test, and the other to a dummy load (a cheap 100W unit). The third port is a SMA jack, with a 40dB "L-pad" attenuator, using a few low-power resistors. My simple construction gave me a flat response out beyond 100MHz.

author: tautech
author_rank: Super Contributor
timestamp: August 14, 2019, 02:08:12 am
content: Quote from: The_Spectrum.A_idiot on August 13, 2019, 10:29:33 pmWelcome to the forum.The datasheet on P5/9 sets out the maximum input parameters under Amplitude and Level:https://www.siglenteu.com/wp-content/uploads/dlm_uploads/2017/10/SSA3000X_DataSheet_DS0703X-E04A.pdfOf important note: Maximum average RF power 30 dBm, 3 minutes, fc≥10 MHz, attenuation >20 dBm, preamp offFurther in the Users manual in 2.1.3 Amplitude (P38-41/95) in 2.1.3.4 Units are the amplitude conversion relationship formulas.https://www.siglenteu.com/download/7918/There is 51dB of user input attenuation available but best practice with strong signals is to use external attenuation to get signals to low levels.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 14, 2019, 03:15:41 am
content: Quote from: radiolistener on August 13, 2019, 10:53:15 pmI dunno about Chinese stuff, but the normal convention with smaller attenuators is that the input to the attenuator is a female "N" connector, & the output a captive male "N", to make it easier to connect directly to a following test instrument.Although the one linked to is getting a bit close to the size where it could damage the SA's input front panel connectors due to its mass & mechanical advantage, I'm pretty certain it would follow that convention.Similar units from the USA, the UK, Japan, & Germany commonly have an arrow silkscreened onto the body, showing the signal path through the device, as do many of the large "thruline loads".Higher power "thrulines" & the like, often have female "N" connectors on both ends--- maybe to stop morons hanging them off the front of SAs, but more probably, so they don't need to use an adaptor to connect to a standard cable.At 2 watts it is doubtful that it would make much difference if the OP got the attenuator input & output mixed up.

author: 0culus
author_rank: Super Contributor
timestamp: August 14, 2019, 03:19:46 am
content: Also a quality directional coupler or RF sampling tee can be helpful. See:

author: vk6zgo
author_rank: Super Contributor
timestamp: August 14, 2019, 03:41:24 am
content: Quote from: 0culus on August 14, 2019, 03:19:46 amYes!It is well worth keeping an eye out for stuff like this, even if the original intended frequency of use is not near the one you are using.When my ex-employer scrapped their old Siemens TV Transmitters, we ferreted through them to find any useful RF "bits".There were several directional couplers, one of which was already "spoken for" for a project we had in mind.The other one was "up for grabs", so I put my hand up.Although the original application was around 200MHz, I have used it up to 470MHz, & sweeping it with a SA & tracking generator shows it is usable out to around 800MHz.It came in handy in a later job, when we had to retune RF amplifier boards that the Chinese manufacturer conveniently "forgot" to tune to our operating frequency!

author: borjam
author_rank: Unknown
timestamp: August 14, 2019, 06:15:35 am
content: The best way is to use a proper dummy load which will dissipate the transmission power with a RF sampling port. That way you can obtain a low level sample of your signal which is safe to feed to a spectrum analyzer.

author: tggzzz
author_rank: Super Contributor
timestamp: August 14, 2019, 07:03:49 am
content: Many spectrum analysers can't tolerate DC on their input. In this case adecentblocking capacitor is an extra safety measure.A blocking capacitor is not a panacea. It will affect the lower frequency visible, if there are large transients when the transmitter is switched on/off they could reach the SA, and many ceramic caps are non-linear and so would create intermod product.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2019, 08:04:38 pm
content: A directional coupler between the amplifier and load is safer than a big attenuator and they are more common than samplers and often more rugged.

author: fourfathom
author_rank: Super Contributor
timestamp: August 14, 2019, 08:19:02 pm
content: Quote from: David Hess on August 14, 2019, 08:04:38 pmSafer in what way? A directional coupler still has to have an appropriate coupling ratio (or additional attenuators), and the frequency range is generally much narrower than a resistive attenuator. I use directional couplers, but I do wonder about your "safer" comment.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2019, 09:10:29 pm
content: Quote from: fourfathom on August 14, 2019, 08:19:02 pmA directional coupler does not have to dissipate the power so there are fewer issues with overheating. This makes power attenuators and terminators less rugged or at least easier to misuse. Samplers have the same advantage but are not as common as directional couplers.Directional couplers and samplers can still of course fail catastrophically.

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:13:36 pm
content: Dear radiolistenerthank you so very much for the help yes indeed after having the onboard attenuator at 50 DB and the external one and transmitting 2 W i get a very small signal well with in the range of the front end of the Spectrum ,Thank you for your timeQuote from: radiolistener on August 13, 2019, 10:53:15 pmThis was very nice to know how to calculate everythingQuote from: radiolistener on August 13, 2019, 10:53:15 pmNo such marking was foundAlthough a friend with more experience told me that there are no such "sides" per say lastly no 'SU' mark exists I used it the way I found convenient the N type male on the instrument and N type female towards my transmitter (although data sheets of high end attenuators have sides per say in the datasheet )anyway it was very helpful and enlightening ,thank you for your time and effort and rely to my questionThe_Spectrum.A_idiot

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:18:50 pm
content: Dear hflemingThank you very much for responding to my post ,Quote from: hfleming on August 13, 2019, 11:26:56 pmFascinating box I may try to do that although for now that i am quite inexperienced with RF I would prefer a ready made commercial solution so that I won't kill anything by mistake ( as far as I know no warranty will cover idiot user mistake )but still thank you so very much for your contributionthank you for your time and effort and rely to my questionThe_Spectrum.A_idiot

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:23:25 pm
content: Dear fourfathomthank you very much for the contribution and the ideaQuote from: fourfathom on August 14, 2019, 01:25:02 amhopefully soon I may try to make this so called (RF sampler lets call it ? ) although I would use N type all over for my convince although the response in my application is a bit more critical to go up to 500 mhzanyhow when I will get more experience into making my own RF circuits I will try to do this ( seems very easy and a simple design with almost nothing could go wrong )thank you for your time and effort and reply to my questionThe_Spectrum.A_idiot

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:27:52 pm
content: Dear tautechThank you so very much for the links , I skimmed though most of them before buying the instrument although now it looks like I need to read it more and more thoroughly to get a better grasp of understanding my instrumentQuote from: tautech on August 14, 2019, 02:08:12 amnow I will most probably buy a more reputable 50db attenuator and call it a day or maybe even an RF sampler ,anyway it looks like I have my work cut out for me so I should get to readingThank you again for pointing out the details i've missed and for the links too ,Thank you in advanceThe_Spectrum.A_idiot

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:32:57 pm
content: Dear vk6zgoThank you for your time and response to point out several things I didn't even thought about ,Quote from: vk6zgo on August 14, 2019, 03:15:41 amThe direction of the attenuator ( yes indeed there are no markings , and indeed that is the most typical way i looked on more expensive attenuators on the web that the male part is to the instrument and female part to the transmitter )regarding the mechanical advantage once I plugged it on the SA I didn't think about it but now I am definitely buying a barrel connector and having a small patch cable for itthank you in advance for your time and ideasThe_Spectrum.A_idiot

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:39:38 pm
content: Dear 0culusThank you very much for the youtube video amazing channel definitely going to watch all his videosQuote from: 0culus on August 14, 2019, 03:19:46 amDamn that capacitive sampler looks amazing ( definitely making one out of copper pipe as the person on the video suggested)thank you so very muchThe_Spectrum.A_idiot

author: 0culus
author_rank: Super Contributor
timestamp: August 14, 2019, 11:46:04 pm
content: Alan is a forum member too; FWIW.

author: hfleming
author_rank: Regular Contributor
timestamp: August 15, 2019, 07:27:34 pm
content: Siglent USA has an app note describing an input protection circuit.https://siglentna.com/application-note/diy-spectrum-analyzer-input-protection/

author: fourfathom
author_rank: Super Contributor
timestamp: August 15, 2019, 07:49:52 pm
content: Quote from: The_Spectrum.A_idiot on August 14, 2019, 09:23:25 pmThe reason my tap was only flat to 100MHz was that I used 1/4/W resistors with long leads and paid no attention to high frequency layout. If you do it right (say smt resistors on microstrip PCB) there's no reason your tap couldn't be usable into the GHz range.And *thank you* for responding so nicely! Many times someone gets an answer and then disappears. We like to hear how it all comes out.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 16, 2019, 04:02:55 am
content: If your sampler varies in output overa wide frequency range, it isn't the end of the world, as sometimes you only need a small part of that range when testing.If you havd a tracking generator with a Spec An, you can determine if the range you need is fairly flat, prior to using it to test a transmitter.Even if there is a bit of a "slope" you can correct for that.This is how we did things back in the old days, when the flatnesss of equipment was not always a given.

author: The_Spectrum.A_idiot
author_rank: Regular Contributor
timestamp: February 06, 2020, 06:27:07 pm
content: Dear All ,I manage to find and order a Bird Model 4275 20mhz-1Ghz RF Sampler so now I even more safe with my measurements ,I would like to thank everyone for their help and comments about this issue I had , and enlightening meThank you in advanceThe_Spectrum.A_idiot