Question: How to protect Spectrum Analyzer input from nearby radio sources?

[Bicurico]

Hi,

This might be a silly question, but better be safe than sorrow.

Imagine I have a spectrum analyzer on my desk. In my case, a Siglent SSA3021X. The input port is rated at MAX +30dBm, which is the equivalent to 1W.

I own several Baofeng radios, some can emmit up to 5W on their antenna (https://baofengtech.com/uv-5r).

Questions:

• What would happen if I have the SSA3021X switched off and at 0.1m to 1m distance start transmitting with 5W on the Baofeng?
• Would I be breaking the input stage of my SSA3021X?
• How can I *safely* measure the minimum distance a radio needs to be operated away from the SSA?
• Does it matter if the SSA is turned on or off?
• How can I protect the SSA? Is it enough to screw an attenuator on the input port or could the transmission still break something "going around" the attenuator?

All opinions are welcome.

Regards,
Vitor

[kirill_ka]

• Does it matter if the SSA is turned on or off?

For example, some analysers switch their mechanical attenuators to the maximum attenuation when powered off. You can still burn the attenuator anyway.

• How can I protect the SSA? Is it enough to screw an attenuator on the input port or could the transmission still break something "going around" the attenuator?

The external attenuator should be ok, if you don't exceed it's specification.

[Wolfgang]

Hi,

you could do the following:

- Always (whenever you can) use a front end attenutor (30dB). That should protect the input even against a multi-Watt RF source
- Put it away from radiating antennas. I know (even happened with an R&S) that the cheap models have shielding problems
- Try it out. Put the attenuator, attach no cable, use your transmitter and see what is going on. Shoud not be much.
  Then, terminate the input with 50Ohms. Still see something: Shielding Problem. Remedy: Get a Boatanchor, old-style Analyzer. They are normall tight. (not serious).

Much success !
  Wolfgang

[Mechatrommer]

put this on...

if you still afraid, you can make circuit to harness radiowave energy to light up your room.

[SeanB]

I have some converter kits that include, as part of the kit, a replaceable picofuse in line with the input. Might not work to save the SA input from damage, but might just reduce it as the 50mA fuse blows in the connector body. I mostly use them as a very nice converter set, able to change from both flavours of connector on the ends, with BNC, N and TNC being the most common terminals I use.

[Wolfgang]

Sorry, but a fuse wont help because it is way too slow. Your mixer diode junctions will be converted to steam before the fuse even gets warm. What you could do if you do not like a big attenuator is a *limiter* (see Mini Circuits Website). That can help fast enough.

[coppercone2]

one way is to power the radio on when your far enough away, walk towards it monitoring the scope and see when the signal gets powerful

[ogden]

Imagine I have a spectrum analyzer on my desk. In my case, a Siglent SSA3021X. The input port is rated at MAX +30dBm, which is the equivalent to 1W.

• What would happen if I have the SSA3021X switched off and at 0.1m to 1m distance start transmitting with 5W on the Baofeng?

To fry SA input you shall directly connect your Baofeng antenna port to SA using coax cable. RF connector of spectrum analyzer is kinda very, very very bad antenna. Indeed it will pick-up some RF radiation if unterminated, but far from destructive levels

[tautech]

I think the SSA model RF inputs are pretty robust as there's zero input protection from the factory and if you consider how they might be shipped and handled on their way to a customer .......airlines, sorting, couriers etc and all using wireless comms of some sort or another.
Just a N-BNC and 50 Ohm terminator should offer input protection in all but the most extreme cases.

Edit to add.
You can try some experiments yourself.....
Dial in the max internal attenuation (-51dB) and with an open input use your Baofeng's at various distances to examine if there is significant signal imposed into the SSA.
It of course would not be wise to leave any wide band devices like an EMI/rf coax loop connected when the SSA is not in use.

[ogden]

Just a N-BNC and 50 Ohm terminator should offer input protection in all but the most extreme cases.

To get 1W of RF into bare N-type connector, you need such an RF power that anything conductive around including SA enclosure, is melting.

[tkamiya]

I was fixing TS700S a while back.  (this is a 2 meter 10 watt radio)  Keying down at 10 watt level and feeding into dummy load and antennas only 10 feet or so away.

If I had nothing connected to specAn, I might see a small blip or something.  If I have open coax connected, I see a bit more.  I never tried connecting an antenna to SpecAn and try the same with mine but have done so in commercial environment.  Then, I saw a significant blip but not even close to damaging anything.

Understandable after such a significant purchase, but I think you are overly concerned.  Spectrum Analyzers are used all the time next to multi-kilowatt transmitter environment.  Besides, if any SAs are affected at such level, it will be useless in any environment involving transmitters.
 

[Bicurico]

Thanks for all the replies!

I know I am a bit paranoid with my SSA (it is actually one SSA3021X and two R&S CRTU sitting on my desk), but it would be really stupid to break them just because I was testing something else that required the Baofeng to transmit at 5W.

I tried putting a 50 Ohm terminator on the RF4 IN of the CRTU. Thi sis the less attenuated input port of this device (and thus the easiest to break). I did not notice a considerable difference on having it attached or not: the most active transmitters nearby (mobile network) can still be seen on the spectrum. This is of course without any cable or antenna attached to the connector (other than the terminator).

My other concern was/is: won't the radiation "go around" the input connector and directly arm whatever sensible component are in the input stage?

Naturally it makes sense what people say: if a simple portable radio is enough to break spectrum analyzer when operated at a distance of 1-2m then that would be a real issue, as one could easily sabotage shops, universities, labs, enemies, etc.

Also, I do have my mobile phone on my desk all the time. I am not sure at what power it transmits. And then there is the WIFI access point, too...

I know that the transmission power decreases on air at the power of two by distance, so probably I can just ignore this.

Anyway, I wanted to have your opinions!

Regards,
Vitor

[Bicurico]

Update:

So I did try it now, coming closer with the transmitting radio from around 5m distance up to next to the SSA's.

In fact, the signal is at 0.5m only around 20dB higher than the noise floor. I guess it is safe to operate the radio, as long as one does not connect it by cable straight to the RF Input.

Regards,
Vitor

[ogden]

My other concern was/is: won't the radiation "go around" the input connector and directly arm whatever sensible component are in the input stage?

Maybe. Depends how good instrument is. But anyway very, very low levels. Meaning it can impact your measurements, add EMI noise. Much more you shall be concerned about EMI impact on device you are measuring.

When you RF-measure something, better switch your mobile (< 1W), wifi transceivers (<=0.25W) off or at least put as far away as possible from your desk. They can impact or even trash your measurements. Especially when you don't know what you are doing.

[rf-loop]

Update:

So I did try it now, coming closer with the transmitting radio from around 5m distance up to next to the SSA's.

In fact, the signal is at 0.5m only around 20dB higher than the noise floor. I guess it is safe to operate the radio, as long as one does not connect it by cable straight to the RF Input.

Regards,
Vitor

Yes exept if you have very extremely high level and enough high frequency field there. But I think you do not want be in this kind of RF fields - perhaps your body limits come well before SA limits (when input N connector just open). Totally other case is if you connect some antenna what can be even some cm wire.

Common warning. Input is ESD fragile. First RF switch is just after input and it is very ESD sensitive! Even when it is 50 ohm but this do not protect if enough energy fast ESD. Many peoples (not you) may think that oh it is 50 ohm sysztemn it is not danger. No, even well charged cable can destroy front end. Old school SA what have first mechanical atrteenuator are much more safe for this. (but also fragile)
Also it need note that 30dBm is accepted max 3min and only if internal attenuator is 20dB or more (datasheet)

[tautech]

Also, I do have my mobile phone on my desk all the time. I am not sure at what power it transmits. And then there is the WIFI access point, too...
Regards,
Vitor

SSA3032X
If I'm using a wide band coax loop my cellphone goes in the next room as it has set off the input level warning in just doing a handshake to the cell tower however these occurrences are setting dependent. 
WiFi is easy to find with a loop and the right settings much like local broadcast stations.
Basic SA knowledge and care of usage will keep your equipment safe.

[tkamiya]

About "go around" issue....

I don't know about Rigol but HP analyzers are built like this.  Back of N connector is completely sealed and shielded.  It goes into a small hardline and into an attenuator.  Those connectors looks like SMA or 3.5mm.  From attenuator, it's all ridged lines.  There is no way sufficient signal can penetrate it to damage it.  I think somewhere on this site, there is a video or thread of "explorative surgery" of your SA. 

But as you proven to yourself, it's fine.  I'm even surprised you get 20db above floor....  If I were you, I'd be more worried about DC into the input.  Mine, some ports are protected to 20V, 50V, and ZERO volt.  I have a DC blocking connector there for it.  If I need a low frequency response, I will have to CONCIOUSLY REMOVE that to work on it.  Otherwise, it stays.

[Bicurico]

Again, thanks for the feedback.


Here some notes:

1) I have DC blocker on all my SA input connectors. The reason is that my main focus is TV/CATV/SAT broadcasting and reception and due to amplifiers, LNB, etc. it is common to have DC present. One of the usual fails is to connect the LNB Loopthrough connector of a satellite receiver to a spectrum analyzer input port! This will certainly break the input stage, as there will be 14/18V present (depending on vertical or horizontal polarity). Thankfully I "only" broke a cheap "SMA 133MHz-4.4GHz simple spectrum analyzer" this way and could easily repair it by replacing the M810 IC.

2) Since my measurements are normally focussed on TV/CATV/SAT, I don't have issues with mobile network or WIFI. For digital terrestrial television measurements there are "LTE blockers" to avoid issues with nearby LTE transmissions.

3) My concern relative to the Baofeng radios was not using them DURING a SA measurement, but if generically using those radios in viccinity of the SA could inadvertantly break the SA (even when it is turned off).

I feel more confortable now, as it doesn't seem that bad as I feared (tested at 0.5m distance with up to 5W, no antenna connected on RF input).

Regards,
Vitor

[hwalker]

At our laboratory we use RF limiters to protect the front end of our spectrum analyzers.  Its much cheaper to replace the limiters than to have the front end of the spectrum analyzers repaired.  SeeedStudio sells a very reasonably priced limiter, https://www.seeedstudio.com/Power-Limiter-p-1444.html.  The main drawback is it is limited to 3 GHz and is only available with sma connectors.  You can search eBay for other RF limiters, but even second hand ones from Agilent are usually 5 times the cost of the ones SeedStudio sells.

- Herb

[Wolfgang]

... Mini Circuits has some as well. Some also with N connectors, and some models up to 7GHz.

https://ww2.minicircuits.com/WebStore/Limiters.html