I really do not want to blow up my new toy


#1

I have just obtained a shiny new DSA815-TG (the new one with the 10Hz RBW) and mostly due to the price am very keen not to blow it up. Here are my thoughts so I would appreciate any suggestions if I have anything wrong.

I intend to put the output of a small handheld Yaesu transmitter (1 or 3 watts) through a 20DB attenuator, then a mini-circuits ZFDC-20-5 directional coupler into a 50ohm dummy load. The SA will take a feed from this. So, I will have 40dB attenuation up front.

I was originally intending to use the exciter at 1W but must plan for a 3W cock-up. The directional coupler has a mid-range limit of 2W so 20dB before it would drop the 3W to 30mW or 15dBm. The SA quotes a maximum input of 20dBm (with the internal attenuator set to 30dB). So, I think this means the mixer can handle -10dBm max.

My thinking is the directional coupler has an attenuation of 20dB which takes the worst-case signal down to -4dBm. Not low enough. Adding another 6dB attenuator would guarantee that running the exciter at high power and if I forget to check the internal attenuator is set above 0dBm I will still be safe.

Is this being over cautious? If I use 3watt and attenuate, then I cannot accidently triple the power. I have lusted after having a spectrum analyser for decades and making the magic smoke is something I really want to avoid.


#2

If you only want to see it working, just dangle a short wire from the input and transmit from your Yaesu in the room. Or if you’re very paranoid, plug in the 20dB attenuator into the spectrum analyzer’s input first, and dangle the wire from the attenuator.

Here’s a hint from an old timer: Did you know that the inner portion of a Type N connector is the same as a BNC? So you can plug a Type N male into a BNC female directly without an adaptor (this would be for quick probing and measurement, obviously, it’s not a strong mechanical connection).


#3

I have same model and I am using this adapter: https://mouser.com/ProductDetail/031-216/
alongside with these series of attenuators: https://mouser.com/Search/Refine?FS=TRUE&N=0+4284645234+18361414+4292890977+4294638670+4292906361

I think you should pay attention to the power because these attenuators are rated at 500mW.


#4

You are not being overly cautious. Connecting a Spectrum Analyzer to a transmitter is probably the most common way to break it. Take your time and get it right. If the frequencies are low enough, look at it on your scope first, to make sure the signal levels are working and that you haven’t blown your attenuators. As a confidence check, I like to set the scope to high-impedance input. First, confirm that the level won’t blow the scope in 50 Ohm mode (signal level not too high, no DC level). Then switch to 50 Ohm mode, and unless you are on an unlucky cable resonance, the signal level will go down by a factor of 2, just from the voltage divider of a 50 Ohm source and 50 Ohm load. Read the scope voltage and convert to dBm.

Once the setup is right, make a nice diagram of it, and tape it up somewhere nearby. That will make it easy to hook up again without reinventing it.

Having attenuators sprinkled throughout the system is really good for reducing reflections, also.

Speaking of unlucky cable resonances, I avoid the trick with the Type N connector and the BNC cable. A customer who was testing one of our prototype Spectrum Analyzers showed me this trick. A few hours later he said that the unit had a problem because it couldn’t receive certain frequencies. I had him get a proper Type-N cable, and the problem went away entirely! There was a really bad reflection that cancelled the signal in the hookup made by hanging the BNC cable in the N connector. A Type N to BNC adapter works much better and will mechanically protect your Type N input.