Spectrum Analyzer and Channel Noise Estimation

Hi everyone,
I'm new in this forum and first of all I want to congratulate with all of you for that support.

I'm working on a engineering thesis project that consist in a few words the usage of the PowerLineCommunication technology, that means, to send a message (a packet modulated in OFDM - 35MHz-90MHz) through the wire where also flow the current supply (DC voltage 12V).

My intent is to "sense" the channel with the spectrum analyzer in order to understand what frequency is better to send my packet (I can choose for example what sub-bands are ON and what are OFF individually).
-I'm not skilled with the spectrum analyzer, so I don't know how to perform the channel estimation.
-Furthermore I'd like to know if the DC 12V could damage the instrument. (It is a E4408B Agilent).

Thanks for the support,
Best,

Andrea

First to start: Yes, you will damage the spectrum analyzer by applying 12 V DC to its RF input. Inside there is a precision RF attenuator with 50 Ohms nominal impedance. DC input will burn the first resistor and repairs are costly.

To understand the spectrum analyzer use, please find Agilent White Paper on Spectrum Analyzers. Operation manual is also good to study first.

Technically any frequency you choose can transmit any signal by OFDM. But the Power LIne Communication is a very unethical application as you send your signals over a wide radiating antenna, and cause interference to all licensed users of "your" frequency band, incuding FM radio (88-108 MHz).

Please find International Rules of Radio Communication issued by ITU in Geneva where you can see all legally licensed users of various frequencies. Your signals will interfere with all of them. This is illegal and unethical. The users paid money for their licences and equipment, to provide often critical services. Your signals will cause harm to them.

After sending your signal to a power line, it will guide them over a long distance and to a wide area. You have no control of such interference.

Hi jiripolivka,
Thanks for the support.
Actually I have read the Agilent Specification but I didn't find about how DC voltage can bear the instrument.

My thesis project is very simple, I have two Power Line Modem Developer's Kit (TMDSPLCKIT-V3) where one send a packet (TX) and the other modem receive this packet (RX) and then simply it generates a PWM signal on site, nothing else.
Everything works but my next step is to "sense" the channel (between TX and RX) in terms of noise/interferences.
In that way I will send the packet from the TX in the range of frequency with lower noise/interferences.
My question is: How can I perform a DC channel sensing?
Thanks for the support,

Best,

Andrea

>> Actually I have read the Agilent Specification but I didn't find about how DC voltage can bear the instrument.

Read the yellow warning label under the input connector. 0V DC MAX
You may get away with a few volts if the input attenuator is in circuit, but don't try your luck dead mixers are very a very common failure.

The "DC channel sensing", if it means that along with a RF signal you also send a DC voltage like +12 V, can be done using a "bias tee". This is a device with a capacitor in the RF line (between RF connectors), while the third port (usually not a RF connector) is connected with one of the RF connectors through a choke which passes the DC "signal" but reflects RF back.

As G4BCH indicated, on a spectrum analyzer there may be a warning label not to apply DC to the RF input. If not sure, set the RF attenuator to MAX and use an ohmmeter to check RF input resistance. It should read 50 Ohms.