# Rigol Spectrum Analyzer !

I’ve kept drooling for this for several years. Today I finally got it ! Moreover, it seems it is fully-option enabled, which is kinda weird because I haven’t ordered for such. However, here it is: the Rigol DSA 815 TG Spectrum Analyzer with tracking generator. This will enhance my lab’s capabilities in RF electronics and Ham projects.

No project will be a good project without the blessing of a cat.

Rigol DSA 815TG — it works !

For those that wonder, here is a very small list of first things that one can do with a spectrum analyser:

Measuring the performance of a crystal filter; Hams use crystal filters in their radios to enhance selectivity, but short of installing the filter there is really no easy way to test one. A spectrum analyzer with a tracking generator makes it fairly easy. Simply feed the tracking generator into the filter input and look at the resulting response from the filter output. A nice additional feature is the “N dB” feature that makes it easy to find the point where the response is N dB down from that at the filter center frequency.

Testing and adjusting duplexers and diplexers; Duplexers are used in repeater installations to allow a single antenna to be used for simultaneous receive and transmit on slightly separated frequencies. In order to do this the duplexer has to be adjusted so that it provides the required attenuation of the transmitter frequency so that it does not interfere with the receiver. Any repeater owner has to deal with tuning a duplexer at some point. A spectrum analyzer with a tracking generator makes this easy. A diplexer allows the use of a single antenna on two different bands, and is a lot smaller than a duplexer. Lots of HAMs use diplexers in their cars to allow a single antenna to be used on both the VHF and UHF bands.

Coaxial stub tuning and coaxial cable measurements; Attach a piece of coaxial cable to the input/output of the tracking generator using a T connector. First, this shows you the frequency where the cable is ¼ wavelength long. ¼ wavelength coaxial stubs are often used to reduce interference from nearby transmitters. You can trim a piece of coaxial cable until you reach the frequency you want just by cutting a small amount from the cable and watching the response on the screen.

Another useful feature using the T-connector technique is that when you have determined the frequency where the cable is ¼ wavelength long you can easily determine the physical length of the cable without actually having to measure it. All you need to know is the frequency where the cable is ¼ wavelength long (read from the screen on the
spec an) and the velocity factor of the cable which is a specification that can be looked up for any type of cable. Then the length of the cable (in feet) is calculated by the formula

L = (246*Vf) / F

where F is the frequency in MHz.

Antenna performance (return loss); The return loss of an antenna is a measure of how close its impedance is to that of the system driving it. You can measure return loss with the spectrum analyzer and the tracking generator if you have a directional coupler. By attaching the reflected port to the spectrum analyzer you can measure the return loss of an attached antenna, and you can calculate VSWR from return loss by the formula

VSWR = (1+(10^RL/20)) / ((10^RL/20)-1)

where RL is the measured return loss.

FM and AM Radio Channel performance; you can demodulate, see the frequency and amplitude deviation, etc…

A quickie for now, tested my 144 MHz transmitter, used a prototype for the APRS beacon project.

My 144 MHz transmitter, prototype for the APRS project, tested on the Rigol DSA 815TG Spectrum Analyzer. Quite ok.

Now — let me put it to work. Will get back with more impressions.

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### One Comment to Rigol Spectrum Analyzer !

1. Florentin says:

I like the cat! 🙂