Wednesday, December 2, 2009

PRC-320 on a spectrum analyzer

Today I went with Ernesto (EA1LQ) to do some testing in order to get the PRC-320 ready for the international Naval Contest.
I got some pictures of the RF output on a spectrum analyzer.
The first picture is CW on 10.0000Mhz with a 10Mhz span. It shows two spurious signals 3.5 Mhz above and below the main signal. These two peaks bacame slightly smaller as the transmission progressed and did not seem particularly worrying, as they appear to be consistent with the service manual specs. They are 40dB below the main signal, so if we assume that the PRC-320 transmits the main signal at its nominal power of 30W, then the spurious signal would be in the range of 3mW. If we are thinking in terms of signal strength, 40 dB represent between 6 and 7 s-meter units.
But we were mainly concerned about the modulation in USB so we analyzed the output in that mode, and the result is presented in the second picture. We used the FM noise from a TV set as an AF generator. We were mainly concerned about the peak that was there, very pronounced, even when we covered the mic. What do you think about it?
One of therese days I will take the PRC-320 to a mountain top near the city and ask some fellow hams to make recordings of the received signal.


  1. Diego, Five questions: (1) What was the power into the analyzer's RF Input at the time if the test?? (2) What is the analyzers internal attenuator set for?? (3) What RF level is represented by the top line of the display? (4) What is the bandwidth setting (5) What is the sweep rate??

    If too much of your signal is seen by the analyzer's 1st mixer, you may generate distortion within the analyzer and make a clean signal look bad. My rule of thumb is to make sure less than -20 dbm of RF is seen by the 1st mixer.

    Larry, K4MLA

  2. Ojala que el día que subas al monte pueda estar yo para ver como suena eso...añademe al messenger si lo usas con mi indicativo a ver si hay suerte y nos puedes avisar.


  3. As you may have noticed, I am not a great expert in this topic, but the fellow ham that is helping me with the analysis (Ernesto - EA1LQ) has just told me that you have made a very insightful point in the second part of your comment.
    I cannot give you precise measures (some may be in the picture) but we did have an external variable attenuator between the transceiver and the analyzer, and Ernesto regulated it to get the proper signal in the analyzer.
    We are mainly concerned with the modulation in SSB that is difficult to understand. We are suspicious about the fact that there may be too much gain that is amplifying the noise, because a large peak also shows when we transmit with the mic covered (I wish I had that concrete picture to show).