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Thread: Spectrum Analyzer

  1. #1
    Join Date
    Oct 2009
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    Phenix City, Alabama
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    Default Spectrum Analyzer

    I've been thinking about building a FM Spectrum Analyzer. It won't be as fancy as those new fangled SDR's. However, no PC is required! It should be in the sub $150 range, maybe less. Here is an example screen:

    Click image for larger version. 

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    Anyone interested?
    Ray H. Dees
    PG-6-14714
    EM72LM

  2. #2
    pjdyer Guest

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    I first became aware of the value of a 88-108 MHz spectrum analyzer in the mid 70s when I noticed an HP Journal from 1968 showing a scan of that in the SF Bay Area, but at something like $8k it remained a pipe dream. I once hung a Wobulator across the oscillator section of the variable tuning cap of the SX-62 - with an ancient RCA150 3" scope I could see two blips (coming and going) of what was likely 92.9 here. Lately, K6STI has been touting http://www.ham-radio.com/k6sti/sam.htm as useful (if one can find the proper functioning unit at a reasonable price - EBay results range widely - and you still need to hang a scope on it). I keep kicking myself for not getting a Precision E-400-C sweep generator from Jeff-Tronics in Ohio in 1975 for $75. So, your project is interesting.

    73, Pat - WA5IYX

  3. #3
    Join Date
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    15 mi NW Phila, PA
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    Default

    I've used a slightly modified Heathkit SB-620 on AM for DX'ing, in fact I still have the unit, although I haven't used it in some time. I found it most useful back in the 1970's and 1980's when there were still numerous Latin American stations on split frequencies, and some of them drifting due to instabilities in temperature control of the transmitter crystals. For FM use, I can see the "curiosity value" of looking at your FM spectrum in whole or in part with various different antenna directions and for those who do FM antenna phasing, for tuning the phasing unit. But given the nature of FM and the stability of today's transmitters, I would think there's a clear application for those who are trying to DX Trans-Atlantic FM but not so much in terms of finding domestic stations off frequency far enough to have usable offset info. Am I missing something else here ?
    Russ Edmunds
    15 mi NW Philadelphia, PA
    WB2BJH -- Grid FN20id
    Yamaha T-80 & Conrad RDS Manager;
    Yamaha T-85 & Conrad RDS Manager;
    Onkyo T450RDS; 4) Tecsun PL-310;
    Sony XDR-S3; Sony ICF2010; all w/ APS-9B @ 15';
    Sony ICF2010 barefoot w/ whip


  4. #4
    pjdyer Guest

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    My main interest in "seeing 88-108" displayed has always been for Es events to see how "sharp" the MUF might be from co-sited TLs and the dynamics of that. Back in the 90s at an Armed Forces Day at Kelly AFB here they had one showing the FM band. I was puzzled at the strong 98.7 blip on it - and soon learned that it was an STA station for the event there (which then became very handy - with other spectators asking what I was listening to!) At the 1992 Central States VHF Society Conference at nearby Kerrville the flea market had an SB-620 for $20. When returning from my room with enough cash I found that W5UWB now had it tucked under his arm ...

    73, Pat - WA5IYX

  5. #5
    Join Date
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    Default

    I hadn't thought of the Es MUF application. It would seem like there would be some further mods required to the SB-620 for FM use although that might not be the case - I'll have to check the documentation I have. I didn't do the initial mods to it - nor did I do the necessary mods to my Hammarlund HQ-150 to interface with it, and I doubt I'd be able to do any further mods.
    Russ Edmunds
    15 mi NW Philadelphia, PA
    WB2BJH -- Grid FN20id
    Yamaha T-80 & Conrad RDS Manager;
    Yamaha T-85 & Conrad RDS Manager;
    Onkyo T450RDS; 4) Tecsun PL-310;
    Sony XDR-S3; Sony ICF2010; all w/ APS-9B @ 15';
    Sony ICF2010 barefoot w/ whip


  6. #6
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    Default

    I've made quite a bit of progress: This picture below shows the actual tuning rate. It is stepping 200 kHz every 60 ms for the entire 20 mHz band. It will go faster, but the longer it is "parked", the more accurate the RSSI is. (I'm still working that portion out.) The "Violet" blips represent STC (Seek/Tune Complete).



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    This is an actual photo of the result:


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    The radio is breadboarded and not in a RFI enclosure. I'm using a three foot piece of wire for the antenna. I'm sure that a vast majority of the spectrum peaks that you see are from equipment in my own shop. However, the bigest peaks are nearby "Flame Throwers".
    Ray H. Dees
    PG-6-14714
    EM72LM

  7. #7
    pjdyer Guest

    Default

    The SB-620 is designed to show, at most, a HF ham band (e.g., 7.0-7.3 MHz) so would, with the proper i.f., probably display only one or two FM broadcast channels at a time.

    N5TX (SK Oct 2011) had a Cushman which could show a few of our local FM stations on it. In the mid 70s, one of them (KISS 99.5) was running so much compression that it looked like a square wave! At the WTFDA Convention in Aug 1985 in New Orleans we visited the WRNO shortwave transmitter (not far from our host) - WA5UUD was the engineer on duty. He turned on one there - nice strong blips every 800-kHz from the locals. BTW, someone quoted the unit at the KAFB AFD at $26k.

    This recently-generated numeric table http://www.qsl.net/wa5iyx/87500-108000_100.txt is the closest that I've come to a spectral scan of 88-108 MHz here. The Seeeduino takes a few seconds with the 100-kHz steps, and only seems to work if in FM mode. Too bad the Serial Monitor display didn't let me do a cut-n-paste of the numbers - transcribing them took a while.

    73, Pat - WA5IYX

  8. #8
    Join Date
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    Default

    The SB-620 as modified for AM BCB has a display bandwidth of slightly more than 20 kHz., so for domestic FM, it would be tow channels max. As discussed here, the uses for FM are different from those I was used to on AM. A broadband S-A is another breed of cat, as the end result display looks not unlike that of an SDR. But with the limited number of useable frequencies here due to locals and IBOC, I have less than half the band available for DX, I'm not sure if I'd pursue this.
    Russ Edmunds
    15 mi NW Philadelphia, PA
    WB2BJH -- Grid FN20id
    Yamaha T-80 & Conrad RDS Manager;
    Yamaha T-85 & Conrad RDS Manager;
    Onkyo T450RDS; 4) Tecsun PL-310;
    Sony XDR-S3; Sony ICF2010; all w/ APS-9B @ 15';
    Sony ICF2010 barefoot w/ whip


  9. #9
    Join Date
    Oct 2009
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    Phenix City, Alabama
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    Default

    Pat:

    You can use HyperTerminal or other similar program to talk to the Arduino. Then you can cut and paste as in:

    9810, 10
    9830, 23
    9850, 14
    9870, 10
    9890, 13
    9910, 37
    9930, 56
    9950, 28
    9970, 14
    9990, 19

    Nice scan by the way!
    Ray H. Dees
    PG-6-14714
    EM72LM

  10. #10
    pjdyer Guest

    Default

    A graphical version http://www.qsl.net/wa5iyx/images/880-1079a.gif - took a while with two graphs from Lotus Works (Win98) being joined by PSP4 on Win7. During strong tropo a lot of those "gaps" will fill up with in-state stations.

    73, Pat - WA5IYX

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