SDR vs FT-991A

Discussion in 'Software Defined Radio (SDR)' started by W4MUF, Jul 16, 2019.

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  1. KF5PAL

    KF5PAL Ham Member QRZ Page

    Honestly, I just read through the Forums from time to time and respond to those that are of interest to me or ones that I have knowledge of. I never really pay attention to the dates. As for my use of the 991A and a RSP1A as a panadapter..."Technically" it's not a panadapter...I have the RSP1A connected to a Rx only antenna via a MFJ-1708B-SDR switch. I use SDRUNO and OmniRig to sync the VFO of the 991A to the RSP1A and vice versa. Though not 100%, for the most part, what I see on the 991A I see/hear on SDRUNO. When I see a signal of interest on SDRUNO, I click on it and it tunes the RSP1A and the 991A to that frequency. I also have a TS-480SAT and on that radio, I did tap and buffer the IF and ran it to the back of the radio to an SMS connector. Aside from HDSDR, most software programs don't deal with that setup to well.
     
  2. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Check it out:


    https://www.reddit.com/r/amateurradio/comments/dw3r3e/modified_my_new_ft991a_to_add_sdr_panadapter/

    Details: https://github.com/rfrht/FT991A-PAT/wiki
    Discussion: https://forums.qrz.com/index.php?th...r-in-ft-991a-need-rf-designers-review.650840/

    \o

    - Rodrigo, PY2RAF.
     
  3. K8XG

    K8XG Ham Member QRZ Page

    I solve this all the time. When the 991A screen is flowing and I see signals on the left or right I want to tune to, I STOP the water from flowing by pressing the SWEEP button and move the VFO over to the line or worm as I call them I want to hear. This is also how you have to do it on the FT991 non-A version.

    I can then start the water flowing again if I wish with another button press. Try it out. Also look at the menu choices available that control these functions and how the VFO tracks.
     
    N5DSF likes this.
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    No mystery going on here. The SDRPlay A/D is only 14 bits, so at its very best only has a dynamic range of roughly 80-84 dB. About what a cheap AM radio would have.

    I own both a SDRPlay RSP2pro and a Airspy HF+, and for HF bands the Airspy is a superior radio as it has 16 bit A/D converter providing roughly 92tp 96 dB dB of dynamic range, or that of a decent entry level HF radio. So nothing is wrong, you got what you paid for. The SDRPlay strong point is a much wider frequency range and better suited to be used on commercial broadcast FM, VHF, and UHF.
     
  5. KC8QVO

    KC8QVO Ham Member QRZ Page

    You didn't read my post. Highlighted the pertinent paragraph.

     
  6. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    @KC8QVO hi Steve,

    How does a panadapter allows you to receive the wide bandwidth and tuning signals of interest that is not the center frequency?

    == IF ==
    It's the Intermediate Frequency. What's the IF? When you tune to a frequency, this signal is converted to the IF center frequency - 69.450 MHz in the FT-991/A. So, if you tuned to 146.410? This is what is being outputted at 69.450. You moved to 7.135? Now this is is what is being output at 69.450.
    After the IF stage, the signal undergoes a massive clip (the famous roofing filter) and cuts it to a few kilohertz wide, 3 kHz for CW/Narrow modes and 12 kHz for FM and wide modes. After this signal clipping, it is then amplified and moved forward into the further stages of the radio.

    There's also another stage prior to the IF frequency, the bandpass filter (a short mention at the end of this post), if you are interested, ask and I can try to explain; but let's put that aside.

    So pay attention here: Entire spectrum -> Conversion to the IF frequency -> Roofing filter -> further stages. Right?

    == THE SDR ==
    So, what's the panadapter magic?
    Let's talk a bit about the SDR now. Overly simplifying it, SDRs are receivers that digitalizes a chunk of spectrum. The chunk amount is the bandwidth.
    The RTL-SDR can reliably digitalize around 2.4 MHz of spectrum, the SDRPlay can digitalize 10 MHz at once - So these are their bandwidth.

    == THE SDR + THE RADIO ==
    Now, let's start to put the things together.
    Remember the IF signal at 69.450? If you feed that signal to your SDR that is tuned to 69.450, the SDR will also see the signal that your radio is tuned at - If your radio is tuned to 146.410, your SDR will happily reproduce that signal if it is tapping the IF signal and the SDR is tuned to 69.450.
    But wait, there's more! The SDR will also see all the signals within its bandwidth around 69.450 - If you are using a RTL-SDR you can see 2.4 MHz wide around 69.450, meaning, you can receive what's between 68.250 and 70.650 - The to 2.4 MHz SDR bandwidth, 1.2 MHz up and 1.2 MHz down from the 69.450 center signal. All at the same time. But wait, there's more: That also translate that you can receive in your SDR from 145.210 to 147.610 - without having to tune your SDR outside 69.450.

    Things gets even more dramatic in HF. If your radio is tuned to 29 MHz, the SDR can see the entire 10m band in a standard RTL-SDR, allowing you to see from 27.8 to 30.2
    So can I tune to some frequency in the panadapter without re-tuning the radio? YEs, if it is within the SDR bandwidth.
    In my HDSDR I configured to left-click, tune only the SDR. If right-click, tune both radio and SDR. Clickity clicky.

    Moving back to that block list: Entire spectrum -> Conversion to the IF frequency -> Roofing filter -> further stages.
    It is crucial to tap the signal prior to the roofing filter, otherwise... You will be narrowed to what the roofing filter spews out - a very narrow spectrum, basically only the signal of interest, blanking out everything else. Roofing filters are very sharp filters.

    == HACKING AROUND ==

    Can I tune outside the 2.4 MHz boundary? Yes you can - Absolutely. But a few gotchas here.
    When the radio transforms the signal to the IF, it undergoes something called Spectral Inversion. Which means, if you want to go up in real frequency, you need to go down in IF frequency.
    The IF is centered at 69.450 right? Now let's say you are still tuned to 146.410, and want to listen to VHF SSB in 144.300 - That falls outside your 2.4 bandwidth (bottom limit 145.210). Now what?
    Then you have to ADD (because the spectral inversion) the the difference to 69.450:
    • 146.410 (actual center frequency) - 144.300 (frequency of interest) = 2110
    • 69.450 (center FI) + 2110 (the above step difference) = 71.560
    Tune your SDR to 71.560 and your 144.300 signal will be right there.

    Can you be extreme and listen to 123.45 in airband while having the radio tuned to 146.410? Yes you can!

    146.410 - 123.45 = 22.960
    69.450 + 22960 = 92.410

    Can you then listen to VHF in radio and 6m in 51.200 in SDR?

    No - you are constrained by the band-pass filters. Check the link for the band limit for each operation.

    Well, that's it Steve - I hope this was of help and clarified - feel free to ask any further questions!

    73 - PY2RAF, Rodrigo.
     
    NY7Z, W9WQA and W7ESR like this.
  7. W7ISS

    W7ISS XML Subscriber QRZ Page

    I built a simple preamp for my sdr v3 dongle. it had weak receive on all bands except for broadcast fm band. with preamp on it actually receives better than my 991a on the outside dipole on 40 meter band. Then I noticed they sell the preamps on ebay for 15$ haha.. still fun building your own stuff :)
     
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    Much less than that, how about $7?
     
  9. VA3VF

    VA3VF Ham Member QRZ Page

    I have a couple of these eBay preamps. They are worth the price for the most part. They are not all equal though.

    My suggestion is the one using the SPF5189Z LNA.
     
  10. W9WQA

    W9WQA Ham Member QRZ Page

    will yaesu ever make a 991a with a 7300 quality display. i really want a 991 but not the current display...
     

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