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DIY downconverter

Adventuresome types amongst us might trying using things like AD831 mixer, ADF4001 LO and one of the various bandpass filter modules that are commonly available ready to go with SMA connectors. Should be able to assemble something for under $80

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  • I am not really interested in > 30 MHz at the moment but since nobody else commented on my idea, I just ordered the stuff above and as time permits, will try it and report
    KA7U
  • Hello James, interesting for me, waiting your report.
    I think to use for downconverter GPSDO like http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301
    How do you think, it's possible?
  • I use a Bodnar GPSDO on three different up/down converters I built for amateur radio use: 2m, 70cm and 23 cm. The more expensive one, http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=234, has two different outputs so is somewhat more flexible. The two outputs are not independent but it may be possible to cover at least two ~20 MHz wide bands and probably three, with careful selection of (integer) synthesizer numbers. Bodnar provides a tool but the SiLabs web site has even more information.

    These GPSDOs can drive 32mA into 50 ohms - which is enough to provide LO for a mid-level passive mixer, perhaps something like https://www.minicircuits.com/pdfs/ZAD-1H+.pdf which has even better IMD performance than the AD831, but which is quite a bit more expensive. You will still need a filter to protect against images though. You could also use one output to provide external KiwiSDR 66.660 MHz clock directly. I do this on two of my Kiwis.

    One other alternative is to make a dual-conversion converter. Convert, say, 10-30 MHz of the Kiwi to 90 - 110 MHz with a 120 MHz output from the Bodnar and with BPF or a 110 MHz LPF . That gives the Kiwi (reverse tuning) aircraft band coverage. Then use the second output to convert again, a 20 MHz span of your choice at frequencies maybe as high as 1 GHz. This also requires an image reject filter but since the image will be 20% away a suitable filter won't be too difficult.

    Even with no preamplifier, conversion loss of either passive or active mixers and simple filters shouldn't raise the noise figure so high as to be unusable. Of course it is also possible to add a low noise preamplifier ahead of most of the filtering in order to increase sensitivity. But for terrestrial signals there's probably not much value in striving for system noise figure much below 3 dB since the antenna temperature/noise will probably be that high or higher.
  • Hello Glenn and thanks for this information. AD831 is more accessible on eBay or AliExpress. For external clock you use J5 on KiwiSDR board? You installed jack or or soldered the cable to board directly?
    PS please check URL on your qrz.com site for "My Web pages", you missed ":" after http
  • AD831 should be fine, though it is nice to have really good GPS reference. If you want to see one in action go to my remote Kiwi which is line-of-sight to US NIST primary frequency standards and look at the results. :) This is with the Bodnar unprotected from drafts too.
    I just soldered small diameter Teflon coax to the board and the other end soldered to a SMB connector on the LAN/PS end of the Kiwi enclosure. I've been concerned about common mode current QRM ingress and also about phase noise purity from an external clock but I've found no difficulty with this arrangement. You can see a blurry photo of it in the Antenna Switch documentation. Try to use the really small Teflon coax, smaller than RG174, if you can and then also use some kind of adhesive to act as a strain relief on the board.
    Thanks for the heads-up on the QRZ page. I didn't know that and it turned out to take three tries to get it to include the ":"!
    If this isn't clear, I can try to open on of my kiwis up and take a better photo.
  • Thanks Glen, photos on the Antenna Switch documentation quite enough.
  • edited July 2019
    http://195.222.165.118:8073/ my AirBand KiwiSDR with passive mixer (QTH near SVO airoport Moscow).
    KA7Ucathalferris
  • Nice work.............. very similar to what I was going to do, but project got pushed aside for now
  • It's working well.

    I have some observations based on my own experience from previously trying to setup something similar.

    You seem to have some switched mode power supply noise in the background, this may possibly be from the LNA ?

    There is still a bot of FM broadcast band image, maybe an extra FM band stop filter on the input would help ?

    https://www.rtl-sdr.com/rtl-sdr-com-broadcast-fm-band-stop-filter-88-108-mhz-reject-now-for-sale/

    Some of the other noise may be due to out of band signals being passed through the convertor and aliasing with the KiWI's 66.66MHz clock. A 30MHz low pass filter between the mixer board and KiWi input may possibly help with this.

    Maybe set the KiWi to operate from 2-32MHz instead of 0-30MHz. This moves the GPSDO DC Local Oscillator frequency further way from the low frequency limit of the KiWi and may help prevent ADC overload if there is any high level Local Oscillator blow-though the mixer.

    Regards,

    Martin - G8JNJ
  • Thanks Martin, it's my first step of this project. I have plan for add LPF after 1-2 month.
  • It's working better than I expected, considering the few parts.
    Encourages me to try something here, thanks.
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