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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 16, 2018 5:45 pm 
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Frank,

To get a quick idea of what it does for image rejection with on the air signals wait until there is a strong signal from WWV on 15 mhz. and then tune your RME with the preselector OFF to around 14.070 and you should pick up the image from WWV. Engage the DB-20 and tune it to 14.070 and you should note a drop in the WWV signal relative to anything (usually a digital signal) on 14.070. The apparent benefit from the DB-20 increases as you go up in frequency where the weakness of a single set of RF tuned circuits with a relatively low IF frequency becomes apparent.

Of course you can easily simulate a test using your signal generator to provide the image interference.

The RME-69 uses high side injection so the HFO should be ~465 Khz. above the dial frequency on all ranges (approx. 465 KHz. as the exact IF frequency is determined by the crystal filter element). This means that the interfering image will be located at 930 Khz. (2 x 465 khz.) above the tuned receiver frequency. For example with the 69 tuned to 14.000 Mhz. the HFO is operating at 14.465 Mhz. creating the 465 Khz. "difference" frequency of the IF. However the received frequency of 14.930 Mhz. (HFO plus instead of minus the IF frequency) also "solves" the equation so if there is a strong station there it will also be received. The amount the image is down is based upon the attenuation slope of the tuned RF circuits.

The DB-20 improves image rejection by adding additional tuned circuits with much more attenuation on the slope where the image lies. With a 465 Khz. IF receiver image rejection is not a problem at lower frequencies where the difference between desired and image frequency is very large, i.e. around 22% removed at 4 Mhz. But if you move up to 30 Mhz. then the desired signal and undesired image are only separated by 3% and the interfering image and desired signal will receive nearly equal amplification from the RF stage. This was the major driving force behind the move to dual conversion because it provided for much better image rejection while allowing a simplified front end (usually one RF stage) which was needed primarily to overcome mixer noise saving the additional circuitry and band switch section along with tracking issues associated with multiple RF stages. Going to a first IF in the 3 Mhz. range provided around 20% separation between the desired signal and the undesired image in the 10 meter band and the first IF could be very simple and often consisted of a simple converter stage that directly fed the second IF.

Rodger WQ9E


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 16, 2018 5:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1772
Location: Eagan, Minnesota, USA
Rodger,

Thank you for this excellent explanation! I am going to print it off, and actually study how this works so I can finally get this into my head. And file it with the notes I have on the rebuilding. Also want to thank you for all of the help along the way with the pre-selector and the radio!

_________________
Frank
KD0RUC


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 Post subject: Re: RME DB-20 Pre-selector.
PostPosted: Apr Mon 16, 2018 9:52 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6673
Frank,

You are welcome! I wish soccer was as easy for me to explain as radio since I am once again coaching my daughter's team but they are now 14 years old which makes things a lot different than it was to coach the group 5 years ago. The difference is I understand radio but I never played soccer growing up :( It is a great group so they are fun to work with and they are being nice to their somewhat inept coach.

And for me radio is REALLY simple compared to hair. My daughter is letting her hair grow out longer like some of the other girls on the team so keeping it out of her face while she plays is important. One would think that for a father with a PhD and a really smart daughter that installing a simple headband wouldn't be an issue but now I realize that restringing a dial cord in a radio infested with brown recluse spiders would be easier for me. I finally figured out that I had to put her hair in a ponytail first (no experience there either!) and then install the headband. It isn't as simple as it looks on Youtube! And somewhere along the way one of the little elastic "scrunchy" things used for holding a ponytail together went flying away at the speed of light not unlike what sometimes happens when trying to take off one of those horseshoe tension clips on switches and controls. Fortunately I bought them in quantity just like I do those clips. By the time her hair was finally in place we were both laughing so hard we were choking. And then I got a text that the games that day were canceled due to standing water on the field. At least "hair styling" will be easier next time... I HOPE!

Rodger WQ9E


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