Re: Spurs for Dummies? #ubitx-help

Tom, wb6b


I've been enjoying all the conversations on improving the power chain. I'm learning a lot from all the technical issues, design parameters and practical issues that affect the performance of the transmitter.

As I'm mostly interested in digital modes on 80 through 17 meters, the power mods are not of burning importance to me. Yet, it still would be nice to transmit WSPR beacons on 15*,12 and 10 metes. I don't because I read about the transmitter spur issue related to the 45Mhz IF minus the transmit frequency.

Some of the issues seem to related to the possibility of people using the uBITX to drive high power amplifiers and the spur level exceeding an absolute limit rather than just the 40 some odd DB down spec.

For us folks who wan't stay with the QRP sprit of the uBITX would it be possible to change the 30Mhz output lowpass filter (K1 pins 14 and 3) to a bandpass filter? The uBITX is already fairly close to meeting the attenuation spec for the spur, so the additional attenuation needs are not great.

Or changing the wiring to leave the 30Mhz lowpass filter always in circuit and use relay K1 pins 14 and 3 to switch in a highness filter?

* The band that is most problematic seems to be 15 meters, so, sadly, that band may best be a receive only band unless another set of relays and filter were added for that one band.  Or, is it possible to extend the 18Mhz lowpass to cover 21Mhz? The tolerances, in this scenario, might be tight so an adjustable capacitor(s) in the filter might be needed to dial these in. 

For the most part I am very happy with the uBITx and when the dust settles I'll likely make some/all of the power chain improvements. 

Or, I may just wait for the uBITX Gen2 to come out and buy another one. These are great inexpensive transceivers, the old one will be a handy second radio to have that is still very good for most needs. 

Tom, wb6b

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