Wednesday, February 9, 2011

JUMA-RX1 possible mod

To the time of writing this post., my JUMA-RX1 kit is still waiting with postal services. Will be pick the kit up tomorrow....

Time to look into some possible modifications. Why, you may ask, why the hack modify a well thought through design? Vy vy simple, the receiver, due to the simplicity of the design, is limited to bands 40m and lower, i.e. 100kHz to 7.5MHz.

So, what is that constraint all about? The design uses a 16F819 as controller. This PIC is limited to a 20MHz clock frequency. The clever bit of OH2NLT's design is to use the 16F819's clock for the AD9833 DDS too. This however limits the DDS output frequency. One possibility would be to have the DDS running at another clock frequency. Such a modification would however require reprogramming of the firmware, in order to adapt to the new DDS clock. Possible, ok, but somewhat intensive.

OH7SV designed a direct conversion receiver with an active switching mixer. Absolutely nothing wrong with this design. If we sacrifice the 136kHz band, we could gain the 30m and 20m bands by doubling the LO. The frequency coverage will move from 100kHz-7.5MHz to 200kHz-15MHz.
There is one downside to this game, the LO frequency steps are also doubled. The 10Hz minimal steps would result in 20Hz QRG change... also frequency readout will have to be doubled to determine the QRG.

Here would be my preferred solution: introduce a second front-end having a low-pass filter, a subharmonic mixer and a low noise op-amp AF preamp. A dual switch toggles the antenna input and the preamp output between the original and the additional circuits. Extracting signal to the subharmonic mixer may require an additional adjustable driver.
Such a mod will preserve 136kHz operation and better fine tuning in direct conversion mode and offer improved performance and wider coverage in subharmonic mode.

Should there be no need for 136kHz and/or 10Hz steps, the whole front-end could be modified easily by changing low-pass filter components and adding a frequency doubler between the DDS and the RX boards.
In case subharmonic advantages are on the wish-list, the MUX-mixer could be replaced by a RA3AAE-mixer (don't forget the low-pass!). In this case the differential preamp should be modified too.

I am sure that even more possibilities would be obvious to the skilled artisan.

Stay tune for practical experience, as soon as I picked up and built the kit.

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