The Beauty of Customization

Okay.. some corrections to the previous posts. The one pin connector that comes off of the TX Audio connector is, of course, PTT. On my sr63ng, anyway... the way the cable dimensions are specified in the instructions.. it's too short to make it to the PTT connector. I suspect it would be on a "real" Softrock 6.3 also. I extended the connection and changed the sex to match the sr63ng be making a 3" cable with pin headers on each end and then I heat shrunk the splice parts for a "permanent" splice. So it had nothing to do with eliminating the LDO regulator for the 3.3V supply for the si570 as I thought based on the missing regulator in the official Softrock 6.3 base kits. The regulator is coming for the "real" Softrock 6.3 RXTX base kits in the "service pack" as stated on the Yahoo groups site.

Okay.. secondly since I'm using a sr63ng... some of the updated parts for the SDR-Cube specific use obviously are not in the unit I already assembled. The sr63ng is almost an exact clone and most of the circuitry is exactly the same except for the fact it uses the much better to build 0805 parts (IMHO) instead of "shoalds" of bent through hole resistors and through hole capacitors.

I discovered tonight that the RX op-amp will have to have the supplied 220 pF capacitors changed to 4700 pF, just like on the Softrock "base kit". The reason for doing this at first is not obvious. The SDR-Cube samples at a rather low 8 KHz to keep processing in line with the DSPic33 processor capabilities. Because of this, a 192 KHz wide filter, like is on the Softrocks/SR clones and most other SDR's that would externally hook to the SDR cube would have aliasing issues with the excessively wide filter on the rig. George calls this "BCB blow by" but the reason it would be happening is indeed aliasing. Mix products would be pretty ugly if it were left at 192 KHz. I also made the realization of why AM and FM would be difficult to do maybe even if the PIC32MX with 2x the processing power was used, with a 16 or 22.1KHz sample rate. To properly do those modes, a Zero IF wouldn't be feasible like it is with USB/LSB and CW. I'll get into that later. AM is doable and maybe NBFM but it would be more interesting to do as there would have to probably be a conversion stage (in software) like the PC based SDRs do at low KHz IF. Of course.. if at some point the sample rate is increased in the future this filter will need to be widened up. :O)

The RC filter uses a 4.99K and a 220 pF cap for the 192 KHz analog filter. 4700 pf / 220 pf is 21.4 times difference. 192 KHz / 8 KHz is roughly a BW reduction of 24 times. Adjusting for standard values.. that's close enough.

The thing I don't like about this is that when using with an external SDR, either the SDR would have to be modified for the SDR-Cube or an additional op-amp filter is needed. I think that additional filter should have been on the DSP board on the cube to keep the Softrock or other SDR front end "standard".

I have "472" 0805's in my collection of chip caps.. so I will "customize" my sr63ng for the cube.. tho I'm not really wild about doing this.

The other "customization" that has been coming out on the Yahoo group is the TX balance adjustments. There is no good way to do this on the cube currently. One suggestion is to add adjustment pots to the DSP board. I will further investigate that. I wonder if a board with I2C or SPI EEPOTs wouldn't be a better solution. This again is an addition that probably should be done to the cube for clean image rejection on TX.

But the main purpose of this post is to show my solutions to the mechanical issues I don't like so far. I decided to tie down the regulator instead of relying on double stick tape. I left the tape on... but left the backing on the back side so it's not stuck to the aluminum. I like the actual physical connection with the nylon Ty-wrap much better. To do this a hole needs to be drilled on the other side of the regulator on the sub-chassis bracket.


Also, on my cube one of the electrolytic capacitors was rubbing on the case. It was actually already starting to pull off the DSP board from the manipulation I've done to mount boards. I initially thought that I would just use a piece of Kapton tape to try to insulate the cap. But once I noticed that it was getting physically damaged I thought I needed to do something about it.

I do testing for a living including a (very little) ESS testing. You can do whatever you like with your cube.. but wedging a PCB in with a connector and a standoff to "bend the board" violates most accepted quality standards... therefore, I respectfully disagree with the "no bracket is needed" statement by the cube designers. Here is what I did to fix both of these issues:

Notice the cutout. That bloody capacitor won't rub now. (Metal nibbler and a Sharpie.) Of course look at the bracket.. that could have fixed it by itself *if* the NUE-PSK mini-DIN connector I/O board would have cleared the ribbon cable on the DSP without the cap rubbing against the case. It didn't.

This fixes both the "floating DSP board" and the "rubbing capacitor" issue. The bracket is 1/16" thick aluminum with a 90 degree corner bent in it. About 3-7/8" long .. 1/4" on the back wall side.. 5/8" long into the DSP card. Metal nibble out the bracket so it doesn't short.. as necessary. The screws are 4-40... nylock nuts. Nothing too special.


The above picture shows the additional screws on the back.

I feel much better about the mechanical situation now. The cabling.. because I intentionally put on longer heat shrink the cables will physically wedge between the boards.. so I feel better about that as well. If they do not properly wedge in place they will be tied down.

Whether or not you choose to do any of this is up to you but I highly recommend it.