Month: August 2019

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TX IMD measurements for a v5 uBITx

Doug K4DSP just finished measuring the two-tone TX IMD on his v5 µBITx. These are 3rd order products, and they were measured relative to either tone. Since the ARRL states their measurements relative to PEP I have included that as well, for those who sleep better at night secure in the belief that their radio is clean 🙂

This is at 10W PEP using 700 and 1900 Hz tones. Doug’s radio puts out 10W from 80 through 20 meters, and falls off to 5W on 10M:

80m -25.5 dBc (-31.5 dB PEP)
60m -22.5 dBc (-28.5 dB PEP)
40m -22.0 dBc (-28.0 dB PEP)
20m -24.5 dBc (-30.5 dB PEP)
17m -21.0 dBc (-27.0 dB PEP)
12m -25.5 dBc (-31.5 dB PEP)
10m -22.0 dBc (-28.0 dB PEP)

As Doug’s flight instructor used to say after one of his landings, “I’ve seen worse, but I’ve seen a lot better.” This is probably not atypical of IRF510s.

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Powering a larger Nextion screen from the raduino

If your larger (4.3″ or greater) Nextion display is flashing, you are probably overloading your Raduino 5v power supply.  This is because the Nextion does draw quite a bit of current.

Mark AJ6CU found a 7805 with a higher rating on Amazon (maybe 1.5A?). He mounted it on a heat sink attached to the rear panel (using the rear panel as a large heat sink) with a small perf board (to mount the appropriate capacitors).  He then brought in a set of 5V wires to replace the 7805 on the one on the Raduino.  This could be a helpful idea for those of you installing Nextion display units.

MVS Sarma suggested using a 78T05 as a replacement to the 7805. This is  a 3amp TO220 device and available very cheaply .   Managing the TO3  is little dìfferent compared to the TO220 on the 7805.
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Fan cowling

Mark AJ6CU has made a 3D template for a fan cowling for your µBITx. The 3D template for the fan shroud can be found on thingiverse here:

https://www.thingiverse.com/thing:3790450

It basically bolts onto the 80mm fan at the rear of the case (you might recognize the “tan” of a Noctua fan — much quieter than the one supplied). It sits on the uBitx motherboard with two slots straddling the heat sinks.

Marks notes that he has not powered on his µBITx yet, but suspects it will keep things nice and cool even in digital modes. This was designed with Fusion 360. There have been several other suggestions for adding ducts for cooling some of the hotter spots in our radios.

I was inspired in this effort by the work by another cooling shroud for the “shorter” of Sunil’s cases. See:

https://www.thingiverse.com/thing:3360539

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Removing the detent in a 100PPR optical encoder

This optical encoder (widely available on eBay or Aliexpress in black or silver) has been used by some constructors as a replacement to the standard mechanical encoder supplied with the µBITx kit.   It is a 100ppr encoder (whereas the one supplied with the µBITx is 24PPR) so may not work perfectly with either the default firmware supplied by HF Signals or Ian Lee KD8CEC firmware (i.e. you may need to experiment with the code for generating the VFO).

With the detent mechanism removed to allow free turning the encoder gives a very smooth tuning action.   With the encoder able to spin freely the protruding crank handle will always come to rest in the down position so it too has to be removed.

The main complaint with these encoders is the detent feature.  To remove the detent, here are the instructions from Robert GM4CID:

Remove the knob handle, then, using a heat gun or hair dryer heat the silver knob insert to soften the adhesive and carefully pry off that silver insert to reveal retaining screws that can be removed. The mechanism will now come apart and you can remove the detent. Replace everything but not the handle.

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Nextion noise in RX

MVS Sarma VU3SMV  noticed noise on his receiver generate from his Nextion display.   He placed a 2.2mH RF choke in series to the 5v line feeding the  Nextion display.   He also placed a 220µF capacitor across the 5v and Gnd terminals for the Nextion.  The hash was reduced to just 10% of its original value.

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3D printers and the uBITx go together

Doug K4DSP hesitates to say that that is µBITx is actually finished, as he keeps thinking of stuff to add to the hardware and firmware! But the cover is on and he’s making QSOs.

Previously Doug had only used his rig on CW, but he used a 3D printer to make a mic case and got it wired up.   His first SSB QSO was with S51DX.

Doug is pleased with how well the little radio “hears”.   While drafting his update for the BITX20 IOGroups list he was listening to G0EVY on 40M SSB who was booming in.

Doug has yet to install the AGC board so he finds the static crashes are “pretty harsh”.   He says it reminds him of his old Heath HW-16 in that regard.

The mic case  design was sourced from Thingiverse:
https://www.thingiverse.com/thing:3044144

It uses the mic element and microswitch that were included in the uBITX kit. He printed it with black filament and then rubbed some white paint into the front to give it some “character”.

Doug printed the uBITX case from here:
https://www.thingiverse.com/thing:2823172

He only used the front and rear panels.  He designed his own middle section to add mounting holes and a baffle for an internal speaker.   He used FreeCAD to do the design.

Doug calibrated the frequency against a 10 MHz GPSDO. Then, based on advice of others, he used a PC audio spectrum analyzer program (Spectrum Lab by DL4YHF) to adjust the BFO frequency. That worked a treat, and he’s really happy with the way the audio sounds now.

He concludes, “It’s just a very pleasing little radio”

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