Testing removal of spurs with additional 45MHz filter

The photo above shows an additional 45MHz filter (15khz passband) inserted in place of R27 (you can’t see the centre wire on the filter, which is attached to the ground end of R13).

Early indications are that this removes all of the offending spurs.   This will make it  a uBITx.net ESSENTIAL MOD.  The mod has, however, yet to be tested by uBITX.net.

Warren WA8TOD has completed spectrum analysis plots for each band, and these have been reproduced below.  The plots show removal of all unwanted spurs.

Conditions for the test:

  •  eBay filter in place of R27. No other changes.
  • Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

Yet to be verified:

  • 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance.  It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.
  • Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance.  Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Reference

Why has KD8CEC’s firmware been so successful?

There are several alternative firmware versions available for use with the µBITx transceiver.   So why has the KD8CEC firmware been so successful with uBITx owners?

Some key reasons put forward by uBITx.net:

  1. KD8CEC firmware is fully compatible with the standard issue kit.  No hardware changes are required to make it work.  This is a critical point of difference with all of the other variants, and probably the most important factor associated with the success of CEC firmware.
  2. No custom wiring changes are required when using the firmware.  This is a further significant factor in widespread adoption.
  3. The firmware fixes problems that come with the factory firmware – although some argue that CW modes are still not fully addressed in the CEC firmware.
  4. Using CEC firmware means no loss of features from the default factory firmware.   Other alternatives offer fewer or different features from the factory firmware.
  5. Users are familiar with the  user interface, as it reflects the default firmware’s “look and feel” with the standard display supplied with the kit.
  6. It is easy to upload a hex file to the Raduino. Constructors without a working knowledge of the Arduino IDE can upload files easily.
  7. All full releases of CEC Firmware are available as open source firmware.  This follows the same structure as the original code, although most of the code has been replaced.  Open source is not released for beta versions (and for good reason).
  8. No additional processor is required, unlike other firmware variants.  A mechanism for adding additional processors has been added in ver 1.097 (Beta).   This promises a future where multiple processor support will be available.  You won’t be locked into a single processor type.
  9. Addition of a Nextion display or additional processor is relatively straightforward.
  10. The firmware on the Nextion display can be edited by others to provide a different “look and feel” or to add or subtract features.   This is independent of the firmware for the transceiver.

Speaker grilles

Barrett K5SSO points out that these speaker covers may be just the trick for your µBITx!

Tim AB0WR used some fine mesh black hardware cloth i.e. screen door wire. He cut a piece out of the hardware cloth that is a little larger than the speaker diameter and run a mounting bolt through from the top of the cabinet, through the hardware cloth, using the speaker to clamp the hardware cloth against the underside of the cabinet.

Reference

10dB spur reduction mod

Raj, VU2ZAP has come up with a fix that reduces the spurs by up to 10 db  and requires ONLY ONE part to be added.   Farhan VU2ESE has come up with an alternative modification.

These mods result in a significant change in the level of spurs above 10MHz with some improvement below this frequency as well.

With Raj’s mod CW may not work anymore and will need some more mods.  With Farhan’s modification CW will still work.

Raj VU2ZAP Instructions

  1. T2 – desolder the transformer wires that go to pin 3 and 5. Pin 1 has a square pad.
  2. Bring out the two wires above board and join them together and solder.
  3. Take a 45Mhz filter- 45M15 or  similar 2 pole  (one crystal only) and solder one end of the filter to the wires of T2 pulled out. The centre filter wire to ground at one end of R26. Check which end of the resistor is grounded.
  4. Solder the third wire of the filter to C10/R27 junction.

This mod prevents the leaked TX signal that gets amplified by the 1st bidirectional amplifier from getting into the first mixer and creating havoc.

Farhan VU2ESE Instructions

  1. Remove R27
  2. Solder the 45Mhz filter two extreme ends to the pads of the resistor.
  3. Solder the center lead of the filter to the nearest ground. R13 is very near with a ground via.

Using the first method (Raj’s solution) the extra filter will work in RX mode as well as TX, but CW is disabled.  In  the second approach, the filter is only used in the TX path.

Folks with DSA815 or better please share your feed back. The filter may work better properly terminated.

Reference

Dumping grounds …

What is it about amateur radio ops that they feel they can dump all over other constructor’s efforts or ideas? I am sure it happens in your local amateur radio club, in your national association and in every email reflector and facebook group.  The IO Groups BITX20 list seems to have one or two trolls at present.

Email groups are a perfect opportunity for trolling activity. It’s best not to respond.  Complain to the moderators instead.  If somebody is putting down their competitor’s product offering, what could be worse?  Don’t support their product.

uBITx.net is not affiliated with the moderators of the BITX20 email list or HF Signals.   The views on this website are those of the site owner.

Cleaning up the transformers in the output stage may fix harmonics

Jim AB7VF suggests that much of the harmonic cruft in the µBITx is from DC current flowing through the L7 and L8 ferrites that effectively lowers the inductance as current increases and allows RF to go everywhere.

Jim replaced the electrolytics as can be seen from the picture ..

He suggests that the transformer is effectively a centre tap.  He wanted tighter coupling between the two windings ….  The electromagnetic field set up by the DC passing through the coil will bias the ferrite “magnetic domains” causing a shift in the B-H field resulting in loss of inductance and the generation of spurs .

When feeding the center tap – current flows up toward the “dot” or start of the top winding “left hand rule” will give you the polarity of the magnetic field around the top coil … current also flows through the bottom coil away from the dot or start of the bottom winding creating a magnetic field opposing the one created by the top coil. The net result is no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.

The following photo shows the 80 meter output of Jim’s unit after doing the L7,L8 mod and the output transformer mod.

Jim suggests putting a proper inductor on the IRF510’s that is NOT affected by DC current flowing through it and you will get legal output on 80m CW.

uBITx.net would be interested in whether this approach works for others in cleaning up the harmonics, because it will be a lot cheaper and easier than sorting the LPFs.

Reference

80m harmonic fix by changing the capacitance of the LPF

Howard WB2VXW previously mentioned that he would try to reduce the harmonics by changing the output to 25 ohms and adding a step up transformer to go back to 50 ohms for the output. This would allow tripling the values of the capacitors at the input and output of the filters, reducing the effect of the stray relay and layout capacitance.

In the end after more simulation, Howard decided just to change the filter characteristics and retain the 50 ohms termination impedance.

Howard was able to come up with a filter that doubles the capacitance.  At least on 80 meters, with this new filter design, the harmonics don’t exceed -45 dB in his tests. Not as much margin as he would like, but legal.

Howard changed the 3 inductors to 1.6 uH by adding 2 turns on each.  He added an extra 1000 pF cap in parallel with the input and output caps, bringing the total to 2000 pF.  He also added a 620 pF cap in parallel with the one of the two paralleled 1000 pF caps in the middle two sections for a total of 2620 pF. (750 pF would have been better, but he didn’t have one).

Howard is asking others to give this ago.   After validation of the 80m LPF redesign he plans to try a similar solution for the 40 meter band.

Reference

Some mods to improve RX sensitivity

George UR4CRG/RX3ARG  suggests some mods to improve RX sensitivity:

  1. Add a parallel capacitor to R12.  Try C=470 pF.  George found in his signal analyser that this gave a 3 dB rise in gain at 30 MHz.  He placed an 0805 SMD (or your could use a 1208 SMD part) over R12.
  2. Add two 100 Ohm resistors to  the collectors of Q11 and Q12.  You will need to cut the PCB tracks to add in the resistors.   The emitter-followers do not like HF signals and a small collector resistor gives light feed back

After these modifications, reception was noticeably more sensitive/

Reference

 

W0EB TSW release forthcoming for 5″ touch screen

W0EB’s uBITX is built with a 5″ RA8875 type Color TFT display connected to a BITeensio Card.  This gives an “uncluttered” display.

This version of the BITeensio TSW firmware is almost ready for release.  It is just awaiting completion of the accompanying instruction manuals  and the arrival of a small  adapter board for connection to the display.  The adapter boards will be supplied in the BITeensio kits upon request.

This display, like the previous 2.8″ Colour touch panel display and the 4 Line I2C display (2004) are capable of utilising an optional external USB Keyboard for rig control and CW. The W2CTX RCP (Remote Control Program) works with this as well.

For information and updates see the TSW website.