Month: September 2018

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External LPF filter board from K5BCQ under test

Kees K5BCQ has received his first set of boards. These boards will fit in the space currently occupied by the on-board LPFs.   The piece jutting out and marked “WOOF” is the space for the TX/RX relay (was K3).  The filter parts need to be transplanted from the main board onto small PCBS that are designed to plug in to the board as illustrated above (they are the same size as QRP-Labs filters).

Kees has already made some modifications for better wireability as follows:

1)   Eliminate R3 on the uBITX board and instead add it to the LPF Relay Board.  This frees up space on the µBITX main board and allows more efficient RF wiring for receive. Since the “old R3″ also switches M1 to M2, this requires modification of the board to short M1 and M2 and insertion of the audio pop mod on a v3 board (a v4 board already has the anti-pop mod included).

2) Replace 3 sets of dual 2N3904 Transistors in the PA driver with 3 sets of dual 2N2222A Transistors. Allison had suggested changing the emitter resistors from 22 ohms to 10 ohms (for dual 2N2222A’s).

3) Add a three 1N4148 diode-OR from TxA,TxB, and TxC driver inputs to a TxD driver to pick that relay, which removes the 10m/12m/(15m) LPF when inserting the proper (A/B/C) LPF during Tx. Yes, that means 4 relays are active during TxA,TxB, or TxC transmit. All four of the LPFs use the existing inductor/capacitor components off the uBITX board. They are just moved over to a small 1.5″ x 0.5″ bare LPF board (that footprint is also the one QRP labs has for their LPF/BPF filters, but they don’t sell blank boards.

4) uBITX Antenna wired to either one of two SMA connector pads.

5) T11 will be connected to the RF input on the LPF Relay Board with about 1″ of coax.

6) The smaller 12V 10 pin DPDT bifurcated contact relays can be found on ebay for about $1 each. They fit nicely.

7) Leave all the traces, just make a cut and remove 1/16” of the trace where needed.

[Photos of installation still to come]

Reference
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Removing M1/M2 audio switching from Relay

In considering adding a new LPF module to replace teh original on board LPF matrix, it is helpful to remove the audio switching from K3.   This reduces RF signal routing on the uBITX board.  Many have found the signal routing via K3 causes RF to get into the audio so this represents good practice.

It is suggested that Audio M1 and M2 are tied together at R70, and does not  involve Tx/Rx switching them with K3. 

Power is Tx/Rx switched to Q70 on v4 boards, and the audio pop modification does the same thing on the v3 board.

The procedure is:

  1. Remove R70
  2. Bridge the front pad to the adjacent track
  3. Cut the adjacent track and short the relay sides of both tracks to ground.
Reference
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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
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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.
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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
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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
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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.

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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