Category: Hardware

All hardware posts

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RF preamp

Woody KZ4AK says “My µBITX has always been a little deaf (couldn’t figure out why) so I built a RX front-end addition”.

Woody also wanted AGC so it became an amplifier followed by an attenuator. As with prior AGC / RF Gain mods, it inserted in the RX antenna trace (Ripley, et.al.). As built, it provides a maximum gain of ~ 8 db (+12db from mmic – 4 db from atten) with a 3.5 db NF, which can then be attenuated to more than (about) 50 db down.  Woody has not measured this exactly… but it really wakes up the RX!

Attenuator Control: >5 volts for minimum attenuation, approaching zero volts for maximum attenuation (at 22 to 5 ma, depending on attenuation setting). For manual control would use a potentiometer and voltage follower for current buffer.

Woody made no attempt to minimize the current draw. It is a bit of a pig, consuming 70 ma + control current (22ma).  Also, no attempt was made to minimize the board physical size.

The circuit uses a MAR3 SM MMIC followed by a HSMP-3816 pin diode array attenuator.

If anyone is interested,  he has a to-scale PDF for the board layout, a parts placement image, full resolution schematic, etc.  Woody can ZIP up a package for you …

He is also currently working on an audio derived AGC amp/controller for this thing. Have it sort of working, but he is not happy with the performance yet.

Reference
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G4USI Go Box

Several constructors have put their µBITx in a Go Box for portable/emergency use, but this is one of the best looking results yet.

Daimon G4USI has used a 3D printer to produce a very professional looking front panel.

This is a re-mix of DU2RK’s uBitx Case, and the re-worked front panel of this case by AngelDMercedes.

Daimon wanted a different case, one he could build into an existing flight case to create a Go-Box for HF.   He remixed the ideas above to create a case which fitted his flight case perfectly, but with every control and function sitting on the top panel.

In the strictest sense this is not a full case. It is a top and two vented sides. There is not a bottom, front or back panel –  the flight case provides the structure.

On the top photo you can see a 12v 5a power supply and mic in the space to the left of the rig.  Daimon now has a home-brew EFHW multi-band antenna, SOTA Beams ATU and miscellaneous portable QRP operating items in the right hand compartment.   Everything, in fact, that he needs for portable QRP HF.

Reference
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A spur fix and BFO leak fix from Ashhar Farhan

Spurs fix (See update below)

Ashhar Farhan VU2ESE, the designer of the µBITx has been experimenting with solutions to the spurs on SSB mode above 18MHz.   He has posted to the list that one spur fix that is now consistently working for him, and that is easy to apply, is to fix the distortion in the 45 MHz amp.

He inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board.   This is on the output of the 45 MHz IF amp going to the front end of the mixer.   The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).

UPDATE But wait … he proposes a better spurs  fix a day or two later

1. Change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair’s standing current and decreases the Harmonic distortions.

2. The 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A ‘T’, rather than a Pi is used. This helps improve isolation on both the RX/TX path.

3. The LPF consists of an L-C-L of 300nH, 47pf, 300nH.

Removing the BFO leak into the local oscillator

Ashhar also found that there is cross talk inside the Si5351, whereby the 12 MHz output from the BFO leaks into the local oscillator.

Ashhar used  a 5µH inductor in series with a 47 pF capacitor placed across C202 of the LPF suppresses this. The inductor comprises 35 turns on a T30-6 toroid (to give 5µH).   However,  another constructor found this blanked out the 30m band, and calculations reveal that there may be an error with the calculation of inductance or capacitance that is waiting to be confirmed.  The filter should be tuned to 12MHz suggesting a slightly small capacitance value of around 35pF

uBITx.net will be looking for verification from constructors that these mods represent a definitive fix for the spurs and carrier leak problems.

Reference

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The cause of spurs

Ashhar VU2ESE and Raj VU2ZAP have nailed the cause of the spurs.  The 45 MHz  IF amplifier is distorting and producing a second harmonic at 90 MHz,  which mixes with the local oscillator to produce a spur at 90 MHz.
Here is an example of how it happens :
  • the spur moves down as you tune up.
  • when the radio is tuned to 28 MHz, the spur is at 17 MHz.  When the uBITx is tuned to 28.150 MHz, the spur is on 16.850 MHz.
  • At 28 MHz, the local oscillator  is at 73 MHz.
  • There is a signal X such that :

X – local oscillator = 17 mhz

Local oscillator = 73Mhz.

X – 73 = 17,  so X must be 90 MHz.

After checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.
Now to confirm a simple solution …  if the problem is in the 45MHz IF, then it may also be possible to address the IMD at the same time.
Reference
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Higher current Axicom relays

Jim W0EB has finally received his >500 mW Axicom relays, after they got misrouted in the mail.

H plugged these into his V4 test bed built up on an aluminium open chassis. Sockets are mica filled 2mm thick with silver plated flat contacts. This leaves the relays 2mm above the PC board.

With the >500mW coils (i.e. with coils that have fewer turns on them) the hope was that this would lessen  the extent of inductive coupling between the switched portion of the circuit and the relay coils.  Unfortunately this does not appear to be the case.  Jim got almost exactly the same results as with the 400 mW relays mounted the same way.

40 meters is still just barely in spec with the 3rd harmonic being -43.8 dBC. The others are also in spec but not much better with 3rd harmonic varying from -46 dBC on 10 meters, -44.5 dBC on 12 & 15 meters , -44.7 dBC on 20 meters and -57.3 dBC on 80 meters.  60 meters wasn’t tested.

Raj VU2ZAP notes that it makes quite a difference between socketed an unsocketed replacements.  It is best not to use sockets for the relays, in order to get the relays as close as possible to the PCB.

Before and after shots of relay replacement as observed by Raj follow:

Before replacement
After replacement

Raj and Jim both note that there is variation between boards in terms of improvement in performance.  The reasons for this are not known.

The conclusions are:

  • relays help, and ubitx.net recommends them, but replacing relays with AXICOM relays may not always be sufficient to reduce “blow by” on some bands or modes
  • there is no point in spending more on Axicom relays with a higher coil current
  • the best option would be to replace the filter section altogether, and there are a number of ways of doing this.
Reference
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Relay board from VK4PP given thumbs up in testing for isolation

Nick VK4PP sent one of his LPFx4  test boards to Warren WA8TOD for blow-by testing.   It looks like the bare PCB has excellent isolation between its input and output, of around 90db.

Matched with RF rated and separate in/out relays this board will perform very well.   The updated pic above shows a few tweaks and more ground stitching.

The only downside is the US$1.50 each relays (x8).    Relay K3 can potentially be reused, or you can source AXICOM relays for a bit more.

Nick is interested in feedback from constructors on the level of demand for these boards.

Reference
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Variable bandwidth filter mod

Broadening the crystal filter

Michael N2ZDB notes that the BITX40 he is building receives somewhat OK but the transmit audio is terrible with distortion and/ narrow bandwidth.

He figured he may have one or two bad 12.0 MHz crystals, as the bandpass is only around 1,700 Hz at best (11.998800 to 11.997100)!  The crystals are from MEC and the letters / numbers on them are: 12.000 G   MEC AH6L1 .  He asked whether anybody knew of where to obtain suitable replacements.

Well, one supplier was disclosed by Lawrence W8LM:

SSSSSSShhhhh!!!!!!  THIS IS MY **SECRET** U.S.A. SOURCE…

between YOU and I ONLY!!!!!

12Mhz Crystals 5 for a $1.00  or 15 cents each in lot 100 quanities…

Right! 5 for a DOLLAR.!!!…. over $75 free shipping, under $75 flat $7 shipping.. They have 1/8″ uBITX volume knobs too..

https://www.allelectronics.com/item/cry-12/12-mhz-crystal-hc18/u/1.html

Also read my review on 50ft. RG58/u cables with BNC’s both ends (GO KIT SPECIAL) … and adjustable 6.8uH inductors..
I usually get my order Calif. to Kansas in 2 days…

Variable bandwidth filter design

Meanwhile, Jim AB7VF, posted a circuit for a variable bandwidth filter for the BITx transceivers.  This is based on a circuit patented by TenTec (the Jones Filter design).  Some constructors may want to try this one out, as it involves only a few additional parts and replacement of C217 -C221.

Reference
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The last challenge: reducing IMD

One of the last challenges to be conquered with the µBITx is the unacceptably high intermodulation distortion produced in the IF stages on transmit.

The IMD seems to be sourced in the Termination Insensitive Amplifiers used as bi-directional IF amps at both 45 and 12 MHz.  

Warren WA8TOD is experimenting with a prototype MMIC amplifier utilising the ERA-3SM+ (80 cents each on eBay) mounted on an RF prototyping board ($2.50 each from SV1AFN) that is the proper size and form factor to replace both transmit TIAs. The MMIC/board measure flat response from 300 Hz to well over 200 MHz, and will provide around 22dB of gain.

This test quantifies the IMD performance of the combo. The reference point on the SA is shifted +6 dB so that the readings reflect the power level that would be achieved with a single tone for convenience of interpretation.

Measurements were taken at Vcc of 3.3 volts and current draw was the recommended 35 ma.

The blue trace shows performance at 0 dBm out and is an acceptable -42 dBc (-36 dB minus 6 dB for a single tone carrier). Performance at – 10 dBm out, the level needed to drive the uBitx driver/PA chain, is an excellent -51 dBc (-45 dB minus 6 dB for a single tone carrier).

Yet to be determined is how to add the LM1117-3.3 regulator to hold the Vcc at the required level.

Reference
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Replacing relays with Axicom brand relays

 

Jim Sheldon W0EB has now replaced all stock relays in one of his µBITx transceivers with Axicom D2n 12v relays.

All five relays have been replaced with Axicom D2n 12v parts.  Photo courtesy Jim W0EB.

This improved filtering by a wide margin on all bands except 40m.   The results:

80 Meters 3.500 MHz
7.000 (2nd harmonic) – 65.7 dBC
14.000 (3rd harmonic) -76.4 dBC

40 Meters 7.000 MHz
14.000 (2nd) -58.0 dBC
21.000 (3rd) – 47.2 dBC

30 meters 10.000 MHz
20.000 (2nd) -76.9 dBC
30.000 (3rd) -75.8 dBC

20 meters 14.000 MHz
28.000 (2nd) -65.2 dBC
42.000 (3rd) -64.8 dBC

Jim didn’t test 17, 12 and 10 meters, but he suspects their harmonic suppression is in the same boat as the rest.

Closeup of the four Axicom relays in the output stage. Photo courtesy Jim W0EB.
Reference

Graph showing results

The graph above provided by Gary AG5TX shows the resultant effects of using Axicom relays (using Jim’s data points).

Gary says:

“Your shared data maybe useful as you have shown the same board and relay with different measured results. For that particular board and the measurement data you gathered, I would say the results ARE encouraging from this viewpoint:
You have 4.5 dBc more margin to FCC spec on your worst case data point (40m 3rd harmonic) with the supplied data set.”

Reference

 

Sourcing Axicom relays

These relays can be obtained from a number of sources.  If you live in the US they can be found at Arrow, Digikey and Mouser for less than US$3 each.  They are also available from Aliexpress for under US$2 each in a set of 10 and from Component HK in Hong Kong for under US$1 each.

More details on using Axicom relays for RF switching

For those interested in the benefits of Axicom relays see this webpage for a switched Bandpass filter bank:

http://www.i1wqrlinkradio.com/antype/ch97/chiave50.htm

There are wealth of tips on this page for those building switched filter banks.

Conclusion

It appears that the Axicom relays are the answer, and Ashhar Farhan has already committed to replacing these relays in production µBITx, presumably when the current µBITx stocks have been depleted.  Hopefully Raj’s mod for removing spurs will also feature in a v4 board upgrade shortly.