The All-band HF Amateur Radio Transceiver: Just waiting to be modified
All hardware posts
Nick VK4PP has 45MHz filter boards to fix spurs available now to order. He has installed one himself and it works fine.
Nick has added them to his ordering page. The price is a stunning ONE cent only + postage. Nick says
“Feel free to take up to 10 and also maybe sample the other warez …”
VK4PP Order Form
Raj VU2ZAP has found good reduction of spurs with the following mods:
1. Replace L5 with an SMD inductor 680nH or 681nH. Mount at right angles to old toroid
2. Replace L7 as above. Note that C211 is bypassed with this mod.
Don’t be distracted by the missing C216 and C210. These parts were never populated.
An additional 45MHz filter is not required and, therefore, you should get almost the same power out as stock boards. If you ALSO use the second 45MHz filter the spurs get further reduced, but with some loss of output power. Raj used Murata inductors 1210 size. 1206 size will do.
With Mikes W0MNE relay fix and this mod, Raj has the uBitxv3 and v4 boards now operating within normal emission standards.
Raj thinks that a combination of high Q, small size and possibly internal shielding contribute to the reason why. Another hypothesis is that the original inductors caused ringing. Raj has also replaced L1-4 with 330nH SMD and that helped a little on one test board. His interest was to fix the problem of spurs with the simplest solution and without major surgery.
Mike W0MNE has come up with a fix for the harmonics problem with the µBITX that is simpler than other fixes …
He has two Ver 3 and one Ver 4 uBiTX rigs. He simply replaced K3, KT1, KT2, and KT3 relays with Axicom relays.
This has knocked down the 3rd, 5th, and 7th harmonic levels from -35dB to below -50dB on all 3 rigs. No board modifications are required, and no changes to the LPF circuitry is required. This may not be reproducible on all rigs, but with 3 rigs showing the same results it may be something constructors should consider.
Mike purchased the relays from Mouser – part # 655-V23105A5403A201. Cost was US$2.89 per relay plus shipping.
Raj VU2ZAP found a picture of the “innards” of the Axicom relay, and suspects the improvement in isolation is the result of the contacts being closer to the ground plane (as the relay coil is on top in this relay, whereas the standard relays supplied with the main board have the relay coil underneath the contacts).
Before the fix on 40m …
And after the fix on his Ver 4 rig …
The following diagram from Henning DK5LV provides an insight into the final four stages of the µBITx PA chain. This stretches from Q90 through to the finals.
Warren WA8TOD has measured the gain and drive requirements of the transmit chain at operating frequency from Q90 through the finals. A sweep generator was inserted at C80 and the level adjusted first for a nominal 1 watt indicated on the wattmeter (Purple trace) and then five watts indicated (Blue).
– Gain of the chain varies from 56 dB at 3.7 MHz to 52 dB at 28.2
– Input levels at C80 were -24 dBm (purple) and -15 dBm (blue); note 9 dB input increase generates 7 dB output increase….. 2 dB com
pression and resulting unacceptable IMD (not shown)
Jack W8TEE and Al AC8GY have been working on a display replacement for the µBITX for quite some time. They now have a preview video that gives an overview of the JackAl system.
There are some software hiccups in the preview that should be fixed soon. We just sent Rev 4.0 of the JackAl board off to the PCB fabricator. They hope that this will be the final version of the board. All SMD’s will be included on the board.
JackAl makes use of either 5″ or 7″ 800×480 displays (US$34-$44) using the Teensy 3.6 microcontroller (US$30) and its companion audio board (US$14) for the DSP.
The Teensy has 1Mb of flash memory and 256K of SRAM, or which they are using less than 20% and 15%, respectively. Jack and Al have also brought out a dozen “empty” pins for experimentation, so along with the I/O pins, there plenty of resources left to play with.
Pay particular attention to the ALS Tuning it uses. It really makes it so much easier to zero in on a station. Details are all in the video.
Warren WA8TOD has now tested the MMIC based wideband amplifier board from SV1AFN https://www.sv1afn.com/wideband-mini-amplifier.html in two different IF stages in the TX chain.
This is a replacement for the 45 MHz transmit amp comprised of Q20 – Q22 on the µBITx main board. Warren removed C20 and C22 and used two short lengths of miniature coax to take the signal off-board to a 7 dB attenuator and then on to the MMIC amplifier, and then back onto the board.
The resulting MMIC amplifier gain was +16 dB to match that of the BI-DI amp on the stock µBITx.
The yellow trace is before the amplifier board was inserted and the purple trace after. 3rd order IMD was reduced by nearly 10 dB over the stock µBITx by using the MMIC amp.
Warren restored the Q20 – Q22 amplifier and moved the MMIC amp to the Q40 – 42 amp with similar results. Here he found that he required the full 23 dB of gain provided by the MMIC to achieve the same level of main signal.
Yellow and purple traces are as before with the new measurement indicated on the blue trace. Results are almost identical indicating replacing these two amps together would provide 19 – 20 dB of IMD improvement which would make the transmitter completely viable and, in fact, better than some commercial radios in terms of IMD.
In both cases the indicated power out from the two tone test was a little over 4 watts.
Warren also tested this MMIC board for its susceptibility to oscillation. He connected a 60 dB attenuator between the input and output while feeding a signal into the input. He then gradually reduced the attenuation one dB at a time until oscillation was visible on a 1.5 GHz spectrum.
The amplifier broke into oscillation very reliably when the attenuation was stepped below -14 dB. Higher than that and it was completely stable.
In the course of these two tests the board was hanging unshielded about 3 inches from the PA heat sinks and the output was a little over 4 watts. In both cases he saw no indication of oscillation.
The spurs can be conquered by a relatively straight forward modification. The circuit diagram for the mod is shown above and the photo below:
The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit. Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.
Warren’s implementation of the filter uses a single transformer on the output of the crystal filter. A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer. He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output. No doubt a through-hole component could also be deployed.
The filter input is attached directly to the other R27 pad. Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.
In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.
A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs. uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.
Glenn VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board). Photos of their board designs follow:
Warren WA8TOD previously raised concerns about the level of intermodulation being generated in the µBITx TX signal path. IMD may not be as pressing an issue as harmonics and spurs, but the µBITx seems to generate more IMD than it should. This can cause audio distortion and splatter (including out of band transmission if you are operating near a band edge). IMD only occurs in SSB modes, not on CW. If you are CW operator you only need to address spurs and harmonics for which there are now solutions.
The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products.
Warren’s µBITx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. His board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.
In order to better understand where the IMD is being introduced in the radio Warren started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points (on the v4 main board) up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.
TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of the measurement configuration.
TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.
TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.
TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.
TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total… too much and probably indicative of low injection levels as has been stated in the past. On the other hand, it is not the primary culprit by far.
TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.
From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be increased by RV1 adjustment to significantly higher levels.
The IMD problem is rooted first in non-linearities in both bi-directional (bi-di) amps and then in both mixers. The focus is likely to be initially on addressing concerns about linearity in the bi-di amps (on TX only of course).
Glenn VK3PE notes that the original article by Wes Hayward and Bob Kopski on bidirectional amplifiers shows a slightly different biasing arrangement and feedback in the first stage compared to that used in uBITX. As shown, gain is 15dB and flat to 100MHz within 1dB.
A more conventional resistive voltage divider is used and the feedback is AC only. Two extra parts are used. The article doesn’t mention IMD though specifically, as a performance target or measure it.
Glenn has plotted the gain difference between uBITX and Hayward versions of the bi-di amplifier after building a prototype.
There is a difference in input levels before output clipping occurs. The biasing arrangement also gives different Iq. Gain is reduced about 4dB in the Hayward version over the uBITX. It accords closely with test results from Hayward’s paper of 15.5dB. Glenn got 16dB of gain at 30MHz.
Haywards paper gives some values for varying the gain to other values also, so there is more experimentation to be done.
Henning DK5LV notes that the designer of the Bi-Di amp states his version is designed for 15 dB gain, which is why he has a series feedback system (680 ohms + cap) from collector to base, and the biasing is done with two extra resistors.
Ashar VU2ESE in designing the µBITx uses only the biasing and feedback with the two resistors.
Henning suggests that the result is that Ashar’s amp has about 20 dB gain, but the frequency response is worse due to the limited fT of the 2N3904 and input reflection (S11). Due to the higher gain, IMD must be worse for the same input level. If the textbook curve applies (3 dB for every 1 dB of input power change) the IMD will be 15 dB worse.
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]
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: