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Many uBitx's have reduced power on 40m CW

 

Evan,

I've changed the thread name to something I think more appropriate.
Should be sufficient to simply state that this is a continuation of the conversation here:
? ??/g/BITX20/topic/75464085#87372

As a summary of what we know thus far:

As shown in these posts from the old thread:
? ? ? ?/g/BITX20/message/87369
? ? ? ?/g/BITX20/message/87328
testpoint TP1 is quite dirty on 40m CW, but not on 30m CW or 80m CW.
This dirty 40m signal results in reduced output from the transmitter.
The reduced output only occurs during CW transmissions, not SSB transmissions.
CLK2 into the first mixer looks clean.
Issue has been seen in both v4 and v6 hardware, most units tested exhibit the problem.

Evan has found that reducing drive from RV1 does not clean up the TP1 signal,
so this is not an issue of coupling from the power amp.
Problem is most likely around the first mixer, the following steps will be taken to isolate the issue:
So, not as a solution but as a way of understanding the problem I might try the following steps:
1)? Remove L31, terminate that mixer port with 50 ohms to ground.? (There's no signal coming from the IF amp in CW mode.)
2)? Add a 6dB pad (150,33,150 ohms like R101,102,103) between T2 and L4 (perhaps? remove L4,C204 and tack it in there, avoids cutting traces)
3)? Replace the mixer with something else, perhaps an ADE-1.? Could be a resonance inside that mixer?

A question for Evan:
In post 87369 you sent this morning, what is being probed in the top two screen shots?

Jerry, KE7ER



On Thu, Apr 1, 2021 at 08:06 AM, Evan Hand wrote:
Dave,

Not sure how to answer your questions.? The final output of the ?BITX is a very nice sinewave.? I do have a good ground through the cold water line as I am in my basement.

The issue that Jerry is helping me with is the lower output on 40meters when using CW.? I had noticed that this is common on all 4 of the ?BITX that I have as well as others have reported.? Personally, I have seen this on 6 radios.? The scope traces are the first that I have seen that could possibly explain the issue.? We are now trying to work out the root cause and possible solution.? Everything is pointing to the mixer, with the next step of removing any connection to the BiDi amps to see if it is a termination issue with the mixer.

It is evident that I really need to start another thread.? The problem with that is there are a number of posts with pictures that are important to the issue that would also need to be copied.

73
Evan
AC9TU
 

Jerry,

First, thank you for porting the conversation to a new thread/hashtag.? The only clarification is that I also have seen the CW power reduced on my v5 ?BITX.? So I personally have experience with 6 rigs, 3 different versions, that all exhibit reduced power on 40meters when compared to either 30 or 20 meters.? Those are 2 v4, one v5, and one v6 that I own and I have worked on 2 other ?BITX that had the issue as well; a v4 and a v6.? I have also seen posts on this board from others that have the same issue.

To answer your question, the top two pictures in post 87369 were at TP2 with the drive (RV1) at full and at minimum on 40meters.? That is showing that I have some coupling through the scope probe, hence I moved to TP1 where the probe is further from the finals.? There the signal did not change with an adjustment of RV1.? There also is the problem with a distorted 40meter signal when compared to a 30meter signal that is the strongest when in CW mode.

As stated in the prior thread it will be tomorrow at the earliest that I will be able to get to removing L31 and terminating with a 50ohm resistor to see if it is reflected signal from the connection/filter output of the BiDi amp.

Again, Thank you Jerry for your help.
73
Evan
AC9TU
 

I'll finish off the move to this new thread by copying over news of the C81 fix from Curt:
???
From post 87380:
Looks like Curt added 330pf across C81 and got good results:
? ??/g/BITX20/message/84646
? ??/g/BITX20/topic/70906819#80608
I'm very curious what TP1 looks like after this fix, I doubt it's any cleaner.
Would feel better if we can find the root problem rather than just boost
the high frequency gain of Q90 up to whatever that poor little 2n3904 has to offer.
Increasing that gain will also boost any crud at TP1.
On the other hand, the crud appears to be all harmonics of 40m, so it may not matter much.
Jerry, KE7ER

From post 87376:
I am thinking it may be the equalizer network, but surprised you would see it on all 4 units. On mine a capacitor adjustment cured it, I was searching in all the wrong places. I had a pronounced dip on 40m.? Search on my call and you should find this cap in question. I think its in the driver to the PA and only affects cw output.??
Curt wb8yyy
 

Jerry

Thanks for finding my info related to a 40m dip and remedy with a capacitor value change. We did a local group build of v4, and out of about 2 dozen units only 2 reported this issue that occurs only on cw. Wait, many of those builders don't do CW, but still I think this is a minority issue. Ashhar himself provided the clue on the equalization network after I drove myself silly trying the find the cause.?

Any spectral uncleanness is not in my judgment directly related, but the adjacent circuitry around the final transmit mixer does collect whatever RF is leaking around, but in my case my rig is compliant on all CW bands and not as good in upper bands on CW. I had thought v5 and v6 were cured in this regard. Of course we realize a distorted sine wave means other frequencies are represented, but of course higher ones are reduced by the low pass filter.?

73 curt
 

Curt,

Agreed, if rig is compliant on all bands and modes with the increased gain at Q90, it's a good fix.
But Evan's scope traces at TP1 for 40m vs 30m and 80m make me?curious as to exactly what's going on.

Too much gain at Q90 could bring on IMD when operating SSB,
this may be hard to quantify without a good spectrum analyzer.
If present, it will cause interference in adjacent channels.
Not too big a deal at QRP power levels, but some are using their uBitx with a fairly big linear.
?
>? compliant on all CW bands and not as good in upper bands on CW. I had thought v5 and v6 were cured in this regard

So upper bands were compliant, but not quite as good in what way?
Just lower power out?? Harmonics present but within regulations?

Jerry, KE7ER



On Thu, Apr 1, 2021 at 06:10 PM, Curt wrote:

Jerry

Thanks for finding my info related to a 40m dip and remedy with a capacitor value change. We did a local group build of v4, and out of about 2 dozen units only 2 reported this issue that occurs only on cw. Wait, many of those builders don't do CW, but still I think this is a minority issue. Ashhar himself provided the clue on the equalization network after I drove myself silly trying the find the cause.?

Any spectral uncleanness is not in my judgment directly related, but the adjacent circuitry around the final transmit mixer does collect whatever RF is leaking around, but in my case my rig is compliant on all CW bands and not as good in upper bands on CW. I had thought v5 and v6 were cured in this regard. Of course we realize a distorted sine wave means other frequencies are represented, but of course higher ones are reduced by the low pass filter.?

73 curt
 

Jerry

On my v4 all harmonics are clean, thanks to installing one of Gordon's relay boards.? But as with many v3 and v4 I have those fixed frequency leakage spurs in the vicinity of 12 meters. Attempts to remedy it with an LC or extra 45 MHz crystal stage have been futile with negative impact to cw operation. Now how well my yagi might provide some helpful dBs is conjecture without a measurement from a local ham. My rig hasn't seen a spectrum analyzer since before the pandemic, but I imagine it hasn't improved.?

73 Curt
 

Curt,

Ah, good.
You have a v3 or v4.

Was thinking you had a v5 or v6.
And that there might be an issue with current rigs not meeting regulations.

So at this point, there is nothing I am especially worried about.
Just curious about the root cause of the slightly low output power on 40m CW.
Note that this does not effect SSB operation.

Jerry, KE7ER



On Thu, Apr 1, 2021 at 07:21 PM, Curt wrote:
Jerry

On my v4 all harmonics are clean, thanks to installing one of Gordon's relay boards.? But as with many v3 and v4 I have those fixed frequency leakage spurs in the vicinity of 12 meters. Attempts to remedy it with an LC or extra 45 MHz crystal stage have been futile with negative impact to cw operation. Now how well my yagi might provide some helpful dBs is conjecture without a measurement from a local ham. My rig hasn't seen a spectrum analyzer since before the pandemic, but I imagine it hasn't improved.?

73 Curt

?

?
 

Jerry and Curt,

Like Jerry, my main reason for pursuing the 40meter CW dip is to understand the root cause.? I do not really use CW, so it is not that I want to operate in that mode so fixing it with Q90 gain does not really meet my need.? It is that I have noticed a pattern that I am trying to understand, as well as verify that it is a pattern and not a figment of my imagination.

I had read the original thread on working to even out the power on all of the bands.? I had read Curt's post before, I had just forgotten it, and as Jerry pointed out, it is a fix and not really the root cause at least as far as the scope traces I have made are pointing out.

My plan of attack later today is to first see if my one "parts" / experimental v4 has exactly the same issue.? It is already in a state that makes it easier to dissect and probe.? If that is the case, then I will proceed to the steps that Jerry has laid out to try to see if it is a mixer termination issue.? I also want to verify that the SSB power measurements did not give 40meters a higher power reading due to any spurious signals associated with the 40meter mixer sensitivity to the CW unbalancing.? I do not have a Rigol/Siglent level of a spectrum analyzer.? Only a TinySA for harmonics and an RSP1A with SA software for close-in spurs and IMD measurements.?

It might take more than a day or two to go through the testing, so my next post on the topic might be after Easter.? Last-minute requests by kids and family have again changed my schedule and plans.

Thank you both for all of the insights that you have given me.? This is FUN!
73
Evan
AC9TU
 

Evan

I am looking forward to your results with your v4, but indeed do enjoy the significant holiday with your family, no rush.?

I do think those with a pronounced dip are rare in v4. Look for Ashhar's post on the intentional equalization network, and perhaps remove its effect if you want to see root cause, which I perceive is even more variation in output power across the bands. We tested each of our rigs for output power and spurious on cw and ssb, the latter using an audio signal generator. Whether the largest unit to unit variation is the driver or PA I don't know.?

Unless you have modified it for spurious, I anticipate you may be out on a few bands for harmonics, and likely have significant ssb spurs in upper HF. I am curious what your equipment sees as I want to get at least a tiny SA, if not both your instruments. If you attack the ssb spur I am most curious.?

I am not aware of anyone introducing an alc circuit, but wonder at the impact. Some have noted poor audio drive, but mine works nicely without speech processing using an old Kenwood HT speaker mike.?

No hurry much to enjoy.

Curt
 

Just to clarify: Is the issue being discussed, some rigs showing low power output on 40m cw and/or distorted output on 40m cw, only occurring on v5 or previous, or is it also occurring on v6?

Clay NF7X
 

Clay,

Just to answer your question with at least one data point, my v6 power output has a dip in the CW portion of 40m. I always assumed it was because of LPF tolerances.

Rafael PU1OWL
 

Clay - This happens on my V6, less than half output on CW opposed to LSB...

73,

Justin B.
KI5GKD
 

No complaints of distortion, just relatively low power out on 40m CW.
The 40m SSB transmissions don't seem to be affected.
The v5 and v6 are pretty much identical, except for the display and connectors.
It seems this problem could occur on most any uBitx:? ?v3,v4,v5,v6.

Not actually much of a problem, 5W on 40m is still quite usable.
And the uBitx is primarily an SSB rig.


It is possible that "fixing" the low output on 40m CW by increasing the gain at Q90
could cause some inter-modulation-distortion on SSB transmissions due
to too much gain.? This has not been verified.

Jerry, KE7ER


On Fri, Apr 2, 2021 at 07:01 AM, Clay Nicolsen wrote:
Just to clarify: Is the issue being discussed, some rigs showing low power output on 40m cw and/or distorted output on 40m cw, only occurring on v5 or previous, or is it also occurring on v6?
 

This is an interesting?report. I havent?noticed it on my ubitx. I should ask HF SIgnals to loan me a standard issue ubitx and check it out.?
As many know, I am working on an SDR. I am using a modified ubitx as the framework. I noticed a few things :
1. Swapping the L5 and L7 to the bottom side of the PCB seems to cure a number of spurs.
There are a few mods that increase the power output, especially on the higher bands:
1. Add a 220 pf across the drains of the IRF510 (this cures many things). You can solder this across the secondary of the T11 (pins 1 and 6)
2. Add 0.1 uf on C261 and C262 (these were left unsoldered from the factory).
3. Change all the through hole 2N3904s to 2N2222A (metal version)
4. Change the SMD 2N3904 at Q90 to a through hole 2N2222A as well (this takes a little bit of contortions of the 2N2222A pins)
These mods work only if you first change the 2N3904s to 2N2222As. The steps 1 and 2 are needed to deal with extra gain of the 2N2222As.
- f

On Fri, Apr 2, 2021 at 8:12 PM Jerry Gaffke via <jgaffke=[email protected]> wrote:
No complaints of distortion, just relatively low power out on 40m CW.
The 40m SSB transmissions don't seem to be affected.
The v5 and v6 are pretty much identical, except for the display and connectors.
It seems this problem could occur on most any uBitx:? ?v3,v4,v5,v6.

Not actually much of a problem, 5W on 40m is still quite usable.
And the uBitx is primarily an SSB rig.


It is possible that "fixing" the low output on 40m CW by increasing the gain at Q90
could cause some inter-modulation-distortion on SSB transmissions due
to too much gain.? This has not been verified.

Jerry, KE7ER


On Fri, Apr 2, 2021 at 07:01 AM, Clay Nicolsen wrote:
Just to clarify: Is the issue being discussed, some rigs showing low power output on 40m cw and/or distorted output on 40m cw, only occurring on v5 or previous, or is it also occurring on v6?
 

All,

I did confirm the same CW mwaveform issues at TP1 on my v4 ?BITX.? Since it is a spare and not assembled into a nice case I will use it to hunt down the possible issues with the 40meter CW waveform distortion.? Since the v4 does not have the LCL low pass filter, I think that it can be eliminated from the study.? I will still try to isolate the connections to the mixer and terminate in a 50ohm load.

Power with 12volts:
3.674MHz - 12.12 Watts
7.074MHz - 5.45 Watts

Here are the waveforms:



73
Evan
AC9TU

80meterV4TP1.BMP

Will B
 

Ashhar Farhan said:

I noticed a few things :
1. Swapping the L5 and L7 to the bottom side of the PCB seems to cure a number of spurs.
There are a few mods that increase the power output, especially on the higher bands:
1. Add a 220 pf across the drains of the IRF510 (this cures many things). You can solder this across the secondary of the T11 (pins 1 and 6)
2. Add 0.1 uf on C261 and C262 (these were left unsoldered from the factory).
3. Change all the through hole 2N3904s to 2N2222A (metal version)
4. Change the SMD 2N3904 at Q90 to a through hole 2N2222A as well (this takes a little bit of contortions of the 2N2222A pins)
These mods work only if you first change the 2N3904s to 2N2222As. The steps 1 and 2 are needed to deal with extra gain of the 2N2222As.

Thanks for that.? Would these be things we could try on a uBitX v6, or is this specifically for the SDR unit you're working on?

Thanks! :-)

Will B
AF7EC
 

These are the changes to all the versions of ubitx. your mileage will vary. i have tried it on my own build, I have asked some colleagues at HF Signals to try them out too, you might want to wait for their results as well.
- f

On Sat, Apr 3, 2021 at 10:30 AM Will B <will.brokenbourgh2877@...> wrote:
Ashhar Farhan said:

I noticed a few things :
1. Swapping the L5 and L7 to the bottom side of the PCB seems to cure a number of spurs.
There are a few mods that increase the power output, especially on the higher bands:
1. Add a 220 pf across the drains of the IRF510 (this cures many things). You can solder this across the secondary of the T11 (pins 1 and 6)
2. Add 0.1 uf on C261 and C262 (these were left unsoldered from the factory).
3. Change all the through hole 2N3904s to 2N2222A (metal version)
4. Change the SMD 2N3904 at Q90 to a through hole 2N2222A as well (this takes a little bit of contortions of the 2N2222A pins)
These mods work only if you first change the 2N3904s to 2N2222As. The steps 1 and 2 are needed to deal with extra gain of the 2N2222As.

Thanks for that.? Would these be things we could try on a uBitX v6, or is this specifically for the SDR unit you're working on?

Thanks! :-)

Will B
AF7EC
 

Jerry and Curt,

I had the chance to pull L4 and C204 this morning and substitute a 47omh resistor (too lazy to look for a 50ohm in my stash).? I believe that the results confirm what Jerry suspected.? Namely that there is a termination issue with the first mixer into the LPF.? Here are the traces of TP1 for both 40 and 80 meters:



Now the question is how do we change the impedance match to the filter.? I have not done this type of design before so could use a few suggestions.? I believe that changing the turn ratio of the transformer would be the correct approach.? Suggestions on approach and any details?
73
Evan
AC9TU
 

I should have stated that based on the already low levels of the signals that I do not think that adding the 6db pad would help.? It would make all of the CW signals lower and all would then be less than 10watts.

The above are my thoughts at this time.
73
Evan
AC9TU
 

Evan,

Excellent!

Yes, a 6dB pad would mean we need an extra 6dB in the power amp somewhere.
The power amp is already pushing the edge of what can be done with 2n3904's,?
more gain would likely require adding another amplifier stage.
Though adding an amplifier stage is a possibility.

We should take a look at Q90, see how close the input impedance of that stage is to 50 ohms.
Also see how well that 50 ohms gets reflected to the mixer end of the L1,2,3,4 filter.
This could be done by circuit analysis or by taking measurements.

The filter is low pass, might have a corner around 35 to 40 mhz.
Anything above that corner will see a high impedance, and bounce back into the filter.
We could experiment with a diplexer there to divert stuff that cannot pass through the filter
into a 50 ohm resistor.

A vague, poorly thought out possibility:
If this really is only an issue with CW transmissions, perhaps figure out a way to do this
that does not involve the mixer.? For example, turn CLK2 off, and have a way to switch CLK1
energy directly into the base of Q90.

Not obvious to me what the best way forward is.

Jerry, KE7ER


On Sat, Apr 3, 2021 at 05:09 AM, Evan Hand wrote:
Now the question is how do we change the impedance match to the filter.? I have not done this type of design before so could use a few suggestions.? I believe that changing the turn ratio of the transformer would be the correct approach.? Suggestions on approach and any details?

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