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80 Meters Harmonics Fix Proposal
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I previously mentioned that I 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. I experimented some more with a simulator, and decided just to change the filter characteristics so I could stay with 50 ohms. I was able to come up with a filter that doubles the capacitance. The improvement is not as great, but at least on 80 meters, the harmonics don't exceed -45 dB in my tests. Not as much margin as I would like, but legal. I changed the 3 inductors to 1.6 uH by adding 2 turns on each. I added an extra 1000 PF cap in parallel with the input and output caps, bringing the total to 2000 pF, and I 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 there. (750 pF would have been better, but I didn't have them). I attached the simulation for the design.?
If someone with good test equipment could validate what I have done, it would be great to get more data points.? After validation of 80 I will try the 40 meter band. My test equipment is not good enough for the higher bands.(My analyzer only works to 30 MHz).? Howard PS my Arduino problems were probably a ground loop. They seem to have gone away by moving some cables. |
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Warren Allgyer
Howard
When I measured 80 meter harmonics the particularly troublesome ones were 5th at 18 MHz and 7th at 25 MHz. In both cases the attenuation of the stock uBitx at those frequencies was limited by the layout of the board and by the coupling of relays sharing both filter input and output in the same frame. In such cases the characteristics of the filters, both existing and any modifications are immaterial. The attenuation of the stock filter supplied is more than adequate. But the harmonics bypass the filter and go via I/O coupling straight to the output. Please measure the 5th and 7th harmonics on CW. I think you will find you have not done much to improve the stock situation. WA8TOD |
开云体育Warren:
The fifth and the 7th are at -45dB now. You are missing my point. The design of the filter directly effects the level of the harmonics. Yes, the stray? capacitance creates a "sneak" path around the filter. However, the input and output capacitors of the filter act as a divider to that "sneak" signal. If you double the output capacitance, you 1/2 the sneak signals amplitude, reducing it 6 dB. If you create a model of each? of the filters in a simulator, and put 8 pF from input to output, you can clearly see the improvement in the attenuation of the filter for the higher output capacitance. All the data I have seen shows that a 6 dB improvement would just make the uBitx legal. Howard On 9/3/2018 2:51 PM, Warren Allgyer wrote: Howard
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Gordon Gibby
开云体育Howard, I get your point. And a six DB improvement makes the device a much better transceiver per the regulationsWith simple unshielded wiring to three external relays, in my tests I found the fifth harmonic of 80 meter CW was basically obliterated, but the seventh only went down by about six dB. ?(I don’t have an explanation for the disparity )?I haven’t tried any
shielded wiring to see if that would make any better — but it certainly might.?
In my case It might be simpler to add in a permanent low pass filter above 15 MHZ in my case. ? ? Your idea of lowering the capacitance is interesting. ?And it’s pretty simple also! ? For many people that might be the easiest solution to the harmonic problem.?
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开云体育Gordon:
Thanks for your concurrence. To prevent others from being confused, I am raising not lowering the capacitance, which I am sure is what you meant. And yes, I am trying to come up with an easy fix that all can implement including HF signals. Howard On 9/3/2018 4:05 PM, Gordon Gibby wrote: Howard, I get your point. And a six DB improvement makes the device a much better transceiver per the regulations
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Gordon Gibby
开云体育Yep, sorry, you were lowering the impedance, raising the capacitance. ? Slip of the tongue. (I use Siri)You might tell us how you changed each value in those filters. ??
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Warren Allgyer
Got it Howard. And I agree with both the strategy and the necessary amount of improvement. On mine, a six dB improvement would make all bands legal for CW harmonics. That would be a nice solution for a CW only radio. Of course you must not use it on SSB for 20 meters and above but perhaps that is not your objective. Good work! ? WA8TOD |
开云体育Warren:
I'm peeling the onion, slowly. I thought the problematic band was 15 Meters for SSB? Gordon: I don't understand what you are asking me. I provided the simulation file for the 80 meter filter. I haven't touched the other bands yet, I was hoping someone would also try the mod and confirm 80 meter CW is now OK. Howard On 9/3/2018 5:05 PM, Warren Allgyer wrote:
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开云体育Yes, I stand corrected, but Warren said
20 meters, not 20 MHz.
On 9/3/2018 5:35 PM, ajparent1/KB1GMX wrote: ?peeling the onion, slowly. I thought the problematic band was 15 Meters for SSB??
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jim
On Monday, September 3, 2018, 11:51:11 AM PDT, Warren Allgyer <allgyer@...> wrote:
Howard When I measured 80 meter harmonics the particularly troublesome ones were 5th at 18 MHz and 7th at 25 MHz. In both cases the attenuation of the stock uBitx at those frequencies was limited by the layout of the board and by the coupling of relays sharing both filter input and output in the same frame. In such cases the characteristics of the filters, both existing and any modifications are immaterial. The attenuation of the stock filter supplied is more than adequate. But the harmonics bypass the filter and go via I/O coupling straight to the output. Please measure the 5th and 7th harmonics on CW. I think you will find you have not done much to improve the stock situation. WA8TOD Just a fyi ...see the picture of the 80 meter output from my unit ...after doing the L7,L8 mod and the output transformer mod . A whole bunch of the cruft flying around in this radio is due to L7 and L8 having DC current flowing through them ...This DC biases the ferrite (shifting the BH curve who knows where) lowering the inductance and allowing RF everywhere .. Put a proper inductor on the IRF510's that is NOT affected by DC current flowing through it and see what you get Jim _._,_._,_
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开云体育Jim:
You can't argue with success! However, I don't quite understand a few things. Why remove the electrolytics, the bulk capacitance can't hurt? Why not center tap the transformer primary and feed the DC there? It should have the same result and is simpler. Howard On 9/3/2018 9:04 PM, jim via Groups.Io wrote:
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Howard,
>>You can't argue with success! However, I don't quite understand a few things. Why remove the electrolytics, the bulk capacitance can't hurt? Why not center tap the transformer primary and feed the DC there? It should have the same result and is simpler. I sorta agree.? The bulk caps should be retained if only relocated. The center tap vs bias transformer goes both ways.? in some cases the bias transformer keeps DC out of the output and aids stability.? it also allows for doing unusual forms of transformers that do not center tap well. Jim is retracing my path and doing a lot of the things I tried.? He is?performing experimental changes and measuring them.? ?Good on that.? ?If he is having fun who am I to stop him.? Allison |
开云体育Allison:
Yes, I understood that. Howard On 9/3/2018 10:53 PM, ajparent1/KB1GMX wrote: Howard,
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jim
Ok?? I did replace the electrolytics as can be seen from the picture ..It is effectively a center tap? I wanted tighter coupleing between the two windings ....The reason why you feed from the center tap is because the elecrtomagnetic 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 ..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 ...Result no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow. Try it and see. can't cost a lot .Maybe it'll work Jim
On Monday, September 3, 2018, 6:27:45 PM PDT, Howard Fidel <sonic1@...> wrote:
Jim: You can't argue with success! However, I don't quite understand a few things. Why remove the electrolytics, the bulk capacitance can't hurt? Why not center tap the transformer primary and feed the DC there? It should have the same result and is simpler. Howard On 9/3/2018 9:04 PM, jim via Groups.Io wrote: Howard When I measured 80 meter harmonics the particularly troublesome ones were 5th at 18 MHz and 7th at 25 MHz. In both cases the attenuation of the stock uBitx at those frequencies was limited by the layout of the board and by the coupling of relays sharing both filter input and output in the same frame. In such cases the characteristics of the filters, both existing and any modifications are immaterial. The attenuation of the stock filter supplied is more than adequate. But the harmonics bypass the filter and go via I/O coupling straight to the output. Please measure the 5th and 7th harmonics on CW. I think you will find you have not done much to improve the stock situation. WA8TOD Just a fyi ...see the picture of the 80 meter
output from my unit ...after doing the L7,L8 mod
and the output transformer mod . A whole bunch of the cruft flying around in
this radio is due to L7 and L8 having DC
current flowing through them ...This DC biases
the ferrite (shifting the BH curve who knows
where) lowering the inductance and allowing RF
everywhere .. Put a proper inductor on the IRF510's
that is NOT affected by DC current flowing
through it and see what you get Jim
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