Topics

2.8 Mhz parasitic signal on 15 and 20m #ubitx

Thomas DD2TH
 

When I use SSB on 20, 18 and 15m, I get a strong signal (20db below the main signal) 2.8Mhz left and right of the tx frequency, as if the carrier is being amplitude modulated with 2.8 Mhz.

I already tried disabling the PA power and measuring the signal at different points before the PA, but this signal isn't there, not even directly at the power MOSFETs. When I enable the PA, I can't measure low power signals on the PCB anymore.

On all other bands, this singnal isn't present. In CW mode, the signal isn't there on any band.

Has anyone observed something similar? I'm completely at a loss. Spectrum analyzer screenshots will follow as soon as I get everything set up again.

Ashhar Farhan
 

Which version of ubitx do you use?


On Sat 22 Jun, 2019, 12:34 PM , <fr.beckenbauer@...> wrote:
When I use SSB on 20, 18 and 15m, I get a strong signal (20db below the main signal) 2.8Mhz left and right of the tx frequency, as if the carrier is being amplitude modulated with 2.8 Mhz.

I already tried disabling the PA power and measuring the signal at different points before the PA, but this signal isn't there, not even directly at the power MOSFETs. When I enable the PA, I can't measure low power signals on the PCB anymore.

On all other bands, this singnal isn't present. In CW mode, the signal isn't there on any band.

Has anyone observed something similar? I'm completely at a loss. Spectrum analyzer screenshots will follow as soon as I get everything set up again.

 

Questions!

1: Which uBitx model is this ?

2: Is the carrier to spur frequency difference constant in separation or does it increase and decrease as you tune ?

Raj

At 22/06/2019, you wrote:
When I use SSB on 20, 18 and 15m, I get a strong signal (20db below the main signal) 2.8Mhz left and right of the tx frequency, as if the carrier is being amplitude modulated with 2.8 Mhz.

I already tried disabling the PA power and measuring the signal at different points before the PA, but this signal isn't there, not even directly at the power MOSFETs. When I enable the PA, I can't measure low power signals on the PCB anymore.

On all other bands, this singnal isn't present. In CW mode, the signal isn't there on any band.

Has anyone observed something similar? I'm completely at a loss. Spectrum analyzer screenshots will follow as soon as I get everything set up again.

Thomas DD2TH
 

the difference is constant, both when tuning and when changing band

it's a V3

Ashhar Farhan
 

Haha, Raj, He hasnt read our emails?


On Sun 23 Jun, 2019, 9:59 AM , <fr.beckenbauer@...> wrote:
the difference is constant, both when tuning and when changing band

it's a V3

 

Very strange issue. Final may be oscillating ?


At 23-06-19, you wrote:
Haha, Raj, He hasnt read our emails?

On Sun 23 Jun, 2019, 9:59 AM , < fr.beckenbauer@...> wrote:
the difference is constant, both when tuning and when changing band

it's a V3

Thomas DD2TH
 

any tips on how to test or mitigate? I have access to a very well equipped HF lab tomorrow.

 

Please edit and leave in parts of the message you are referring to in your reply. In my case
when I dont see trailing message content I dont bother to read or reply as I cant be bothered
to figure it out who anyone is talking to.

Your problem is interesting, please show us a spectrum of it. I have not encountered any such
fixed spacing spurs, they always move closer and farther from carier as I tune.

Raj

At 23-06-19, you wrote:
any tips on how to test or mitigate? I have access to a very well equipped HF lab tomorrow.

Thomas DD2TH
 

Please edit and leave in parts of the message you are referring to in your reply. In my case
when I dont see trailing message content I dont bother to read or reply as I cant be bothered
to figure it out who anyone is talking to.
I'm not used to this mailing list stuff, I tried to treat it like reddit, sorry.

Back to the topic: I feel so stupid. The Spurs *do* change when I tune.

On 20m, the 3rd harmonic of the VFO (59 MHz * 3 = 177 MHz) is mixed with the 4th harmonic of the 1st IF (45 MHz * 4 = 180 MHz), resulting in spurs 3 MHz (180 - 177 = 3) left and right of the signal when I tune to 14.0 MHz. When I tune to 14.05 MHz, the spurs are 2.85 MHz from the carrier, when I tune to 14.10 MHz they are 2.7 MHz away, and so on.


On 15m, the 2nd harmonic of the VFO (66 MHz *2 = 132 MHz) is mixed with the 3rd harmonic of the 1st IF (45 MHz * 3 = 135 MHz), resulting in spurs 3 MHz (135 - 132 = 3) left and right of the signal when I tune to 21.0 MHz. When I tune to 21.05 Mhz, the spurs are 2.90 MHz from the carrier, when I tune to 21.1 MHz they're at 2.80 MHz, and so on. 

These mixing products are generated in the final mixer. This also explains why the products aren't there in CW mode, since the IF isn't used at all in CW mode, and the frequency is directly generated by the VFO. Maybe add a note to the block diagram, currently it says the VFO tunes from 45-75 MHz, which only is the case in SSB mode.

I modded the PA to RD16HHF1s following DF3FY's instructions, checked the output afterwards, but except for the known harmonics issues it was clean. I replaced the LPF relays with name brand parts to fix that, checked the harmonics afterwards (now at -45dB, nice), and this weird signal still wasn't there. Then I took everything apart to put the bitx into a metal case, and this signal appeared, now I can't get it to go away.

The problem is that I don't remember if I tested everything using SSB mode with a 1khz signal from my computer (tune button in WSJT-X is convenient for this, also for finding the sweet spot for NF gain and so on), or if I got lazy at some point and used CW mode, or at which stage I used which.

I don't know how the unmodified design prevents these signals from getting strong enough to matter, do the IRF510s just not have enough gain at these high frequencies? BPFs at the output would completely remove them, of course.
On 15m the signal was weak enough to be within legal limits (around -45dB), but still very noticeable. On 10m, I saw some spurs at similar distance to the carrier, but they were at like -60dB, so nothing to worry about at all.

DD2TH

Alan de G1FXB
 


DD2TH

If you found the DF3FY reworked PA through ubitx.net there is also the following which apply to Version 3 &4.
A higher B/W flatter response PA will amplify what ever is on it's input, both in band and out that much better.

http://ubitx.net/category/hardware/harmonics-and-spurs/
http://ubitx.net/spectral-purity/

T
here were many threads prior to the release of the V5 revision around October last year, perhaps search this forum for the posts

Understanding Spurious Emissions
si5351a CLK drive levels in uBITX #ubitx
Spurs for Dummies? #ubitx-help

or made by
Alison,
KB1GMX
Henning, DK5LV
Warren, WA8TOD

I don't hold a German licence,
I would verify your countries requirements for spurious emissions is similar to?? FCC @ -43dBc spec
as not all countries are.


Alan

On 29/06/2019 15:40, fr.beckenbauer@... wrote:
Please edit and leave in parts of the message you are referring to in your reply. In my case
when I dont see trailing message content I dont bother to read or reply as I cant be bothered
to figure it out who anyone is talking to.
I'm not used to this mailing list stuff, I tried to treat it like reddit, sorry.

Back to the topic: I feel so stupid. The Spurs *do* change when I tune.

On 20m, the 3rd harmonic of the VFO (59 MHz * 3 = 177 MHz) is mixed with the 4th harmonic of the 1st IF (45 MHz * 4 = 180 MHz), resulting in spurs 3 MHz (180 - 177 = 3) left and right of the signal when I tune to 14.0 MHz. When I tune to 14.05 MHz, the spurs are 2.85 MHz from the carrier, when I tune to 14.10 MHz they are 2.7 MHz away, and so on.


On 15m, the 2nd harmonic of the VFO (66 MHz *2 = 132 MHz) is mixed with the 3rd harmonic of the 1st IF (45 MHz * 3 = 135 MHz), resulting in spurs 3 MHz (135 - 132 = 3) left and right of the signal when I tune to 21.0 MHz. When I tune to 21.05 Mhz, the spurs are 2.90 MHz from the carrier, when I tune to 21.1 MHz they're at 2.80 MHz, and so on.??

These mixing products are generated in the final mixer. This also explains why the products aren't there in CW mode, since the IF isn't used at all in CW mode, and the frequency is directly generated by the VFO. Maybe add a note to the block diagram, currently it says the VFO tunes from 45-75 MHz, which only is the case in SSB mode.

I modded the PA to RD16HHF1s following DF3FY's instructions, checked the output afterwards, but except for the known harmonics issues it was clean. I replaced the LPF relays with name brand parts to fix that, checked the harmonics afterwards (now at -45dB, nice), and this weird signal still wasn't there. Then I took everything apart to put the bitx into a metal case, and this signal appeared, now I can't get it to go away.

The problem is that I don't remember if I tested everything using SSB mode with a 1khz signal from my computer (tune button in WSJT-X is convenient for this, also for finding the sweet spot for NF gain and so on), or if I got lazy at some point and used CW mode, or at which stage I used which.

I don't know how the unmodified design prevents these signals from getting strong enough to matter, do the IRF510s just not have enough gain at these high frequencies? BPFs at the output would completely remove them, of course.
On 15m the signal was weak enough to be within legal limits (around -45dB), but still very noticeable. On 10m, I saw some spurs at similar distance to the carrier, but they were at like -60dB, so nothing to worry about at all.

DD2TH

 

The whole problem is that the final tx > BiDi amp picks up signal from the finals/relays and remixes
them with carrier and creates a mess. Your problem could come from a not properly grounded case
or unshielded output wire to connector. Make sure no output RF wires come near the BiDi stage
Q20/21/22 .

At 29-06-19, you wrote:

I modded the PA to RD16HHF1s following DF3FY's instructions, checked the output afterwards, but except for the known harmonics issues it was clean. I replaced the LPF relays with name brand parts to fix that, checked the harmonics afterwards (now at -45dB, nice), and this weird signal still wasn't there. Then I took everything apart to put the bitx into a metal case, and this signal appeared, now I can't get it to go away.

DD2TH

Thomas DD2TH
 

The legal requirement for in Germany is -40dB below carrier for the total power of all unwanted emissions added up.

I've already done the L5, L7 mod, although I used würth inductors we had in the lab instead. It helped the overall situation, with fewer spurs appearing, but these two characteristic spurs, which are generated in the way I calculated earlier, stay, since they are generated after the roofing filter.

I'll try adding the v5 LPF after the 45Mhz preamp next. There looks to be enough space on the board for the mod to be simple. I don't know how I missed that option.

I didn't think the spurs would be generated in the preamp, since when I reduced the drive by doubling R22, all that changed was the output power, the spurs stayed at the same level relative to the carrier.

I'll report back tomorrow if the LPF helped.

DD2TH, 73

On Sun, Jun 30, 2019 at 12:07 AM, Alan de G1FXB wrote:

DD2TH

If you found the DF3FY reworked PA through ubitx.net there is also the following which apply to Version 3 &4. A higher B/W flatter response PA will amplify what ever is on it's input, both in band and out that much better.

http://ubitx.net/category/hardware/harmonics-and-spurs/ http://ubitx.net/spectral-purity/

There were many threads prior to the release of the V5 revision around October last year, perhaps search this forum for the posts

Understanding Spurious Emissions si5351a CLK drive levels in uBITX #ubitx Spurs for Dummies? #ubitx-help

or made by Alison, KB1GMX Henning, DK5LV Warren, WA8TOD

I don't hold a German licence, I would verify your countries requirements for spurious emissions is similar to?? FCC @ -43dBc spec as not all countries are.

Alan

On 29/06/2019 15:40, fr.beckenbauer@... wrote: > > Please edit and leave in parts of the message you are referring to > in your reply. In my case > when I dont see trailing message content I dont bother to read or > reply as I cant be bothered > to figure it out who anyone is talking to. > > I'm not used to this mailing list stuff, I tried to treat it like > reddit, sorry. > > Back to the topic: I feel so stupid. The Spurs do change when I tune. > > On 20m, the 3rd harmonic of the VFO (59 MHz * 3 = 177 MHz) is mixed > with the 4th harmonic of the 1st IF (45 MHz * 4 = 180 MHz), resulting > in spurs 3 MHz (180 - 177 = 3) left and right of the signal when I > tune to 14.0 MHz. When I tune to 14.05 MHz, the spurs are 2.85 MHz > from the carrier, when I tune to 14.10 MHz they are 2.7 MHz away, and > so on. > > > On 15m, the 2nd harmonic of the VFO (66 MHz *2 = 132 MHz) is mixed > with the 3rd harmonic of the 1st IF (45 MHz * 3 = 135 MHz), resulting > in spurs 3 MHz (135 - 132 = 3) left and right of the signal when I > tune to 21.0 MHz. When I tune to 21.05 Mhz, the spurs are 2.90 MHz > from the carrier, when I tune to 21.1 MHz they're at 2.80 MHz, and so on. > > These mixing products are generated in the final mixer. This also > explains why the products aren't there in CW mode, since the IF isn't > used at all in CW mode, and the frequency is directly generated by the > VFO. Maybe add a note to the block diagram, currently it says the VFO > tunes from 45-75 MHz, which only is the case in SSB mode. > > I modded the PA to RD16HHF1s following DF3FY's instructions, checked > the output afterwards, but except for the known harmonics issues it > was clean. I replaced the LPF relays with name brand parts to fix > that, checked the harmonics afterwards (now at -45dB, nice), and this > weird signal still wasn't there. Then I took everything apart to put > the bitx into a metal case, and this signal appeared, now I can't get > it to go away. > > The problem is that I don't remember if I tested everything using SSB > mode with a 1khz signal from my computer (tune button in WSJT-X is > convenient for this, also for finding the sweet spot for NF gain and > so on), or if I got lazy at some point and used CW mode, or at which > stage I used which. > > I don't know how the unmodified design prevents these signals from > getting strong enough to matter, do the IRF510s just not have enough > gain at these high frequencies? BPFs at the output would completely > remove them, of course. > On 15m the signal was weak enough to be within legal limits (around > -45dB), but still very noticeable. On 10m, I saw some spurs at similar > distance to the carrier, but they were at like -60dB, so nothing to > worry about at all. > > DD2TH >

Thomas DD2TH
 

So, I just installed a LPF after the amplifier to the final mixer as exists on the v5 bitx, and it didn't change my spurs whatsoever.

jim
 

Just out of idle curiosity, do you have access to another 12 volt supply..apply separate power to ubitx and the FET finals

Just to see....

Jim


On Monday, July 1, 2019, 10:25:55 AM PDT, Thomas DD2TH <fr.beckenbauer@...> wrote:


So, I just installed a LPF after the amplifier to the final mixer as exists on the v5 bitx, and it didn't change my spurs whatsoever.

Thomas DD2TH
 

On Tue, Jul 2, 2019 at 01:24 AM, jim wrote:
Just out of idle curiosity, do you have access to another 12 volt supply..apply separate power to ubitx and the FET finals

Just to see....

Jim


On Monday, July 1, 2019, 10:25:55 AM PDT, Thomas DD2TH <fr.beckenbauer@...> wrote:
 
 
So, I just installed a LPF after the amplifier to the final mixer as exists on the v5 bitx, and it didn't change my spurs whatsoever.

 I've been running off a lead acid battery to eliminate power supply noise for these tests

Lawrence Galea
 

That red wire touching that toroid could be picking up rf and coupling it to the toroid and generating the spurii


On Tue, Jul 2, 2019 at 9:54 AM Thomas DD2TH <fr.beckenbauer@...> wrote:
On Tue, Jul 2, 2019 at 01:24 AM, jim wrote:
Just out of idle curiosity, do you have access to another 12 volt supply..apply separate power to ubitx and the FET finals

Just to see....

Jim


On Monday, July 1, 2019, 10:25:55 AM PDT, Thomas DD2TH <fr.beckenbauer@...> wrote:
 
 
So, I just installed a LPF after the amplifier to the final mixer as exists on the v5 bitx, and it didn't change my spurs whatsoever.

 I've been running off a lead acid battery to eliminate power supply noise for these tests

Arv Evans
 


Some thoughts...
If the 2.8 MHz spurious is rough sounding and drifts a bit then it could be some sort of parasitic
oscillation.  If it is stable and a pure note then it is probably related to internal or external mixing
between VFO, BFO, or external stable frequency sources.  If the 2.8 frequency changes with tuning
then the rigs VFO is somehow involved, but it may be multiples of the VFO that is mixing with
something else to arrive at that frequency. 

Scope probing of mixer inputs and outputs might show if a mixer is balanced or unbalanced.
Mixer imbalance usually results from one side of the transformer having a bad connection or
a defective diode in one side. 

Improper biasing of an amplifier stage could be causing it to act as a mixer.  DC voltage levels
could isolate such a situation.  An open or wrong value RF bypass capacitor can also lead to
increased mixing products from a supposedly linear amplifier stage. 

Arv  K7HKL
_._


On Tue, Jul 2, 2019 at 3:04 AM Lawrence Galea <9h1avlaw@...> wrote:
That red wire touching that toroid could be picking up rf and coupling it to the toroid and generating the spurii

On Tue, Jul 2, 2019 at 9:54 AM Thomas DD2TH <fr.beckenbauer@...> wrote:
On Tue, Jul 2, 2019 at 01:24 AM, jim wrote:
Just out of idle curiosity, do you have access to another 12 volt supply..apply separate power to ubitx and the FET finals

Just to see....

Jim


On Monday, July 1, 2019, 10:25:55 AM PDT, Thomas DD2TH <fr.beckenbauer@...> wrote:
 
 
So, I just installed a LPF after the amplifier to the final mixer as exists on the v5 bitx, and it didn't change my spurs whatsoever.

 I've been running off a lead acid battery to eliminate power supply noise for these tests