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.

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

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.

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

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

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.

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 >