Ashhar Farhan VU2ESE responds to recent posting on the BITX20 IO Groups list surrounding issues with spectral purity for the µBITx. His comments are reproduced in full below
Let me put some of the discussions on harmonic and spur purity of ubitx in context. This is going to be a long read, so bear with me as we plod through this. At the outset, I must thank Allison, Arv, Raj and Warren for their deep dives into these challenges.
Before I deep dive into these numbers, let me sum it up for those who just want the headline :
The ubitx put out harmonics less than 2 milliwatts on all the bands. That’s well below what a ‘commercially designed’, well behaved, FCC compliant, 100 watt commerical set would spew out. There are some easy fixes.
Now, onto the details:
Let’s consider a no-name, standard issue, 100 watt HF transceiver with the harmonics below the required -43 dbc level. 100 watts is +50dbm. +50dbm – 43 dbc = +7dbm. That is about 5 milliwatts in harmonics. This is what we live with on air with all those factory built rigs. 5 milliwatts of harmonics. If you add a linear it can even get worse. An SB-220 after an Elecraft K2, will put out almost a watt of harmonics.
Now, consider our 10 watt ubitx. You can see the scans done by Warren. They are on https://groups.io/g/BITX20/album?id=65861. He has helpfully drawn a green line across the scans. The green line is the -43dbc line. Apart from the actual signal, everything else must stay below the green line as per the FCC guideline.
Let’s sum up the report :
10 meters : SSB & CW are good (https://groups.io/g/BITX20/photo/65861/0?p=Name,,,20,1,0,0)
12 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/1?p=Name,,,20,1,0,0)
15 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/2?p=Name,,,20,1,0,0)
17 meters: SSB & CW are good (https://groups.io/g/BITX20/photo/65861/3?p=Name,,,20,1,0,0)
20 meters: SSB harmonics are higher by 4db, CW is good (https://groups.io/g/BITX20/photo/65861/4?p=Name,,,20,1,0,0).
30 meters: CW harmonics are 4db above the required level. SSB is not allowed anyway (https://groups.io/g/BITX20/photo/65861/5?p=Name,,,20,1,0,0)
40 meters: SSB is 3db above the green line, CW is even higher by 10 db (https://groups.io/g/BITX20/photo/65861/6?p=Name,,,20,1,0,0)
80 meters: SSB is within the spec, CW is out of spec by 5 db (https://groups.io/g/BITX20/photo/65861/8?p=Name,,,20,1,0,0)
To sum it up:
SSB: it is within the rulebook everywhere except on 20M & 40M by 3 db.
CW: it is within the rulebook except on on 30M, 40M & 80 M.
So, what’s the problem and how is it cured?
First: the trouble seems to be more with CW than SSB harmonics. We can reduce the CW level by decreasing the CW drive level. This is determined by the amount of current that flows into the front-end diode mixer by changing the R105 to a higher value resistor. I guess one can also do this in software by reducing the CLOCK #2 current to 2 ma from 8 ma when in CW. This will clear CW for all the bands.
Second: the alignment as it comes out from HF signals will show far better harmonic suppression than being reported here. For instance, the -38 dbc on 20 meters and -30 db on 40 meters will almost disappear if you balance it out with the bias on the IRF510s. The factory alignment works like this : First crank up both the IRF510s for 100 ma standing current on each, then tweak one of them to null out the harmonics. It is like balancing out the carrier on diode modulator.
Third: below 14 MHz, the IRF510s are distorting with too much drive. If you back off the drive to adjust to a leve of 7 watts, the harmonics will climb down to be within the spec. (Remember that the harmonics are not present at the output of the 45Mhz-to-HF mixer, they are generated in the power chain). This was also Allison’s suggestion.
We use 3 section low pass filters made out of micrometals’ toroids and C0G caps from AVX. Most commercial sets use only 2 section low pass filters. And yes, we can change the relays. I doubt if the problem is the relays. If it were so, it would be worse on the higher bands. The fact that the harmonics are climbing at the lower frequencies points to overdrive, not filter design. The filter layout etc is less critical on the lower bands as opposed to higher bands.
I guess that we we really need is a good transmit side ALC to cut the drive on lower bands. The gain varies by as more than 10 db across the spectrum from 3 Mhz to 30 MHz. Given that we want to keep the design to use what is available in a regular junkbox, this is a challenge to overcome with IRF510s and general purpose transistors. Allison has tried using 2N2222As with some success.
So, in a nutshell, here is the summary:
The ubitx remains less polluting than a 100 watt commercial rig that works within the FCC requirements. The harmonics at 14 Mhz and below can be controlled by backing off the drive level.
I would like to hear contrary views and figures. As I said before, unlike a commercially made radio, ubitx will thrive on critical feedback and design suggestions. This only gets better with time, like any good open source design.
Thanks all for the inputs.