Trial to control CW power (and reduce harmonics as a result) by unbalancing the 2nd mixer #ubitx
After a week working on this mod I need help as I can't come up with a logical explanation as to what is happening.
My uBitx is a V3. It has a few mods and works well, good receive sensitivity, Tx power within range. Previous mods which could be of interest for this question: Allison’s 2N2222A as pre-driver, drivers and BFR106 as Q90, Axicom relays, Raj L5/L7 replacement with designated SMT inductors.
The mod is to move the unbalancing from clock #2 mixer to clock #1 mixer, then adjust the software to generate a 45Mhz(+ or -) on clock #1 and a (45Mhz + Tx target freq) on clock #2.
This is not my idea and was mentioned before by Jerry Gaffke and others (E.g. https://groups.io/g/BITX20/topic/16656657#45805)
Details of the mod:
1. Disconnect (cut trace) from R105 to the common connection of C10, R27 and T2 (3,5).
2. Connect via a wire the disconnected side of R105 to the common connection of T4 (3,5), C211.
I passed the wire under the board by drilling two small holes next the two connection points to ensure minimal pick-up of RF.
3. Change the software to, when in TX CW mode, disable clock #0, generate a clock #1 at “SECOND_OSC_USB” - “usbCarrier” = 45Mhz (+ or -) and clock #2 at (that same 45Mhz signal + target Tx frequency).
I get a signal on a control receiver at the expected frequency, so the software logic seems to be ok.
But two problems:
a) I get like a permanent unbalancing of clock #1 mixer. The difference between key up and key down is only about 3dB.
b) It generates a much lower power level: example at 40M it delivers 12W in CW mode in stock configuration, I get 1W with this mod on key-down (and 1/2W on key up). At 10M I only get 100mW and 200mW (Key up/ key down respectively) which is too low for my auto-tune.
I checked the voltages on the common connection of now R105, T4 (3,5), C211 and I get 0.2mV DC on key-up and 245mV DC on key down which I believe is consistent with the diodes forward bias. This should have the mixer go from a well balanced state to a well unbalanced state.
I checked that by moving the 45Mhz clock #1 signal around the peak of the 45Mhz crystal filter I would get a variation in output power and would get a peak where expected and it worked as such. The further from the filter peak, the lower the output CW power.
I tried to reduce the values of R104 and R105 by 1/3. This reduced the power on key-up and on key-down by about the same ratio, so not a win either on the unbalance issue.
I checked that the first mixer was not left “floating” and that this could create a problem. I added a 1.2Kohms resistor from T2(3,5) to ground. It made no difference when I injected clock #2 at the TX frequency. I would get a very low signal on the control receiver, proving that the mixer of clock #2 is balanced with and without that resistance.
1. what can create this reduce difference between balanced and balanced clock #1 mixer?
2. Do we expect that a same level clock #1 signal that the one generated by clock #2, passing through the 45Mhz filter then amplified by Q20,Q21,Q22 stage would be of a lower level than the clock #2 signal coming through the first mixer? As far as I remember the conversion losses of a ring mixer is around 5 to 6 dB. Here we have a difference of around 10dB so something else is at play I suspect.
Thanks and best 73, John (VK2ETA)