Trial to control CW power (and reduce harmonics as a result) by unbalancing the 2nd mixer #ubitx

John (vk2eta)
 

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.

This mod summary: in a stock uBitx, on CW key-down, the mixer fed by clock #2 is unbalanced (the output of the Arduino CW-KEY is set high).

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.
The TX frequency is generated as a beat between the 45Mhz and the (45Mhz + Tx freq) in the clock #2 mixer. CW power control is done using the slope of the 45Mhz filter to reduce the amplitude of the 45Mhz signal.

Prime personal objective: control CW power level (for my integrated auto-tuner).

Side benefit: Reduce the level of harmonics present in CW mode as we currently feed a square wave directly at the transmitting frequency. With this mod the 45Mhz filter will filter the clock #1 harmonics, and the beat of the harmonics of clock #2 and clock #1 should be well above the L1-L4/C200-C204 filter. This is particularly of value for the 80,40-10M band where the harmonics in CW mode are more of an issue. The drawback is that we may now have spurs in a similar manner as SSB signal, although the first mixer is not used in this case.

 

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).

 

Results:

 

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.

 

Further tests:

 

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.

 

Questions:

 

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.

 

Ideas welcomed.

 

Thanks and best 73, John (VK2ETA)

 

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