Control CW TX power by unbalancing the second mixer

John (VK2ETA)  has been working on a successful modification to get the clock #1 mixer to unbalance on CW key down.  This involves the following steps:

1. Re-wire the T4 transformer input and output as per T2. That means as Jerry said to “… cut traces to T4 pins 3,5 and 6.  Swap them so T4 pins 3,5 are in from R47, pin 6 is out to C211.”

2. Disconnect (cut trace) from R105 to the common connection of C10, R27 and T2 (3,5).

3. Connect via a wire the disconnected side of R105 to the common connection of T4 (3,5), C211.

John used an audio shielded cable, and passed the wire under the board by drilling two small holes next the two connection points to ensure minimal pick-up of RF.

Pictures of R105 trace cut and wire to T4. (partially obscured by the hot melt glue on the toroids as I use my rig /P and /PM).

Picture of T4 traces swapped and cable from R105. (The line from the yellow toroid is a hot melt glue filament, not a wandering wire..hihi)

4. Change the software.  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).

The difference in signal strength between key-up and key-down as seen on a control receiver is from way below S0 to S9+20dB or so, giving a dynamic range of over 70dB (indicative value in light of the basic test method).

The output power in CW mode can now be controlled finely by shifting the 45MHz clock slightly along the slope of the Xtal filter. For example I go from 14W max to between 1.5 and 2 watts by shifting the clock #1 frequency by 30Khz on the 20M band. On the 10M band, I need a shift of about 10Khz to reduce the 8 watts out to the same level.

Now, thanks to a bit of programming, John has full control of SSB as well as CW power across all bands. Great for his built-in ATU.

If someone with a spectrum analyzer could check the implications for the harmonics and spurs that would be a plus. John would expect in CW mode that the harmonic for 80, 40 and 30M bands would reduce, but some spurs to appear since we have the beat of two clocks now.  If there is interest John could modify Farhan’s code to match that modification (or publish some code snippets).

By the way, before he  did the mod above, he also tried to put a trim-pot between R105 / Ground / slider to T4 (3,5) and even when turned all the way to zero (slider to ground) it would still constantly unbalance the clock #1 mixer. So the change in topology as described above is the only way he could get it to work as intended.

Reference

Power output from uBITx plotted

The power output of the µBITx varies according to frequency.

Richard Pushman provided a chart showing the power output of the rig by frequency from 2MHz to 30Mhz for two different supply voltages.   The shape of the peaks and troughs is, of course, influenced by the LPF filters in the output stage.

Reference

Low power on 80m?

Allison KB1GMX notes that if you have a v4 board and experiencing low power output on lower bands (80m and 40m) you may want to check the emitter of Q90 to see if a .01uf or a 470pf or 220PF is there.

There was a change to lower the value from .01 to 470 pf to level the output
power so 80M wasn’t 15-20W when 20m was maybe 8 watts output.

Some have experienced  that low bands work very poorly with a 470pF or 220pF capacitor.  You may want to increase the capacitor back to 0.01µF or perhaps 1000pf.

Reference

Evening up power out using KB1GMX and K9HZ solutions

Mike N6CMY has been doing some mods on his µBITx, including some of Allison’s mods by substituting 2n2222’s and a BFR106 for the 3904’s on the RF lineup (pre-driver and driver stages).

In terms of output power the results are impressive. But in terms of levelling  up the output… well not so good.

Mike measured 12W on 10M and over 35W  on 40M!  He was a bit taken aback at this point!

He finally bit the bullet and used K9HZ’s multi relay/pot method for levelling the response.

Mike was reluctant to use such a brute-force solution for a subtle problem but he now measures 15W on 80, 40, 20 M and 12 on 10M. VICTORY!!

Reference

Attenuation on TX to flatten the curve (Phone only)

John VK2ETA has uploaded his latest version of a variation on Ian KD8CEC’s software with a default of “stock standard” which means it should run on units “factory” wired.

It includes the “OPTION_ALC” which use a table and interpolation to set the attenuation for 3 levels (Low, High, Max) for SSB power output.

Not a complex code at all (just search for the “option_alc” keywork). This should give up to 50dB of attenuation. This should be plenty to control the power even for WSPR low power users.

You will probably need to attenuate the signal for “Max” power on 40 and 80m otherwise you will get well past the targeted 16-17W on the lower frequencies.

Note that on 10m, you will not get 10w out.   You will need to follow Allison’s work on the power chain to see if a solution can be found to the fall-off in power out on 10m.

John’s software can be found here.

Reference