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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)

 

John (vk2eta)
 

Sorry for the typo and bad sentence. The first question should read:

1. What can create this reduced difference between the unbalanced and balanced state of the clock #1 mixer?

73, John

Ted
 

Here - let me suggest an alternative:

1. Cut trace on either side of  R104.
2. Add a 10k trim pot in series with R104.
3. Adjust new trimmer for desired CW carrier output.
4. Enjoy favourite cold beverage.

This works exceedingly well, unless by chance you're looking for a software answer so that "tune" RF is lower than operating RF.  This hardware solution works fine on v.3, 4, or 5.

Ted
K3RTA

Dexter N Muir
 

A possibility for the CW just changing amplitude: is the 'unbalancer' driven from an Arduino output? That could be going from low to high - either of which would unbalance. I'm not an Arduino expert, but could that output be configured to be "off=open"? The alternative would be a transistor in there to make it go either (the greater) to open.
73 de ZL2DEX

John (vk2eta)
 

Thank you Ted.

That may well be my final option.

I was hoping for a software controlled power level as I already do in with the SSB signal by using the gentle (relative to an audio passband) slope of the 45Mhz filter to flatten the power output over the 80m-10m range.

But I can live with say 2 watts for tuning at 80M and 1 watt at 10M. Still very usable and low enough to safeguard the finals during tuning.

73, John

John (vk2eta)
 

Thank you for the reply Dexter.

The strange thing is that the control voltage at the mixer swings from a virtually zero value (0.2mV DC on key up) and a full conduction value at 245mV DC on key down.

The 245mV value did not change significantly when I reduced the R105/R104 values so it seems to indicate that the mixer diodes are fully conducting on Key down as expected.

Also something I didn't mentioned before: when I disconnect the wire between T4(3,5), that is the clock #1 mixer, and R105 I get no unbalancing of the mixer and a very weak transmitted signal as expected on key-up (or key-down for that matter as it is disconnected).

So it is as if the clock #1 mixer would be much more sensitive to unbalancing than the clock #2 mixer.

Scratching my head on this one.

73, John

Ted
 

John,

RE:  "The 245mV value did not change significantly when I reduced the R105/R104 values....."

If you're looking for a power change via R104, you must increase that resistance value (from the stock 2.2k up to to 3k, 5k, 8k, etc) in order to decrease CW power output.  I've shorted R104 outright, and power remains at its full potential. Only by increasing value does anything move away from stock output there.

Just making sure that you're getting to where you meant to be on this.



Best regards,

Ted
K3RTA

 

John,

Try a higher value for R104 like 100K preset or pot. The current the diodes need to conduct is very small IMHO.

Raj

At 09/04/2019, you wrote:
Thank you for the reply Dexter.

The strange thing is that the control voltage at the mixer swings from a virtually zero value (0.2mV DC on key up) and a full conduction value at 245mV DC on key down.

The 245mV value did not change significantly when I reduced the R105/R104 values so it seems to indicate that the mixer diodes are fully conducting on Key down as expected.

Also something I didn't mentioned before: when I disconnect the wire between T4(3,5), that is the clock #1 mixer, and R105 I get no unbalancing of the mixer and a very weak transmitted signal as expected on key-up (or key-down for that matter as it is disconnected).

So it is as if the clock #1 mixer would be much more sensitive to unbalancing than the clock #2 mixer.

Scratching my head on this one.

73, John

Jerry Gaffke
 

John,

Topology is different between first and second mixers on the uBitx.
I'd do your mod as proposed, but 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.

Not yet obvious to me why it would matter.
But Farhan likely made that first mixer at T2 different than all 
other mixers on the uBitx and Bitx40 for a reason. 

Jerry, KE7ER

Dexter N Muir
 

It seems the slightest leakage from "open" unbalances the mixer -and an Arduino output doesn't deliver that cleanly. Physically switching the key, direct from the panel socket or by relay, may be the only way to get clean CW by this method.
73, Dex ZL2DEX

Jerry Gaffke
 

Maybe.
Except it is working just fine at the first mixer on thousands of uBitx's.


On Tue, Apr 9, 2019 at 02:07 PM, Dexter N Muir wrote:
It seems the slightest leakage from "open" unbalances the mixer -and an Arduino output doesn't deliver that cleanly. Physically switching the key, direct from the panel socket or by relay, may be the only way to get clean CW by this method.
73, Dex ZL2DEX

John (vk2eta)
 

Thank you all for your replies. Much appreciated.

I will do more work based on your input and report back on the results.

73, John (VK2ETA)

John (vk2eta)
 

It now works, and works well.

Thanks Jerry, the different topology between T2 and T4 was the key issue with the unwanted unbalancing. I can't see why, but that is exactly what is happening.

So the successful modification to get the clock #1 mixer to unbalance on CW key down is the following:

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.

I 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 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).


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 to a bit of programming and I have full control of SSB as well as CW power across bands. Great for my built-in ATU.

If someone with a spectrum analyzer could check the implications for the harmonics and spurs that would be a plus. I 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 I can modify Farhan's code to match that modification (or publish some code snippets).

By the way, before I did the mod above, I 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 I could get it to work as intended.

73, John (VK2ETA)

Jerry Gaffke
 

With John's mod of post    https://groups.io/g/BITX20/message/67981
any spurs and harmonics when operating CW should be
very similar to what is seen when operating SSB. 

If CW signal into the 45mhz IF is too large, distortion (and spurs) will increase.
To correct this, could replace R104 with something like a 5k pot,
top pin of pot to CW-KEY, bottom pin to ground, wiper pin to R105.
(Remember, with John's mod, R105 now goes to T4 pins 3,5.)
Adjust this pot such that CW transmit power is equal to SSB peak power.

If we are lucky, the mixer at D3,4 will want the same CW-KEY injection level
across all rigs, and we can use fixed resistors instead of a pot.
But that may well not be the case.

Jerry, KE7ER


On Wed, Apr 17, 2019 at 08:31 PM, John (vk2eta) wrote:
If someone with a spectrum analyzer could check the implications for the harmonics and spurs that would be a plus. I 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 I can modify Farhan's code to match that modification (or publish some code snippets).
 

Jerry Gaffke
 

Could be that John's method of varying transmit power when transmitting CW
will make the use of fixed resistors to set the  D3,4 injection level OK.
But if that mixer or the crystal filter is getting overloaded on some rigs, 
may need the pot.


On Thu, Apr 18, 2019 at 09:38 AM, Jerry Gaffke wrote:
With John's mod of post    https://groups.io/g/BITX20/message/67981
any spurs and harmonics when operating CW should be
very similar to what is seen when operating SSB. 

If CW signal into the 45mhz IF is too large, distortion (and spurs) will increase.
To correct this, could replace R104 with something like a 5k pot,
top pin of pot to CW-KEY, bottom pin to ground, wiper pin to R105.
(Remember, with John's mod, R105 now goes to T4 pins 3,5.)
Adjust this pot such that CW transmit power is equal to SSB peak power.

If we are lucky, the mixer at D3,4 will want the same CW-KEY injection level
across all rigs, and we can use fixed resistors instead of a pot.
But that may well not be the case.

Jerry, KE7ER

Jerry Gaffke
 

I'm suspicious about the CW-KEY injection level because 
the Bitx40 saw significant variation between rigs when Allard did 
something similar to unbalance the modulator for CW transmissions.
This was solved by going to a fairly high level of injection,
pushing the power amp into non-linear operation.
So it's like all the simple QRP CW transmitters out there with a class C amp.
The Bitx40  transmit LPF seems to work well to remove any harmonics,
and the 40m bandpass filter between the Bitx40's  first mixer
and the power amp removes the spurs.
Neither of these is true of the uBitx, so we can't use the same trick.

Jerry, KE7ER


On Thu, Apr 18, 2019 at 10:29 AM, Jerry Gaffke wrote:
If we are lucky, the mixer at D3,4 will want the same CW-KEY injection level
across all rigs, and we can use fixed resistors instead of a pot.
But that may well not be the case.

John (vk2eta)
 

Thanks Jerry, all very good points.

I really need to invest in a scope and an SA.

I didn't notice any difference in the CW output power after this mod (but I didn't record the before power).

I will check if there is a difference between SSB and CW power levels. That should give me an indication.

Also please tell me if that statement is correct: since the clock #1 signal passes through the xtal filter this should strip its harmonics to pretty low levels (xtal filter overload excepted as you mentioned above. Not sure at what levels this starts occurring).

All the best,

73, John

Jerry Gaffke
 

John,

Yes, the 45mhz crystal filter should strip out the harmonics coming out of that mixer,
leaving just a pure 45mhz carrier when transmitting CW

For SSB, both of the level 7 diode ring mixers want an incoming signal that is 10 dB or so below
the 7dBm local oscillator, so about -3 dBm max.  Beyond that it starts to distort. 
The output of the mixer is about 6 dB below the signal coming in.
The 45mhz crystal has a loss of 3 to 5 dB I'd think, the 45mhz IF amp has a gain
somewhere between 10 and 16 dB (the hot transistors on v5 might put that one 
closer to 16 dB).  

Of course, our si5351 can't quite deliver 7dBm, but the uBitx pushes things a bit, driving the mixers
hard because we don't have enough gain in the power amp.
Anyways, the paragraph above should get you in the right ballpark regarding signal levels.

I'm liking the looks of this AD8307 RF probe, very clever how they make use of a cheap digital voltmeter module:
    https://qrpguys.com/qrpguys-digital-field-strength-meter
That thing could be calibrated, you could read dBm from it using a hand calculator.
Better yet would be if the uBitx had pads for an optional AD8307 feeding a Nano analog pin.
There are AD8307 chips at $0.50 each on ebay, and they generally work fine.

Jerry, KE7ER
 


On Thu, Apr 18, 2019 at 05:23 PM, John (vk2eta) wrote:
Also please tell me if that statement is correct: since the clock #1 signal passes through the xtal filter this should strip its harmonics to pretty low levels (xtal filter overload excepted as you mentioned above. Not sure at what levels this starts occurring).

John (vk2eta)
 

Thanks Jerry,

Yes that gives me good reference points. So assuming the clocks #1 and #2 are at the same level, it looks like after this mod the signal at T2 (3,5) could be around 10 to 16dB too high.

I like that QRPGuys probe.

All the best,

73, John

Jerry Gaffke
 

Signal level into the D1,2 mixer can be a bit higher than it is into the D3,4 mixer
when transmitting SSB.  But not by much.

Assume the D3,4 mixer has a signal loss of 6dB, the 45mhz crystal has a loss of 4dB,
and that the 2n3904 based v3,v4 uBitx 45mhz IF amp has a gain of 10dB.
(Gain is less than the designed for 16dB because the 2n3904's
can't give that much gain at 45mhz.  I have no idea what v5 does.)
We wind up with the same level of SSB signal into the D1,2 mixer
as we had going into the D3,4 mixer.  


On Thu, Apr 18, 2019 at 08:15 PM, John (vk2eta) wrote:
Yes that gives me good reference points. So assuming the clocks #1 and #2 are at the same level, it looks like after this mod the signal at T2 (3,5) could be around 10 to 16dB too high.