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Questions about adjusting QDX RF output by changing supply voltage

 

Greetings all,

?

I've no wish to stir up the debate about which supply voltage QDXs should be built for, but in reading the manual, I saw where it says that the power slider in WSJT-X has no effect on the QDX and that any RF output reduction should be accomplished by lowering the supply voltage. So...

?

1) For those who have done the 12V build and adjust the supply voltage to change the RF output, what problems (if any) have you had?

?

2) Is the QDX tolerant of supply voltage change while powered up or do I need to turn it off before I make a change?

?

An example of why I would want to reduce the RF output is that I normally run ~1W when operating as a WSPR beacon, yet I usually operate FT8 at 2.5-3W, sometimes going up to 5W.

?

I appreciate the information!

?

Charles Johnson

KF4AYT
 

Hi Charles.

Mine is built for 12 volts like yours. I usually run it down at 9 or 10 volts input while doing something like WSPR (1-2 watts out) and it doesn't get very hot. I run it at 12.0 or maybe slightly reduced (11.0) when?running FT-8 (approx. 4 watts out) and again no big heating issues. I am probably overly cautious compared to some.?

Yes, you can adjust the supply voltage while it's powered up. I would caution you to try not to do something I once did. My power supply goes up to 35 volts or so and I quickly discovered once I had adjusted the 10s of volts setting by mistake. I usually adjust the 10ths of a volt setting. I had it at 30 volts for 3 seconds while receiving and fortunately the regulators and all survived. I don't recommend testing yours that way :-)... good luck ...

73 ... Ron

On Fri, Aug 12, 2022 at 9:57 AM Charles Johnson <cjohnson@...> wrote:

Greetings all,

?

I've no wish to stir up the debate about which supply voltage QDXs should be built for, but in reading the manual, I saw where it says that the power slider in WSJT-X has no effect on the QDX and that any RF output reduction should be accomplished by lowering the supply voltage. So...

?

1) For those who have done the 12V build and adjust the supply voltage to change the RF output, what problems (if any) have you had?

?

2) Is the QDX tolerant of supply voltage change while powered up or do I need to turn it off before I make a change?

?

An example of why I would want to reduce the RF output is that I normally run ~1W when operating as a WSPR beacon, yet I usually operate FT8 at 2.5-3W, sometimes going up to 5W.

?

I appreciate the information!

?

Charles Johnson

KF4AYT
 

On Fri, Aug 12, 2022 at 05:57 PM, Charles Johnson wrote:

1) For those who have done the 12V build and adjust the supply voltage to change the RF output, what problems (if any) have you had?

?

2) Is the QDX tolerant of supply voltage change while powered up or do I need to turn it off before I make a change?

Charles.? There is no problem in changing supply voltage while the? QDX is on or indeed transmitting.? That is provided you can do this without any spikes that take the voltage above the appropriate supply level (e.g. 9 or 12V).? I am thinking here about maybe a defective variable resistor which causes brief high voltage excursions as it is adjusted! The clever bits are all fed off 5V or 3.3V regulators which easily handle variations in input voltage.
I regularly switch my QDX between 12V (the supply I built it for) and about 8.2V in my remote "bottom of the? garden" installation.? I use 2 Grove MOSFET switches (see attachments), one of which switches the full 12V to the QDX (note: Schottky diode D3 short circuited in this case) or the other which has a series of silicon power diodes as a voltage dropper. The switches are controlled by my Arduino telemetry/remote control system from the shack PC.
The attached diagram and picture show the arrangement.? Note that the Grove MOSFET switch is a commercial unit.? I have added some RF decoupling to input and output and also use 200uF of electrolytics on the output of the switches to ensure continuity of supply in case of brief periods when both switches are open (not that I believe this happens from the way the Arduino is programmed) and also to maintain a low impedance source for the QDX.? This extra capacitance is almost certainly not really necessary, but it doesn't cost much to add it in, so I did!
My main use of the "reduced power" feature is to check the antenna match at the start of an operating session to ensure that the load is OK before switching to full power, and also when changing bands.

Peter Lee
G3SPL
 

¿ªÔÆÌåÓý

Peter,

Could you leave the diode string in-line at all times and short it out with the Grove MOSFET swich, or do you need to be able to turn all the power off at times?

73, Willie N1JBJ

On Aug 12, 2022, at 8:52 PM, G3SPL <leepd60@...> wrote:

On Fri, Aug 12, 2022 at 05:57 PM, Charles Johnson wrote:

1) For those who have done the 12V build and adjust the supply voltage to change the RF output, what problems (if any) have you had?

?

2) Is the QDX tolerant of supply voltage change while powered up or do I need to turn it off before I make a change?

Charles.? There is no problem in changing supply voltage while the? QDX is on or indeed transmitting.? That is provided you can do this without any spikes that take the voltage above the appropriate supply level (e.g. 9 or 12V).? I am thinking here about maybe a defective variable resistor which causes brief high voltage excursions as it is adjusted! The clever bits are all fed off 5V or 3.3V regulators which easily handle variations in input voltage.
I regularly switch my QDX between 12V (the supply I built it for) and about 8.2V in my remote "bottom of the? garden" installation.? I use 2 Grove MOSFET switches (see attachments), one of which switches the full 12V to the QDX (note: Schottky diode D3 short circuited in this case) or the other which has a series of silicon power diodes as a voltage dropper. The switches are controlled by my Arduino telemetry/remote control system from the shack PC.
The attached diagram and picture show the arrangement.? Note that the Grove MOSFET switch is a commercial unit.? I have added some RF decoupling to input and output and also use 200uF of electrolytics on the output of the switches to ensure continuity of supply in case of brief periods when both switches are open (not that I believe this happens from the way the Arduino is programmed) and also to maintain a low impedance source for the QDX.? This extra capacitance is almost certainly not really necessary, but it doesn't cost much to add it in, so I did!
My main use of the "reduced power" feature is to check the antenna match at the start of an operating session to ensure that the load is OK before switching to full power, and also when changing bands.

Peter Lee
G3SPL
 

On Sat, Aug 13, 2022 at 10:53 AM, William Smith wrote:
Peter,
?
Could you leave the diode string in-line at all times and short it out with the Grove MOSFET swich, or do you need to be able to turn all the power off at times?
?
73, Willie N1JBJ
Willie.? Yes, you can absolutely operate the switches in series, with the second switch just short circuiting the diode droppers. In that case, you would need to short circuit D3 in both of the Grove MOSFET boards as this Schottky diode drops about 0.3 to 0.4 volts otherwise. Doing it my way saved me about 10mm of wire for the short circuit!? Oh, and of course the series arrangement has another MOSFET "on" resistance added which drops a few more millivolts.? Either way is fine.

Peter Lee
G3SPL
 

¿ªÔÆÌåÓý

Hi Peter,

Sorry, what I meant was to use only one Grove MOSFET board, to supply 12V to the QDX when it's on.

When that one MOSFET board is off, the QDX is powered by the string of diodes.

Put them in parallel, so the minimum voltage is "diode string" and the maximum voltage is "MOSFET Board On".

Does that make more sense?

73, Willie N1JBJ

On Aug 13, 2022, at 11:43 AM, G3SPL <leepd60@...> wrote:

On Sat, Aug 13, 2022 at 10:53 AM, William Smith wrote:
Peter,
?
Could you leave the diode string in-line at all times and short it out with the Grove MOSFET swich, or do you need to be able to turn all the power off at times?
?
73, Willie N1JBJ
Willie.? Yes, you can absolutely operate the switches in series, with the second switch just short circuiting the diode droppers. In that case, you would need to short circuit D3 in both of the Grove MOSFET boards as this Schottky diode drops about 0.3 to 0.4 volts otherwise. Doing it my way saved me about 10mm of wire for the short circuit!? Oh, and of course the series arrangement has another MOSFET "on" resistance added which drops a few more millivolts.? Either way is fine.

Peter Lee
G3SPL
 

On Sat, Aug 13, 2022 at 04:49 PM, William Smith wrote:
Hi Peter,
?
Sorry, what I meant was to use only one Grove MOSFET board, to supply 12V to the QDX when it's on.
?
When that one MOSFET board is off, the QDX is powered by the string of diodes.
Yes, that would certainly work, but I needed to be able to turn off the power completely by remote control (this was so that I could switch to using an SDR receiver instead of the QDX over the USB extender).? Hence I needed 2 MOSFET switches and they could be in parallel or series, it doesn't make much difference.? Incidentally, the Grove MOSFET boards are pretty cheap - only ?3.80 each here i.e. a handful of dollars, so they didn't break the bank!
?
--
Peter Lee
G3SPL

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