Topics

PA voltage and RF output

Sasi Nayar
 

Hi all

I am not sure if this has been asked before, although my search of the archives came up with nothing.

I understand that the BITX40 Ver3 PA stage can be supplied with 12-24VDC. Has some one actually measured the current drawn and RF output across the voltage range of 12 to 24V? I do recollect seeing somewhere an RF output of 17W at 20V (current drawn = 1.8A) - not sure where. Any leads would be appreciated.

73

Sasi

VK5SN

John Smith
 

Amps X Volts = Watts. I supply 25V into final, draw 1 Amp, MFJ-9213 meter indicates 24 watts output with 1khz tone into mic for tuning. This is with a tuned 40M dipole and not a dummy load. Plus a much bigger heatsink and fan powered by buck converter from 25V input stepped down to about 7V for 12V 60MM computer fan without noise, including TX. Using main board 12V induces a lot of noise into audio from switching circuitry in brushless fan. Use final PA power for fans.

Does that help?

Sasi Nayar
 

Thanks John. Usful to know you are getting around 24W RF output for 25V into the final. I am trying to ascertain the increment in RF power with an increase in Voltage in to the PA stage (someone has documented this somewhere but I just connot recollect where).  The objective here is to set the step-up voltage module for an RF output of around 20W without pushing it too far. I was planning on beefing up the heat sink and adding a little fan

73

Sasi

VK5SN

John Smith
 

It kinda sounds like you are going to use a boost converter to increase voltage from the 12V input from the main board to power the PA. If that is the case you will have a lot of noise in the audio from both the boost converter and the fan. it's best to keep them separate if you step up, or down voltage and, or use a fan too. Is that what you mean by step-up voltage module?

I think it's better to have 12V 1A for the main board and Raduino. And use a higher voltage supply, (mine it 31V 1.6A) and step it down for the PA, then step down again for the fan. This will keep switching, and induction noise out of the radio audio. Keep the PA current draw at or under 1 amp, and under 25V, you will not push it too far. Adjust R136 for current draw which will also affect RF watts output.


Dr Fred Hambrecht
 

How did he get 96% efficiency?

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Sasi Nayar
Sent: Tuesday, March 21, 2017 8:49 PM
To: BITX20@groups.io
Subject: Re: [BITX20] PA voltage and RF output

 

Thanks John. Usful to know you are getting around 24W RF output for 25V into the final. I am trying to ascertain the increment in RF power with an increase in Voltage in to the PA stage (someone has documented this somewhere but I just connot recollect where).  The objective here is to set the step-up voltage module for an RF output of around 20W without pushing it too far. I was planning on beefing up the heat sink and adding a little fan

73

Sasi

VK5SN

William Kimber <zl1tao@...>
 

He never said he  did.  Power is voltage x current.  Are you assuming 1 Amp?

If it is 1.25 amp then not 96%.

Cheers,

Will


On 03/22/2017 04:01 PM, Dr Fred Hambrecht wrote:

How did he get 96% efficiency?

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Sasi Nayar
Sent: Tuesday, March 21, 2017 8:49 PM
To: BITX20@groups.io
Subject: Re: [BITX20] PA voltage and RF output

 

Thanks John. Usful to know you are getting around 24W RF output for 25V into the final. I am trying to ascertain the increment in RF power with an increase in Voltage in to the PA stage (someone has documented this somewhere but I just connot recollect where).  The objective here is to set the step-up voltage module for an RF output of around 20W without pushing it too far. I was planning on beefing up the heat sink and adding a little fan

73

Sasi

VK5SN


 

It will be more like 2A, when you double the voltage the current will double and power X4

Efficiency will be more like 50%

Raj, vu2zap


At 22/03/2017, you wrote:

He never said he  did.  Power is voltage x current.  Are you assuming 1 Amp?

If it is 1.25 amp then not 96%.

Cheers,

Will

On 03/22/2017 04:01 PM, Dr Fred Hambrecht wrote:
How did he get 96% efficiency?
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From: BITX20@groups.io [ mailto:BITX20@groups.io] On Behalf Of Sasi Nayar
Sent: Tuesday, March 21, 2017 8:49 PM
To: BITX20@groups.io
Subject: Re: [BITX20] PA voltage and RF output
Â

Thanks John. Usful to know you are getting around 24W RF output for 25V into the final. I am trying to ascertain the increment in RF power with an increase in Voltage in to the PA stage (someone has documented this somewhere but I just connot recollect where).  The objective here is to set the step-up voltage module for an RF output of around 20W without pushing it too far. I was planning on beefing up the heat sink and adding a little fan

73

Sasi

VK5SN

Sasi Nayar
 

Reading what has been covered here in the past, I was not going to power either the fan or the convertor (<http://www.ebay.com.au/itm/262455133912?_trksid=p2050601.m570.l6004&_trkparms=gh1g%3DI262455133912.N41.S1.R1.TR1>) off the main board. They are going to be powered directly off the 12V input. I have drawn up the schematics in a way that I will be able to switch to run the PA stage on either 12V or 24V. The fan will have its own switch - probably only run it at when the PA is run at 24V.

This is all being planned whilst waiting for the boards to be delivered in a couple of weeks.
73
Sasi
VK5SN

John Backo
 

Be careful.

Volts x Amps = power for DC.
For AC it is a bit different. Volts = Pk to Pk/2 x .707.

Efficiency is usually measured as DC power input vs. AC power output.
Fred is right to question the 96% apparent efficiency figure.
Raj is right.; the real figure is more like 40-50%.

If the power meter is correct in proclaiming 24W RF output, the DC power
input is probably more like 50W, or 24v x 2A. By the same figure,
12v x 1A input gives about 5W RF output.

This assumes a 50 ohm system with a near 1:1 SWR.

john
AD5YE

John Smith
 

As a disclaimer effort, I listed the model number of the watt meter in case of known unreliability. I don't have anything better to compare it with. And I mentioned a 1khz tone into the mic because of steady maximum drive above my voice fluctuations. I can make the meter peak at 24W with voice but it flutters around 10W with conversational speech.

24W max with tone. Around 10W or so while talking. Does that sound better?