Topics

RF POWER OSCILLATION


Rubens Kamimura
 

For all,

I would like an opinion, I have uBITX v4, the RF power is oscillating, now with more intensity, as it is a QRP, I believe that a variation of 2 to 3 watts is not a good practice in the 40m (7074kHz) in FT8.
--
73's 
PY2PVB
Rubens 


Evan Hand
 

Rubens,
My first thought would be to verify that I see the power fluctuations in the CW mode.  That would help in isolating between the RF power amplifier section and the IF/Audio sections of the radio.

I would also verify how you measure the power out.  Do you see a fluctuation when using the FT8 tune function?  I have seen a power fluctuation on all of my rigs when running FT8 and measuring the power out with an SWR/Power meter.  I do not see the same fluctuations in the tune mode.

One other thought is that you may be operating at one of the SSB filter edges, or that the BFO is not aligned with the filter.  Remember that the SSB filter is only 2.7kHZ wide.  So in the FT8 software, you need to enable the "fake it" functions to allow the frequency to change if you go outside of the passband of the filter.  The alternative is to only work at audio frequencies between 300 and 3kHz.

Above are just ideas on areas to look to see if you do have a problem.
73
Evan
AC9TU


Curt
 

Rubens

I see the amplitude changing on the power indicator within another transceiver on ft8, so I wonder if the simple indicator is not following the modulation. I concur with Evan to have you evaluate it on cw, and watch your transmit frequency (I usually fix my transmit frequency to one spot, and only move to other ops frequency when he doesn't respond).

If you want to check adjustment of your ssb modulation you need a simple audio oscillator.

Curt wb8yyy


Rubens Kamimura
 

Hi,

I'm cleaning up the stains on the garage floor right now. But, I will test in CW, really a good idea, thanks.

73's
PY2PVB
Rubens 


Em sex., 15 de jan. de 2021 às 11:02, Curt via groups.io <wb8yyy=yahoo.com@groups.io> escreveu:

Rubens

I see the amplitude changing on the power indicator within another transceiver on ft8, so I wonder if the simple indicator is not following the modulation. I concur with Evan to have you evaluate it on cw, and watch your transmit frequency (I usually fix my transmit frequency to one spot, and only move to other ops frequency when he doesn't respond).

If you want to check adjustment of your ssb modulation you need a simple audio oscillator.

Curt wb8yyy


Arv Evans
 

A few others have noticed this.  It seems from their posts that this only occurs
when using digital modes that interface the uBITX via analog audio input. 
SSB mode uses a different signal path than plain CW on a uBITX.  If you
are not seeing the RF output level variation using CW but are seeing it during
digital modes with audio interfaces, it may be that the audio input is changing. 
If this is the case, then it may be that the input is changing frequency and driving
your RF off frequency far enough for the IF filter to be blocking part of the
signal, or that it is driving far enough off frequency for the RF PA filter to be
affecting power output.  Both level and frequency are critical for digital modes
that are introduced to the uBITX as an audio signal.

Arv
_._


On Fri, Jan 15, 2021 at 6:05 AM Rubens Kamimura <rubens.kamimura@...> wrote:
For all,

I would like an opinion, I have uBITX v4, the RF power is oscillating, now with more intensity, as it is a QRP, I believe that a variation of 2 to 3 watts is not a good practice in the 40m (7074kHz) in FT8.
--
73's 
PY2PVB
Rubens 


Rafael Diniz
 

Using VARA, I really had to spend some time tuning the frequency offset
and BFO carefully, optimized to provide the >= 2.3 kHz bandwidth for the
VARA OFDM signal. Also the line-out level was a very important setting -
it needs to be very low, as the mic input is very sensitive.

Rafael

On 1/15/21 5:01 PM, Arv Evans wrote:
A few others have noticed this.  It seems from their posts that this
only occurs
when using digital modes that interface the uBITX via analog audio
input. 
SSB mode uses a different signal path than plain CW on a uBITX.  If you
are not seeing the RF output level variation using CW but are seeing
it during
digital modes with audio interfaces, it may be that the audio input is
changing. 
If this is the case, then it may be that the input is changing
frequency and driving
your RF off frequency far enough for the IF filter to be blocking part
of the
signal, or that it is driving far enough off frequency for the RF PA
filter to be
affecting power output.  Both level and frequency are critical for
digital modes
that are introduced to the uBITX as an audio signal.

Arv
_._


On Fri, Jan 15, 2021 at 6:05 AM Rubens Kamimura
<rubens.kamimura@gmail.com <mailto:rubens.kamimura@gmail.com>> wrote:

For all,

I would like an opinion, I have uBITX v4, the RF power is
oscillating, now with more intensity, as it is a QRP, I believe
that a variation of 2 to 3 watts is not a good practice in the 40m
(7074kHz) in FT8.
--
73's 
PY2PVB
Rubens 


Rubens Kamimura
 

TU
Tanks
--
73's PY2PVB

Rubens Kamimura
Técnico em Eletrônica e Eletrotécnica - Aposentado
CFT/CRT-SP: 80254934820
Cel. +55 (18) 9-9819-2225 WhatsApp


Em sex., 15 de jan. de 2021 às 17:01, Arv Evans <arvid.evans@...> escreveu:

A few others have noticed this.  It seems from their posts that this only occurs
when using digital modes that interface the uBITX via analog audio input. 
SSB mode uses a different signal path than plain CW on a uBITX.  If you
are not seeing the RF output level variation using CW but are seeing it during
digital modes with audio interfaces, it may be that the audio input is changing. 
If this is the case, then it may be that the input is changing frequency and driving
your RF off frequency far enough for the IF filter to be blocking part of the
signal, or that it is driving far enough off frequency for the RF PA filter to be
affecting power output.  Both level and frequency are critical for digital modes
that are introduced to the uBITX as an audio signal.

Arv
_._


On Fri, Jan 15, 2021 at 6:05 AM Rubens Kamimura <rubens.kamimura@...> wrote:
For all,

I would like an opinion, I have uBITX v4, the RF power is oscillating, now with more intensity, as it is a QRP, I believe that a variation of 2 to 3 watts is not a good practice in the 40m (7074kHz) in FT8.
--
73's 
PY2PVB
Rubens