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XV50 PA Bias

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g3ymc

XV50 PA Bias

I am aligning my new XV50, latest build using the RA30H0608M module and
having issues with setting the quiescent bias of the pa as per the
manual. I see others have had this same problem but I can find no
answers.

The manaul calls for R39 to be adjusted for 20mV on the DMM across
TP3/4 (across the 20mohm current sense resistor R17). With R39 at its
maximum I get 16mV corresponding to a total current of 800mA as against
the required 1A. This is confirmed by the digital current meter on the
12V psu powering it and my K2, which increases from 0.5A to 1.3A when I
enable transmit on the XV50.

Before anybody asks, I DO have link W1 fitted, and in fact I have the
correct bias voltage on pin 2 of the output module. However there is
where I suspect the problem as the maximum voltage achievable there is
4.7V.

I found a datasheet of the RA30H0608M, albeit a preliminary one which
has a few sheets 'in preparation'. It does not give any figures for
standing bias versus the bias volts but suggests it must be somewhere
between 4.5 and 5V (there is no power output below 4.5V). The voltage
at the R29/R39 junction is limited by the current flowing through R28
and the module bias current (typically 1mA). So I am wondering whether
some rf modules need a bit more than 4.7V to get the correct bias
current of around 1A. Maybe reducing the value of R39 (56 ohms) is the
answer. Or is mine near enough for good linearity?

Any help welcome.

73 Dave G3YMC

http://davesergeant.com

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Don Wilhelm-4

Re: XV50 PA Bias

Dave,

The PA bias control usually has much greater range than is needed, and
the PA bias input should draw very little current.
Check the resistance of R39. Is there a chance that you swapped R39 and
R10 during the assembly?

73,
Don W3FPR

On 10/24/2014 6:46 AM, Dave Sergeant wrote:

> I am aligning my new XV50, latest build using the RA30H0608M module and
> having issues with setting the quiescent bias of the pa as per the
> manual. I see others have had this same problem but I can find no
> answers.
>
> The manaul calls for R39 to be adjusted for 20mV on the DMM across
> TP3/4 (across the 20mohm current sense resistor R17). With R39 at its
> maximum I get 16mV corresponding to a total current of 800mA as against
> the required 1A. This is confirmed by the digital current meter on the
> 12V psu powering it and my K2, which increases from 0.5A to 1.3A when I
> enable transmit on the XV50.
>
> Before anybody asks, I DO have link W1 fitted, and in fact I have the
> correct bias voltage on pin 2 of the output module. However there is
> where I suspect the problem as the maximum voltage achievable there is
> 4.7V.
>
> I found a datasheet of the RA30H0608M, albeit a preliminary one which
> has a few sheets 'in preparation'. It does not give any figures for
> standing bias versus the bias volts but suggests it must be somewhere
> between 4.5 and 5V (there is no power output below 4.5V). The voltage
> at the R29/R39 junction is limited by the current flowing through R28
> and the module bias current (typically 1mA). So I am wondering whether
> some rf modules need a bit more than 4.7V to get the correct bias
> current of around 1A. Maybe reducing the value of R39 (56 ohms) is the
> answer. Or is mine near enough for good linearity?
>
>

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g3ymc

Re: XV50 PA Bias

R39 is correct (10k pot) as is R10 (100k) and all the measurements
confirm this. The adjustment range, measured at pin 2 of the amplifier
module, is 2.5-4.7V, also what is expected from the circuit. The
maximum voltage is limited to 4.7V because of the 5mA flowing through
R28 which swamps the 1mA bias current.

What I would like to know is what the bias voltage should be on a
correctly set up XV50. The datasheet seems to indicate it must be at
least greater than 4.5V. It seems my amp needs more than 4.7V to get
the correct bias, and judging from previous posts on this subject I
don't think I am alone.

73 Dave G3YMC

On 24 Oct 2014 at 7:46, Don Wilhelm wrote:

> Dave,
>
> The PA bias control usually has much greater range than is needed, and
> the PA bias input should draw very little current. Check the resistance
> of R39. Is there a chance that you swapped R39 and R10 during the
> assembly?
>
> 73,
> Don W3FPR
>
> On 10/24/2014 6:46 AM, Dave Sergeant wrote:
> > I am aligning my new XV50, latest build using the RA30H0608M module
> > and having issues with setting the quiescent bias of the pa as per the
> > manual. I see others have had this same problem but I can find no
> > answers.
> >
> > The manaul calls for R39 to be adjusted for 20mV on the DMM across
> > TP3/4 (across the 20mohm current sense resistor R17). With R39 at its
> > maximum I get 16mV corresponding to a total current of 800mA as
> > against the required 1A. This is confirmed by the digital current
> > meter on the 12V psu powering it and my K2, which increases from 0.5A
> > to 1.3A when I enable transmit on the XV50.
> >
> > Before anybody asks, I DO have link W1 fitted, and in fact I have the
> > correct bias voltage on pin 2 of the output module. However there is
> > where I suspect the problem as the maximum voltage achievable there is
> > 4.7V.
> >
> > I found a datasheet of the RA30H0608M, albeit a preliminary one which
> > has a few sheets 'in preparation'. It does not give any figures for
> > standing bias versus the bias volts but suggests it must be somewhere
> > between 4.5 and 5V (there is no power output below 4.5V). The voltage
> > at the R29/R39 junction is limited by the current flowing through R28
> > and the module bias current (typically 1mA). So I am wondering whether
> > some rf modules need a bit more than 4.7V to get the correct bias
> > current of around 1A. Maybe reducing the value of R39 (56 ohms) is the
> > answer. Or is mine near enough for good linearity?
> >
> >
>

http://davesergeant.com

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Don Wilhelm-4

Re: XV50 PA Bias

Dave,

R39 for the XV50 should be a 1k pot. Typo or incorrect component?
The bias voltage is not the important factor, important is the ability
to produce the proper idling current - that is why the instructions tell
you to measure the voltage between TP3 and TP4 when setting the bias.
The voltage at the bias pin will typically be in the 4.7 volt range, but
there is no problem if it is higher or lower.

73,
Don W3FPR

On 10/24/2014 8:49 AM, Dave Sergeant wrote:

g3ymc

Re: XV50 PA Bias

In reply to this post by g3ymc

An update on my XV50.

Having failed to get the pa bias greater than around 500mA I decided to
press ahead with the rest of the alignment. It is now basically working
but only at the 5-10W level. If I increase the drive to 20W it
effectively limits at that level, no more than 20W can be obtained and
it is VERY flat topped on SSB. I assume this is because the pa is
non-linear because of the incorrect bias. But it appears reasonably
linear with the input attenuation R22 turned down to the lower power.

I also did the mod to my KAT2, changing the 47 ohm to 470 ohm - the
power control on the K2 now works correctly.

I intend to leave it set at the 5-10W level and get some reports from
locals - that is the sort of power I intend to use anyway.

I see there have been various similar postings on the reflector over
the past few years with no real answers forthcoming. Looking at the
datasheet of the RA30H0608M it is only specified down to 60MHz and the
performance at 50MHz is totally unspecified. I suspect I, and others
who have had these sort of issues, have devices at the edge of the spec
and the only cure is to try a replacement, which really is pot luck.

73 Dave G3YMC

On 24 Oct 2014 at 13:49, Dave Sergeant wrote:

> R39 is correct (10k pot) as is R10 (100k) and all the measurements
> confirm this. The adjustment range, measured at pin 2 of the amplifier
> module, is 2.5-4.7V, also what is expected from the circuit. The maximum
> voltage is limited to 4.7V because of the 5mA flowing through R28 which
> swamps the 1mA bias current.
>
> What I would like to know is what the bias voltage should be on a
> correctly set up XV50. The datasheet seems to indicate it must be at
> least greater than 4.5V. It seems my amp needs more than 4.7V to get the
> correct bias, and judging from previous posts on this subject I don't
> think I am alone.
>
> 73 Dave G3YMC
>
> On 24 Oct 2014 at 7:46, Don Wilhelm wrote:
>
> > Dave,
> >
> > The PA bias control usually has much greater range than is needed, and
> > the PA bias input should draw very little current. Check the
> > resistance of R39. Is there a chance that you swapped R39 and R10
> > during the assembly?
> >
> > 73,
> > Don W3FPR
> >
> > On 10/24/2014 6:46 AM, Dave Sergeant wrote:
> > > I am aligning my new XV50, latest build using the RA30H0608M module
> > > and having issues with setting the quiescent bias of the pa as per
> > > the manual. I see others have had this same problem but I can find
> > > no answers.
> > >
> > > The manaul calls for R39 to be adjusted for 20mV on the DMM across
> > > TP3/4 (across the 20mohm current sense resistor R17). With R39 at
> > > its maximum I get 16mV corresponding to a total current of 800mA as
> > > against the required 1A. This is confirmed by the digital current
> > > meter on the 12V psu powering it and my K2, which increases from
> > > 0.5A to 1.3A when I enable transmit on the XV50.
> > >
> > > Before anybody asks, I DO have link W1 fitted, and in fact I have
> > > the correct bias voltage on pin 2 of the output module. However
> > > there is where I suspect the problem as the maximum voltage
> > > achievable there is 4.7V.
> > >
> > > I found a datasheet of the RA30H0608M, albeit a preliminary one
> > > which has a few sheets 'in preparation'. It does not give any
> > > figures for standing bias versus the bias volts but suggests it must
> > > be somewhere between 4.5 and 5V (there is no power output below
> > > 4.5V). The voltage at the R29/R39 junction is limited by the current
> > > flowing through R28 and the module bias current (typically 1mA). So
> > > I am wondering whether some rf modules need a bit more than 4.7V to
> > > get the correct bias current of around 1A. Maybe reducing the value
> > > of R39 (56 ohms) is the answer. Or is mine near enough for good
> > > linearity?