[hpsdr] Pennywhistle bias setting

[Graham / KE9H]

Hi Gerry:

The RF amplifier transistors in PennyWhistle are LDMOS field effect 
transistors.
Generally the most linear transistor type available for linear amplifier 
service.
Also the most rugged RF power transistors I have ever worked with.
So feel free to experiment with the bias.  You won't hurt anything, as
long as you keep it between zero and several amps per transistor. (Spec
sheet says 5 Amps max per transistor, on a room temperature heat sink.)

The manufacturer's recommended quiescent current, per transistor, is 500 mA
Some portion of that will convert to RF power when you are processing a
signal.  Saturated CW efficiency of the amplifier will be around 50 percent.

I recommend 450 mA, since the  current will rise slightly with a hot 
heatsink.
So, 450 mA is the "set and forget" value.

As David McQuate said, the bias issue is with regards to close-in 
intermodulation.
Which is an issue for SSB and linear services.  If you have the test 
equipment,
sometimes you can find a "sweetspot" as low as 375 mA quiescent current 
where there
is a dip in intermodulation products.  But I would expect the optimum 
linearity point
to be somewhere between 375 and 550 mA.

For CW service, you can drop the quiescent current back to 100 mA. (or 
lower).
Just don't run SSB in that bias condition.  Won't hurt anything, but 
your SSB signal will
be distorted. If you do drop the current low for CW, your maximum power will
drop, and drain efficiency will drop, also.  Your total "battery life" 
might go up,
because you are not dissipating all that quiescent power between the 
dits and
dashes.

Even harmonics are affected by the balance between the transistors, so keep
the currents about the same for both transistors to minimize second 
harmonics,
where ever you do set it.

--- Graham / KE9H

==

David McQuate wrote:
> Filtering can attenuate harmonics, but cannot suppress the results of 
> intermodulation distortion produced by the amplifier.  Both KISS 
> Konsole and PowerSDR can produce a two-tone test signal.  The bias can 
> be adjusted by observing the amplifier output on a spectrum analyzer 
> for minimum IMD.  KISS Konsole can run in full duplex mode so that it 
> can simultaneously transmit and receive, allowing IMD adjustment with 
> no additional test equipment.   (Be sure to use plenty of attenuation 
> between Pennywhistle output and Mercury input!)
> 73,
> Dave
> wa8ywq
==
>
> Gerald J. Herman wrote:
>> Finished Pennywhistle assembly and going through Adjustments. The 
>> quiescent current is to be set at 450 mA per transistor which seems 
>> rather high to me. This amounts to approximately 12 watts of power in 
>> the quiescent state. I suspect the effect of lower currents and their 
>> effects on harmonics was investigated by someone who has the 
>> appropriate equipment. Since filtering is required after the 
>> amplifier anyway to make it legal, would it be possible to run lower 
>> currents and reduce the quiescent heating and power draw, and still 
>> be legal?
>>  
>> Wonderful kit and a joy to assemble. Thanks to everyone involved.
>>  
>> 73,
>>  
>> Gerry
>> KR8W
>>  

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