[hpsdr] Pennywhistle bias setting

[Gerald J. Herman]

Dave and Graham,

Thank you very much for helping me understand the issues and possibilities 
regarding the bias settings on Pennywhistle. Thanks again.

73,

KR8W
Gerry


--------------------------------------------------
From: "Graham / KE9H" <KE9H at austin.rr.com>
Sent: Saturday, December 26, 2009 6:06 PM
To: "David McQuate" <mcquate at sonic.net>
Cc: <hpsdr at openhpsdr.org>; <gherman at wwnet.net>
Subject: Re: [hpsdr] Pennywhistle bias setting

> 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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