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Has anyone experimented with increasing the final PA bias to improve Flex1500 3rd order IMD?

Bruce McIntyre
Bruce McIntyre Member
edited March 2017 in FLEX Series (Legacy) Radios
Is it possible to increase the final PA bias to improve 3rd order IMD? For use with amplifiers. A 5w final could easily run classA with the proper heatsink. Mine is on warranty so I will not mess with it - looking for info only.

Answers

  • Jeff Gray
    Jeff Gray Member
    edited December 2015
    https://www.hobbypcb.com/products/hardrock-50-hf-power-amp Makes an amp kit. 3-5 watts will get you 50 watts out. They also have an optional internal ATU kit for the amp as well. 

  • Bruce McIntyre
    Bruce McIntyre Member
    edited April 2015
    Hi Jeff, thanks for the info. I have a big FET solid state 20db gain amp - it will do 500watts driven by the Flex1500 / what I'm concerned about is the 3rd order IMD spec of the Flex 1500 .
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2017
    It isn't that simple.  Changing the bias to tweak the IMD would be only applicable for that one frequency and it would not be retained once you power cycled the radio, as the user does not have any way to store the data in the EEPROM.  In general, running the 1500 at approximate 3W will and not over-driving the mic input will result isn the best IMD on all bands.
  • Bruce McIntyre
    Bruce McIntyre Member
    edited March 2017
    Ok but the Flex1500 does not have software adjustable bias - it's adjusted in hardware by a **** for the driver and another **** for the final. That's what I'm getting at - the issues are heat (probably not a problem with QRP), but with higher bias will the amp be stable, and will the existing bias regulators be able to supply the extra bias current?
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    The bias is adjusted via software control.  This control is not available to the user.
  • Bruce McIntyre
    Bruce McIntyre Member
    edited April 2015
    Refer to pages 11 and 12 of the Flex 1500 service manual. It describes manual PA bias setting by pots. Advise?
  • Eric Daniels
    Eric Daniels Member
    edited January 2016
    Some things you just can't fix in software.

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