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Flex 6500 and DEMI transverter 70 cm

Jim    W3IPO
Jim W3IPO Member
I will be using a  L432-28 transverter with the Flex-6500
The Flex has the GPSDO option installed .

The transverter I have ordered has 3 methods for using a 10MHz reference:

1. Built in crystal oscillator.

2.  Built in VHF synthesizer locked to a 10MHz standard.

3. And the option to use an external reference which in my case is to use the GPS reference from the Flex-6500.

It seems that I may NOT need the transverter with the synthesizer option, since I already the reference available from the Flex 6500.

Am I right that having BOTH the built in synthesizer in the transverter and using the reference from the Flex is duplication and it is  not necessary to get the transverter synthesizer option?

Caution: I am a total Newbie and am prone to erroneous thoughts and dangerous actions!

Jim Lauridson

Completed · Last Updated


  • Al K0VM
    Al K0VM Retired Member ✭✭✭
    edited November 2017
    I would normally think that the synthesizer is required even if you use an external reference..

    AL, K0VM
  • Stu Phillips - K6TU
    Stu Phillips - K6TU Member ✭✭
    edited May 2020

    I took a quick look at the DEM site...

    The default XTAL oscillator drives the LO that is required to mix the 28MHz signals up/down from 432 MHz.  The crystal will be pretty stable but will drift slightly with temperature - looks like DEM uses a 101.00 MHz crystal which will be multiplied up 4x to generate the LO for 432.

    Ao any drift in the crystal will appear 4x on 432 - if you have reasonable temperature control in the environment you have the transverter in, it will probably be fine for typical use.

    If you want really good stability and frequency accuracy, then you should add the synthesized LO in the same unit together with the switch - that would enable you to lock the synthesizer to the 10 MHz output of your 6500 assuming you have the GPSDO option fitted.

    In all cases, you do need either the crystal OR the synthesizer to generate the LO UNLESS you already have an external LO which generates the 404 MHz needed by the transverter which from your description you dont! :-)

    Most transverter vendors have an option for a version which uses an external LO because a lot of serious UHF/microwave operators already have their own LO generator - typically locked to some 10 MHz reference source.

    Unless you are going to get into EME, I would think the crystal oscillator version would be fine at 432...  if you get into 10 GHz down the road (as I did), the LO is multiplied many many more times (like 92x) and the crystal drift even with an integral heater isn't good enough for either accuracy or stability WITHOUT an external 10 Mhz reference or a stabilized LO.

    Hope this helps
    Stu K6TU
  • Jim    W3IPO
    Jim W3IPO Member
    edited October 2014
    thanks, Al and Stu.

    Am I right in understanding that a GPS bases reference such as the GPSDO option with the Flex 6500 is now considered the best reference source?

  • Stu Phillips - K6TU
    Stu Phillips - K6TU Member ✭✭
    edited June 2020

    For fixed operation, a GPSDO is a very cost effective reference source.  If you move around like a microwave rover station does, then the time it takes the GPS to lock up and discipline the 10 MHz oscillator can be an issue especially if the GPS needs to acquire the almanac due to a loss of power.

    So for my microwave roving, I have a 10 MHz ovened oscillator that I always keep powered on - its very stable in the short term - a few days - but has to be re-calibrated from time to time as aging sets in.

    Stu K6TU
  • Patrick
    Patrick WH6HI KauaiMember ✭✭✭
    edited June 2016
    There would be no problem using GPS in a mobile situation.  GPS will have no problem locking it's channels and giving a good stable time, frequency, speed and Altitude.  
  • Stu Phillips - K6TU
    Stu Phillips - K6TU Member ✭✭
    edited August 2016

    A GPSDO expects to be in a single position in order to be able to provide the highest level of accuracy.

    Stu K6TU
  • Jim    W3IPO
    Jim W3IPO Member
    edited October 2014
    Thanks everyone.
    Good info all the way around.
    I am going to opt for the synthesizer option.  
    I already have the GPSDO option so it will be used also.

    I have dreams of EME, but that is in the future.
    Right now I am having a lot of fun with the 6500.
    What a great radio!!!
  • Jim    W3IPO
    Jim W3IPO Member
    edited April 2018
    Here is a reply I just received from DEMI tech support.  It clarifies my misunderstanding of the LO options for the L432-28 transverter:

    The L432-28 transverter is available with two types of Local oscillator designs. Our standard and a synthesized option version are available.  The standard oscillator is a Xtal based unit that is self sustaining in that it does not require any frequency reference. It is stable and accurate for normal SSB and CW QSO's and is the simplest to use. It may not meet your requirements if digital operation is planned. The standard LO will drift because of temperature change as much as 400 Hz. during operation.
    This is why we offer a synthesized version called the A32- option.  This synthesizer option is used in place of the standard local Oscillator and can be ordered with an internal 10 MHz clock but for the best stability and accuracy, it requires an external  10 MHz input such as from your Flex 6500.

    The best possible performance than can be obtained would be the L432-28 with the A-32 option connected to your GPS 10Mhz standard.

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