Fix: Uneven TX output across bands

ubitx.net suggests you consider addressing the power fall off issue as frequency increases 

Why do something about the power fall off issue?

One of the issues that many constructors raise about the µBITx is the uneven output across the bands (from 3.5 MHz to 30 MHz).   The output decreases from over 10w on 80m to only 1-2 watts on 10m.   This fall off in power has been found to be caused by:

  • necessary compromises in the design of the push-pull PA stage
  • pre-driver and driver power fall-off, probably due to the relatively low fT of the transistors.

You are likely to want to do something about the power fall off because the rig may put out too much power on lower frequencies (jeopardising the finals) or not enough on higher frequencies (making it more difficult to make contacts).  In these days at the bottom of the sunspot cycle every watt counts.

A number of solutions have been put forward including:

Constructors are welcome to explore the full range of options described here, but ubitx.net thinks that the most sensible approach is the most obvious approach: control the drive level by band.   This involves any of the following options:

  • installing a potentiometer on front or back panel to control drive level manually (i.e. in place of RV1)
  • using a digital attenuator (e.g. PE4312 or HMC470) controlled by the Raduino (some code would need to be developed and I/O lines found)
  • use  the relay and potentiometer solution as implemented by Bill K9HZ in place of RV1.

Relay and potentiometer solution

Three relays are driven off the LPF trigger transistors.  The relays select one of four potentiometers, depending on the band grouping activated in software to select the relevant LPF.

While it may not be ideal to add in three additional relays and four potentiometers, this 7 component solution is probably the most straight forward automated solution available.  It is pretty much guaranteed to work.

Remember that more than one relay may be activated at a time (depending on the band grouping).  Band groupings are 80/60, 40/30, 20/17, 15/ 12/10).  However, only a single potentiometer is ever in circuit.  This allows output of your TXCVR to be  set within a watt or two across all bands to provide around 10w PEP output as per the intended µBITx specification.

You can access the outputs of the relay drivers from the transistor side of each of D11, D12 and D13.   Remove RD1, and replace with four potentiometers switched via the 3 relays (one potentiometer is activated when the relays are all off, the other 3 potentiometers are switched in with the relays).

FURTHER DETAILS, CIRCUIT DIAGRAM, AND PHOTOS TO COME.

IS ANYBODY INTERESTED IN PUTTING TOGETHER A CIRCUIT BOARD TO ASSIST CONSTRUCTORS IN EFFECTING THIS MOD? 


This is a ubitx.net recommended fix.  This solution has yet to be independently verified, but ubitx.net sees no reason why it SHOULD NOT work EFFECTIVELY.