Welcome to m0rjd.co.uk
Bob, M0RJD, is a British Amateur Radio Full Licensee, mainly interested
in constructing and designing Amateur Radio equipment.
This picture of me "calling CQ" was taken some years ago by a work
colleague. See if you can guess where the cable goes...
I wasn't licensed then, so there is no RF, which may be why I didn't
hear any replies?
This website is a Work In Progress, please return from time to time.
Currently in progress is a PICASTAR (STAR) H.F. transceiver, which uses
a DSP system for everything beyond the 2nd IF amplifier (Receive) or up
to the same point (Transmit). The RF front-end is modern yet
consisting of Band Filters, DDS LO, H-Mode mixer, crystal roofing
filter and switching second mixer. The 2nd IF is at 15kHz for
by the DSP unit, which produces audio output for SSB and CW.
I am now commissioning my STAR.
During this commissioning process I found that the LO signal to the
H-mode mixer was much
too low and set about increasing it to the recommended +7dBm. Whilst
doing this it became obvious that many other constructors had suffered
the same problem, using a variety of methods to achieve their goal.
Although I managed to reach the magic target by 'changing things', this
did not satisfy a desire to understand why it was a problem and to
devise a universal solution in place of the ad-hoc methods previously
used. See more of this
I have concluded that, although the DSP methods are excellent, some of
the analogue/RF circuitry has errors. This shows how much
attention was put on getting the DSP working through many code
iterations, sometimes at the expense of the remaining circuit, which
been put together from chip-manufacturers' application notes or
copied from others. This has sometimes resulted in poor interfacing
elements, and in some cases the perpetuation of originally flawed
This is a great shame, because it means that the excellent DSP is not
being presented with the best available signal.
No-one would expect that there would be gratuitous impedance mismatch,
incorrect operating points, matching circuits that don't match, and
just plain design errors, but all these exist in the design.
Over the years some improvements were introduced to PICASTAR, such as
mixer and switching 2nd mixer to replace expensive diode mixer modules;
better & more modern DDS, and so on.
Some introduce new problems, such as lack of LO amplitude, and one has
actually introduced a mismatch.
For many of us, the attraction of PICASTAR was the use of inbuilt DSP
for the 2nd IF and subsequent processing, the DDS-based 1st LO, and the
stable state of the design. Whilst the design is stable, in that it has
no official modifications for several years, it is not free from
problems. It might work well anyway; you might not perceive any
trouble whatsoever in operation. Many units have been constructed to
the published design and are in regular use.
By far the most common problem encountered by recent constructors has
been the difficulty of obtaining the requisite LO amplitude. This was
my own experience.
Whilst in the process of commissioning my own ComboSTAR, as a newcomer
the scene, I have uncovered several areas that are in need of change,
either to remove design errors or to avoid pitfalls. Other people have
problems in some areas also.
If you are put off by this, then please read on with an open mind.
PICASTAR has first class capability, with superb results from the DSP.
The main design problems can
be rectified quite easily - I have a
personal vested interest in ensuring that, wherever possible, changes
require daughter boards or PCB modifications other than component
For now I have dedicated these web pages to identifying and correcting
the problems on PICASTAR, solely with a view to getting the best
performance possible by avoiding marginality. The correction of
design problems can only result in a better STAR.; There
is neither need nor excuse for perpetuating design errors of the past.
Look at any small MDS improvement as a bonus!
The changes I personally propose here are all reversible by reinstating
original components, should you wish. The notes are all aimed
users of the ComboBoard, a large PCB from Glenn,
VK3PE which gathers the original sub-module
collection together on a single PCB, but are equally applicable to the
original modular build method.
Read on and see what you think.
If you know of an area of the design or a part of this site that is in
need of tidying or correction, please let me
know and I will consider either adding your
changes to this website, properly crediting you, or add a link to your
own information. Naturally it is best if not only do you identify
a problem or quirk, but also propose a cure!
The links below address problems and
Squarer part of First Mixer (recently added
and rather rambling)
Low Amplitude from DDS / First LO,
Poor design of 2nd IF,
Mismatches in 1st IF
bidirectional amplifier and surrounding circuit,
Bias point of Bus Switches used as analogue
Ferrite Transformers page to be added maybe.
In the meantime, please don't be put off by these things. Many
constructors have built PICASTARs and are getting very good results
from them. But that isn't a good reason for leaving wrongs wrong
when they can be made right.
I have put these items on this web-site to make them accessible to
anyone who may be interested or just curious.
Please be aware that any alterations that you make to the published
Picastar design will make it
non-standard and could hinder
fault-finding. I suggest you don't make any changes to the
published design without first getting the unit functioning; change
only small sections at a time and verify that the changes don't upset
things for you.
None of these items will ever be
incorporated in the standard design, which has long been frozen.
On the Picaprojects Yahoo! group
"Having given due notice that my time
is going to be devoted entirely
to PICaYAGI, please do not expect any ability to devote my time to any
of your issues with the closed/sealed/finished projects.
They all work without reservation as
This is not from any desire to not
support published projects - but
several years on each has to come to an end as they must if we are ever
to move on. So if you have 'found' these projects several years after
they were finished - or have taken several excess years to finish -
please be aware that I am now not prepared to swap my present
commitments to the present work for the old stuff. This last para is
written because I am being continuously and unreasonably pestered.
The stuff on this website is a way of interchanging 'new' information on the
design whilst avoiding Peter G3XJP being 'pestered continuously and
unreasonably'. As he says, he is completely and unreservedly content