Satgen 415  Rx Frequency Converter  by GM4IHJ  (BID SGEN415)  8 Mar 97

Over the past 20 years, Amsats have gradually moved up to ever higher
frequencies. Each time this has happened, it has been common practise
to add a Frequency converter (Eg 435/28), and indeed this has usually
been  absolutely necessary, because no suitable receivers were
available for the new band. Examples of this occured with the move to
Oscar 8J in the late 70s and early 80s when no 435 MHz receivers were
available, and , more recently with Pacsat Oscar 16, Dove Oscar 17,
and Oscar 13 mode S when no 2401 MHz receivers were available at a
sensible price.

There is absolutely nothing wrong with using a converter, but users
should be aware that their inherent simplicity can lead to problems
which may not at first sight be obvious. Eg a recent design of an S
band receiver converter features conversion from 2400 MHz to 435 MHz.
Which sounds fine until you realise the S band uplink is on 435 MHz,
and you will need to take great care that.you do not desensitize your
receiver as you transmit. The S band converter at IHJ is 2400 to 144
MHz, which poses no break through problems. Less obvious to newcomers
to converters, is their use of Xtal local oscillators which need to
multiply the Xtal fundamental say 80 or 100 times in order to get an S
band mixer input of 2256 MHz. The first time you use a converter you
need to check just where it actually tunes . A 30Hz error in the
fundamental can give you a 2.4 kHz error in your reception frequency.
So please be aware , that as a first step with any converter you need
to know just how much tuning error it produces. Once you know this it
should stay the same provided you carefully employ a warm up procedure
before you go on the air , to let your converter heat up to normal
operating temperature . A procedure which can avoid an error of
several kHz, on start up. At IHJ the S band converter has an offset
when warmed up of 5 kHz and the J band converter of older design has
an offset of 6 kHz. Labels on the  converters warn the operator ( who
would otherwise forget it ). 

The above remarks apply to commercially built converters. This is not
true of Home Brew equipment. What looks good on a QST page, can turn
into a high offset, drifting, nightmare , if it is not carefully
constructed. Particular attention must be given to power supply
stability, and the converter must be protected from air currents and
vibration. Some expensive commercial units use thermostatic control of
temperature, but this is not normally a feature of amateur commercial
units, but could be included in improved Home Brew equipment.
Then provided these simple rules are followed , you should have a very
useful piece of equipment for following the progress of Amsats up to 
these new , qrm free, higher bands. Those who wait for suitable
commercial S band receivers may have to wait a very long time. It was
3 years before J band receivers became available , and the first ones
were insensitive electronic doorstops, such that IHJ has stuck to
converter reception, even now 17 years later. Indeed the test on Mars
Survey produced nothing on the 70cm receivers , whereas the JAMSAT
435/28 converter showed a faint dopplering trace on the FFTDSP4.2m
display.