Re: HERMES Lite 2

Andy G4JNT

In fact it grows faster than exponentially.   The rate of growth of number of sproggies is near-enough a factorial function of the number of frequencies (conversions) present.  Factorial being the fastest mathematical function there is - I think.

 The Inmarsat-3 satellite transponder - which went through its design phase in the late 1980s had multiple (reprogrammable) local oscillators to steer uplinks blocks of frequency to L-band blocks to allocate to spot beams, going via an IF in the VHF region to use SAW filters.   The sheer number of possible interactions that had to be thought about was horrific.  And computer processing power back then was orders of magnitude lower than it is now; some calculations of high order IMPs  took overnight on a PC of that era.  

On Fri, 29 Jan 2021 at 15:55, Chris Bartram G4DGU <chris@...> wrote:
I have a Hermes Lite 1 (essentially much the same as the HL2, but minus
much of the analogue I/O circuitry).  I built mine a few years ago with
the idea of using it as a microwave IF. Although with HB input filters
and QUISK it gave a pretty good account of itself on HF, the conversion
from the 28MHz region to the 144MHz IF of most of my transverters was
the deciding factor and I didn't take the project further.

Why? If there's one thing that the design of receivers for type-approved
use has taught me is to minimise the number of frequency conversions in
any system. Apart from the effect on dynamic performance, Frequency
conversion nearly always shows-up gotchas, and multiple conversion
shows-up multiple gotchas. These will often multiply exponentially and
oft-times can't be be easily designed-out. Also the acquisition of a
FDM-DUO transceiver at a very reasonable price finally made the decision
for me ...

I'm not suggesting that the HL1 or 2 are in any way junk. They're not! I
think the concept using the CATV modem chip and the FPGA design are
really neat. In the last few weeks I have been looking at my HL1
wondering what I could use it for. I have ideas, but I'm determined to
finish my current projects first!


Chris G4DGU

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