Comments below.....
On 6/2/2021 9:32 AM, W8JH wrote:
I am looking to add the DEMI (Q5) transverter to my K3s and have no
experience with transverters other than the internal one in my KX3. In the
future, I may wish to progress to 2m EME as well.
Here are some choices I need to make and I would appreciate any advice:
Common IF vs Split IF. I'm thinking split IF with a K3s?
I would advise using split signal paths. All my transverters are rigged
up for a 0 dBm 28 MHz signal level on TX. I always plan a system that
has power levels such that if you leave the exciter rf drive level at
max by mistake, it still will not impact anything or blow anything out.
Common RF or Split RF. Common RF for one antenna connection?
IF drive level of -20 to 0dBm vs 0dBm to +20dBm. I think the K3s
transverter output is 0dBm so I'm not sure what to think here.
PTT polarity low instead of high I'm thinking.
PTT set for ground on transmit standard for all bands.
High stability Local Oscillator vs. standard.
This is an individual choice as all crystals are not created equally. I
have a K3 with the garden variety ref oscillator. It is more than stable
enough for digital modes. The problem is typically the LO in the
transverter itself. Crystals drift as temperatures change. I have a 10
GHz transverter with a crystal oscillator and the drift from cold start
to warmup is about 2 kHz at 10 GHz. After it is warmed up, I see only a
few hundred cycles of ambiguity from day to day. It was stable enough
to make me not change over to a synthesized LO since sideband noise on
the synthesizers was not so hot. Others have had more trouble with
crystals drfiting all over the place. Not all crystals are created
equally. Cleaning the crystal blanks affects drift drastically.
Production methods vary.
Recently, I made some digital contacts on 222 MHz and found that my old
crystal oscillator in my DEMI transverter drifted a bit. I saw a 10-12
Hz drift after each JT65 transmit sequence. The problem is the power
amplifier inside the trnsverter that heats up the crystal over time. My
solution was to put a small 12 volt fan on the heatsink just above the
hybrid amplifier location. It cured the drift. After employing the fan,
people assumed that I was running a ref locked local oscillator.
Measurements indicate a drift of just a few Hz over time. At higher
frequencies, (over 432 MHz) some sort of reference oscillator
arrangement is needed for digital modes. Cheap 10 MHz sources are
available on EBay. Just make sure that whatever ref locked oscillator
you use, that it does not produce bad LO noise sidebands. I have a
proportional oven 10 MHz crystal oscillator made by Wenzel Associates.
It was cheap on EBay, and more than stable enough for any digital modes.
You do not need a rubidium standard at all.
I could get a custom IF and I think there is some slight advantage to a
14mHz custom but I'm guessing the 28mHz standard IF is good enough?
I use 28 MHz on 50 thru 432. On 902 and above, I standardized on a 145
MHz IF for everything up to 10 GHz. 24 GHz has double conversion.
73
Dave K1WHS
Thanks in advance for any advice or references.
-----
73,
Joe, W8JH
K3s, KPA 500, KAT 500 and KX3 happy user.
--
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