> This excepts the BFO crystals, which I have found to be a source of
> frequency drift during transmit (within Ten-Tec specs, but still excessive
> for the various HF digital modes), hence the substitution of
> better-tolerance crystals.
John...an excellent assessment of what is occurring in the Omni Six/Six
Plus. Ss you point out, substitution with better 9 MHz BFO crystals goes a
long way to 1) improve overall frequency stability, and 2) in conjunction
with a couple other mods, greatly reduce or completely eliminate the effect
of the so-called "micro-chirp."
Between changing the two BFO crystals and modifying the master oscillator to
the newer (ovenless) TCXO, my Omni's stability from start-up is less than 30
Hz on all bands. Others have attained even greater stability by changing
the BFO trimmer caps to ceramic NP0 types.
As you point out, the effect of a highly stabilized master oscillator (e.g.,
HP Z3801A) will be masked by the deficiencies in the Omni's BFO circuit.
Nevertheless, 30 Hz of absolute frequency stability given the stated mods is
very good and well worth the compromise of using Ten-Tec crystal mixing
scheme.
In the mean time, I am enjoying 10 (-9) absolute frequency stability on my
RX-340 receiver with the Z3801A. I use the RX-340 to stream the 14.178 MHz
audio users' group on most mornings until approximately 10AM. Those of you
with a high-speed broadband connection (I stream at 64 Kbps) can listen to
the RX-340 in a flat 6 kHz audio passband by using Microsoft Media Player.
To activate the stream, copy and paste this URL when you select "Open URL"
from the Media Player's "file" menu:
http://24.129.98.118:8080
Those of you with a FFT spectral analysis program (e.g., SpectraPlus) can
even measure the bandwidth of the receiver and the bandwidth of received
signals with a high degree of accuracy. All of this can be measured from
your PC. During quiet band conditions, the FFT display will show an
absolutely flat passband from 20Hz to 6.4 kHz (the upper DSP limit of the
selected RX-340 filter). I would like to hear back from those of you who
try this. I have had it running for the past year or so, but I would like
to see how well the encoder handles simultaneous unicast IP streams.
Thanks!
-Paul, W9AC
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