Here are some further comparisons (bearing
in mind I don’t own a R75!):
The RA1792 has flat sensitivity from about
100kHz – 30MHz (about 0.3uV for 12dB SNR in SSB bandwidth), and rolls off
about 20dB down to 10kHz, so works quite well for VLF. The R75 only tunes down
to 30kHz, but does also cover 30MHz – 60 MHz; I don’t know what the
LF sensitivity is like, although I think it is reduced below 1MHz. The R75 has
various preamp and attenuator settings, while the RA1792 has fixed front-end
gain (although you can link out the RF amplifier). The RA1792 has no preselector,
although it seems to cope well on HF.
The RA1792 has, as standard, 300Hz, 1kHz,
3kHz, 6kHz and “bypass” 16kHz bandwidths (for some reason, mine has
a 150Hz filter instead of the bypass setting), plus dedicated USB and LSB
filters. The bandwidths are specified as -3dB rather than -6dB as is normally
the case for amateur rigs, so they would be slightly wider in amateur terms.
The BFO can be set to +/- 8kHz in 10Hz steps. So, if the RA1792 audio output is
to be processed by DSP, it is quite flexible - for example, you could use a 6kHz
BW and have audio output from 2 – 8kHz. The R75 has 2.1kHz, 6kHz and
12kHz as standard – I think CW filters are an extra. You can only set the
BFO offset between 300 and 900Hz.
The skirt selectivity of the R75 should be
good, since it has cascaded 9MHz and 455kHz filters. Signals a few kHz off tune
are only suppressed by about 60-70dB on my RA1792.
The R75 has an RS232 port and there is
remote control software available. The RA1792 has a non-standard interface that
can be wired for RS232 or RS422 – apparently, there is some software
available for download, though I have not tried using it.
The RA1792 has 455kHz IF output, reference
frequency input/output, audio line out, muting, agc and various other things on
the back panel, so it is easier to get at the internal signal paths if you plan
to do hardware experimentation.
The R75 is quite compact and runs off 12V
DC. The RA1792 is in a 19” rack case and requires AC mains, so much less
The 0.5ppm TCXO is an extra on the R75,
the RA1792 has a 0.1ppm OCXO as standard.
The only real complaints I have about the
RA1792 are the ergonomics of the controls and display – Even after using
it for years, I still find I often press the wrong button and suddenly find it
on a completely different frequency and operating mode!
Cheers, Jim Moritz
73 de M0BMU