[Lowfer] R75 Frequency Accuracy

[Douglas D. Williams]

You guys are funny, and also correct in that the spurs are weak and
mostly covered up by band noise when you connect an actual antenna.

There is also the matter of the phase noise improvement when you use a
quality VLF/LF upconverter as opposed to directly receiving with the
R-75 (or most other HF receiver).

I quote from John Reed's Lowdown article, comparing the R-75's native
LF characteristics VS using it with the AMRAD upconverter:

"This is a huge improvement in both the sensitivity and especially the
spurious response content of the receiving system. The spurs now only
number three and these are all generated by the upconverter. The
sensitivity is excellent, right down to 10 KHz. Below 10 KHz
upconverter oscillator noise increases rapidly. Synthesizer phase
noise is now completely gone."
And:

"The conversion loss in the upconverter is less than a dB. For this
reason, as well as the small loss in the low pass filter, the
sensitivity of the whole system with filter and upconverter is very
close to that of the receiver itself in the 3 to 3.5 MHz band.
Sensitivity now averages -139 dB and LO phase noise is non-existent."

Now whether or not this makes any practical difference in *your*
(meaning anyone reading this) individual receive situation.....I
cannot say.

Certainly there is the limitation that the R-75 cannot tune below 30
kHz, but what's down there really, besides very strong military MSK
signals? Well, in Europe, Stefan, DK7FC, and others, have been having
success with the "dreamer's band" at ~8.9 kHz. Perhaps someday we will
have a TA reception there, or a USA station will decide to try some
serious experimenting in that band.

Back to receivers, I have only owned one receiver that I can say,
without reservation, that has no need for an upconverter for VLF/LF
reception, and that is the Winradio Excalibur Pro G33DDC software
defined receiver. It is a bit expensive for "just a receiver", but I
am continually amazed at it's VLF/LF performance. The only
shortcoming, IMO, is lack of an input for a GPS (or other extremely
high stability) frequeny reference. Out of the box, it has a
manufacturer claimed frequency stability of 0.5 ppm, and I tend to
believe that, but an input for a high stability reference would be
nice.

73, Doug KB4OER













On Wed, Jan 23, 2013 at 10:11 PM, Dexter McIntyre W4DEX
<dexter.mc at gmail.com> wrote:
>
> Douglas D. Williams wrote:
>>
>> The main shortcomings of the R-75 for direct VLF/LF receive occur
>> mostly below 200 kHz, where there are numerous spurs that can be heard with
>> nothing but a shielded 50 ohm "dummy load" connected to the antenna input
>> jack, and as noted in two separate Lowdown reviews of two different R-75
>> receivers.
>
> I'll have to check mine but for the past several years I've never encountered a spur at any frequency of interest.  For many years I used several different high dollar selective level receivers for LF reception.  I've found the R-75 to be as good as any SLM while receiving at 50, 60, 137, 185khz and up.
>
> Dex
>
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