Ok Paul
The Navy has extra IF , the slow motion tune is another down
conversion to , I think 50KHz IF ?
I was not involved with the remote versions , but , yes the boards are
similar , the ra1778 has a memory board , which makes servicing a
problem , but the 1.4 MHz free tune bfo is a little under hand
for such a price ! the ra6790 bfo is locked in 10 Hz steps , main tune
1 Hz , so its possible to tune to 1 Hz , its reasonably stable , or can
input a external reference , I use ma1723 exciter for mf .
Tnx-73 Graham
ps , there is a working Atalanta at gb4fpr . I have the Electra in the
loft octal version hihi
--------------------------------------------------
From: "REEVES Paul" <[email protected]>
Sent: Friday, August 08, 2014 12:47 PM
To: <[email protected]>
Subject: RE: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
Hi Graham,
I don't think the '117 had a beam deflection mixer either but I'm not
sure. I've got one in the garage awaiting (for several years now....)
renovation - will have to have a look. I didn't think the Navy version had
any significant differences apart from possible mods (a 7360 fits the bill
there) or external preselector for co-site tx operation but I think the
only one I have seen was at the museum at HMS Collingwood - and no
details, alas.
All the rest of my Racal bits are 1772 and relatives, generally remote
control versions (6775? etc) - all work ok and show no signs of
deterioration (yet...). At least they provide spare boards for the 1772 if
needed!
I have used the AFEDRI on 136 - worked quite well but was behind a
preselector adapted from a set of front-end coils for a Marconi
'Atalanta'. Proper coils and capacitors -keeps all the rubbish out!
73s
Paul G8GJA
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Graham
Sent: 08 August 2014 12:13
To: [email protected]
Subject: Re: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
Hi Paul,
Yes , the later version , may be the 117 or the fine tune version
for
the Navy used the 7360 , probably about as good as it gets , I
spent some time on production test when the ra1771/2 was first
produced , that was long ago , I have a ra1778 and ra6790gm , with
the
lf front end , the start this year , I sold all the ra17 set up ,
ra17, pan adaptor , lf adaptor [with rad-haz mixers] ssb adaptor , all of
which , I had not used for decades, all had failed in one way or the
other , working or not , seems little difference in the price , must
of used 3 cubic yards of cardboard packing ! the sets are now all
round the world .
Noted on the SDR , yes , I assume thats needed due to lack of 'bits'
,
there was an idea to use a 24 bit a/d as a 0>1 MHz sdr , aimed at
the low frequency bands , but as yet , waiting ..
one of the £5 dongles would work as pan adaptor , showing the 2/3
MHz IF out , only problem, if the ra17 has the original IF amp ,
then
the LO carrier also shows on the trace , 100KHz from the tune point
,
I fitter the modification , but the rx-noise increased , due to the
second valve ,
Q Have you used the AFEDRIxx on 136 . 477 ?
73-Graham
G0NBD
--------------------------------------------------
From: "REEVES Paul" <[email protected]>
Sent: Friday, August 08, 2014 7:49 AM
To: <[email protected]>
Subject: RE: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
Hi Graham,
RA17 with beam deflection mixer? Surely not.......
I would certainly agree with you about the RA1772 (and extended family)
but I like using both - and a (fairly) complete set of RA17/MA79 plus
accessories really beats anything else in terms of looks! And it glows in
the dark too:)
I use an AFEDRIxx too. Works fine but really needs a preselector on HF
unless used as a panadaptor (behind an RA17 perhaps....), fortunately
Racal made nice preselector units too.
73s
Paul G8GJA
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Graham
Sent: 07 August 2014 21:36
To: [email protected]
Subject: Re: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
Yes Tobias,
All is not as it seems , but as you say , adding filtering , reduces
the
problems , and with direct conversion , the noise floor is the
lowest possible , stability is at maximum , having only 1 oscillator ,
In
have good decode results on 477 using the £5 dongle , behind the
TX
atu and inv L ae
For the £150 sdr , the description lists 80 msps , which , I assume
moves the image problems out side the HF spectrum ? at 12 bits ,
that
starts to provide a reasonable , post processed dynamic range ?
12- bit 80 MSPS A/D conversion
I would not link the barlow-wadley , too closely to lack dynamic
range , may be a lack of engineering integrity , the RA17 with
pentode
rf stage , then later cascode , with beam deflection mixer , was
reasonable , though there was a pre selector for use at close
tx/rx
sites , the ra1771/1772 was the first to better the ra17 , but
again
that also tends to question the models before .. the ra1772/1 is
fitted with rf-pre selector , for those 'unexpected' situations , the
ra6790gm , with no pre-amp is the closest I have seen to a
bullet
proof front end , that runs , where the ra1778 needs the
pre-selector
But in terms of noise etc , one of these 'reasonable sdr's' may be
better .. I don't think I would recommend any one go down the racal
path these days !
73-G,
--------------------------------------------------
From: "Tobias DG3LV" <[email protected]>
Sent: Thursday, August 07, 2014 4:16 PM
To: <[email protected]>
Subject: Re: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
Hi Graham !
This type of device has a dynamic range of 48dB at maximum (8 Bit), so
they need help from a narrow-band frontend plus input-attenuators to
reduce the load to the A/D-converter. The conversion speed is variable
from 1 to 3 Megasamples/second, leading to a Nyqist-frequency of 0.5 to
1.5 MHz.
This filter should have a deep attenuation above the Nyqist-frequency to
avoid unwanted reception at harmonics/aliases of the sampling frequency
(=
"undersampling").
The advertized usage of 100kHz to 30MHz (in direct-sampling method) is
based on this normally unwanted harmonics/aliases, i.e. the receiver
uses
a method that has become "famous" with the ancient "Barlow-Wadley"
all-band receivers. (at least equivalent to). Their
(large-signal-)problems of the past are reborn at the direct-sampling
method of these DVB-T sticks. Without narrow-band selective frontends
this
is just a "proof of concept" and not a usable receiver.
For the use at 136 kHz and 475 kHz a steep lowpass-filter (7 to 9 pole
Tscheby with toroids) at (e.g.) 500kHz would be mandatory. An actual
bandpass may not be necessary. Equipped with such filters the lack of
resolution (8 Bit) will become more acceptable. Using a pre-amplifier
without using filters will do no good.
When home-brewed, such filters will not cost much, but it takes time,
measurement-tools and effort to build and tune them.
73 de dg3lv Tobias
Am 07.08.2014 14:13, schrieb Graham:
Receivers for LF and MF136 KHz@ 477 KHz
A question,
Startingat the£5dongleriggedfordirectsample , as
acheapeffectivestartingpoint
A pre amp and pre - selector [ band pass filter ] would help for
lf/mf
Whatwouldbein a scaleofincreasingperformance[ notcost !]
be seen asreasonable in terms ofconfigurationand hardware ?
Any particular equipments stand out as good cost/performance
choices ?
Tnx
Graham
G0NBD
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