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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