Hi Graham !
The foremost problem of the Barlow-Wadley receiver is the generation of
the LO by a comb harmonic oszillator (in German :
"Lattenzaungenerator"). Receiving an unknown signal you could never be
shure, from which of the 1MHz wide Bands the signal actually originated.
The installed IF-bandpassfilters were much too wide and the resulted
purity of the LO was poor. At best it was just 1 MHz up or 1MHz down
from the selected MHz band, but large signals got through even farer
away. And the mixer did his job very well, it mixed everything with
everything. In the 1940s, when Mr. Wadley developed his Patent, this was
certainly not an issue. But even in the 1970s, when I listened first to
such a radio, the bands were much too crowded already.
http://en.wikipedia.org/wiki/Wadley_Loop
http://televideo.ws/wadley.html
Using a direct conversion receiver/transmitter (at any samplerate) you
must use a nyquist-filter (=anti-aliasing filter). Without such you will
receive the noise of all the "harmonics" and thus your noise-figure is
degrading a lot.
73 de dg3lv Tobias
Am 07.08.2014 22:35, schrieb Graham:
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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