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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