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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