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Re: LF: Receivers for LF and MF 136 KHz @ 477KHz ???

To: <[email protected]>
Subject: Re: LF: Receivers for LF and MF 136 KHz @ 477KHz ???
From: "Graham" <[email protected]>
Date: Thu, 7 Aug 2014 21:35:44 +0100
Importance: Normal
In-reply-to: <[email protected]>
References: <F2308B40167B42B486EAD4657D9D78F6@AGB> <[email protected]>
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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,

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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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