Message-ID: <74531A06DA1A40B3AA3D0DDBD8B438E7@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <COL122-W41D63A568F076A1A89DC9FD3000@phx.gbl>, <002e01ca2057$32c9a5c0$0900a8c0@AM> <4A8B9246.29046.10FABD@dave.davesergeant.com>
In-Reply-To: <4A8B9246.29046.10FABD@dave.davesergeant.com>
Date: Wed, 19 Aug 2009 17:36:58 +0100
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Subject: Re: LF: Re: 500kHz qrg range
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Dave,

I used to have a notch filter for  radio Merseyside on 1485 , but then it 
started sending on other frequency's as well , so I made the  high pass 
filter from the  arral handbook .. that solved the rf levels zapping the HF 
rx , but  doesn't stop all the intermod  products that  appear on  160 and 
80 mtrs , at least the  harmonics fell out side the  ham bands in those days 
.. just when the  decca beacon on 1.9 mhz  finally beeped its last beep and 
160  became usable , up popped 5 live 24 hr gibberish .. remember when the 
mw tx stations closed after midnight !

500  has not  presented a problem as the  ae is quite  short and with the 
tuner, it  is quite high Q .. it seems  a bit of a tall order for a 
selective filter in front of the rx ...... its not really  desirable to 
have  high selectivity in the Ae ccts as the  higher the  Q the more the s/n 
ration is  degraded by simple ringing due to  'noise'  which at 500  there 
appears to be a lot of .. I think the  trend was set by the  'need' (desire) 
to publish huge image response figures for HF tx/rx's  ?

Detune , attenuate , digitise and do the filtering in software , I think it 
was Andy  g4gnt who gave a very well presented lecture at the  Eggham  HFC 
,,utilising the  AD convertors from the  new 3GL  phones and  logic block 
down convertors to  produce a 'new wave'  (sorry)  receiver  that would be 
very  hard to beat with  conventional method's  , my rx pumps nearly  7 
watts of rf into the  mixer to  get just close to that kind of  dynamic 
range !  .. sadly we are yet to see a kit for such a pure digital device 
..ie no pre down converting  mixer  .. they know who they are ..!

G ..

--------------------------------------------------
From: "Dave Sergeant" <dave@davesergeant.com>
Sent: Wednesday, August 19, 2009 6:48 AM
To: <rsgb_lf_group@blacksheep.org>
Subject: LF: Re: 500kHz qrg range

>
> On 18 Aug 2009 at 23:56, Alan Melia wrote:
>
>> However I suspect that the worst problem in Europe might well be 500kHz
>> exactly, which is 16 times the TV line frequency. I dont think modern
>> sets radiate so much at this frequency that they used to with CRTs, but
>> I still have one or two CRT around me.
>
> In fact it is 32 times the line frequency - 15.625kHz x 32 =500kHz
> I do have a weakish carrier there, so that is probably what it is
> coming from.
>
> What puzzles me about this thread is why you need such a narrow receive
> filter anyway. Blocking/intermod from broadcast stations is often
> stated as a problem on 500kHz but I get no problems here with my Datong
> UC1 (which has a tunable preselector but pretty broadband). If it is a
> problem then presumably a high pass filter passing above (say) 450kHz
> with a high attenuation below, or a notch at the frequency of a local
> LW station would surely be the better solution - or a better receive
> mixer.
>
> 73 Dave G3YMC
>
> http://www.davesergeant.com
>
>
>



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