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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