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Message-ID: <65AECDF1F89AD411900400508BFC869F0D74EF@pdw-mail-1.dera.gov.uk>
From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: LF Receiver
Date: Mon, 8 Jan 2001 12:40:49 -0000
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Albertoo et al - 
I was keeping away from having to use fast DSP and digital receiver
technology for a piece of standalone signal measurement test kit  to use
in the field without a PC attached.   130dB at that frequency
separation, Fc = 160k to Fs = 900kHz,  is no big deal provided proper
care is taken on layout.   

Then, as we are only dealing with narrow data, I felt an A/D converter
sampled at a low rate, supplying samples via the COM port to a PC would
be a simpler beast to write software for than being forced into the
soundcard route.   I do not like windoze programming and am not prepared
to go the soundcard route - there are plenty of others writing S/W for
that and don't want to reinvent the wheel.   This unit will probably
sample the 300Hz bandwidth and output the data at 1000 S/s. ALso, only
certain soundcards can be guaranteed to have the performance needed.
Already have three suitable mechanical filters spare, as well as
numerous ones of higher bandwidth - and I need to do something with them
all !

Anyway, direct zero IF conversion will still need extensive input
filtering for anti-alliassing and in this case will be even more
stringent unless an artificially high sampling rate is employed.  For
example, assuming 80dB spurious levels, the input signal has to be at
-80dB at half the sampling rate separation from the edge of the
passband.   If a high speed A/D is considered - lets sample at 100kHz
for now - then we need -80dB at 50kHz away from the centre of the band.
(In fact, within this allias bandwidth are some very strong signals, so
the 80dB requirement may have to become 100dB anyway)  so we end up with
filtering more stringent than the double conversion approach.  And at
least to get rid of 1MHz signals, more filtering can be thrown at the
finished unit without degrading in band performance.

Andy G4JNT
An analogue engineer at heart, with an extensive junk box !


> -----Original Message-----
> From: Alberto di Bene [mailto:dibene@usa.net]
> Sent: 2001-01-08 11:15
> To: rsgb_lf_group@blacksheep.org/'
> Subject: LF: RE: LF Receiver
> 
> 
>> Andy, to solve one for all the image response problems, have 
> you considered
> going to the zero IF approach ? This method, using a single 
> DDS for the conversion,
> and a couple of mixers to generate the I and Q components, 
> will allow you to enter
> into a stereo 24-bit ADC and then to do all of the 
> processing, whatever it may be,
> digitally in a DSP chip, keeping the analog portion to a bare 
> minimum. You will also
> save the expense of the mechanical filters.
> 
> 73   Alberto    I2PHD
> 
> 
> 
> 


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