Content-Transfer-Encoding: 8bit
Message-ID: <65AECDF1F89AD411900400508BFC869F0D74EE@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 10:03:41 -0000
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

This Rx is intended as much a piece of test equipment as a portable
receiver.  For the IF gain  the Analog Devices AD603,    has 0-40dB gain
with a voltage control of 0-1 V, laser trimmed for 0.3dB linearity so a
voltmeter can be used to read off gain setting.  No AGC will be used,
and a switchable 20dB attenuator for strong in passband signals.   By
operating the front end device (a MAR-3 modamp) at constant current and
employing an SRA3H diode ring mixer with 50mW of LO signal, the whole Rx
can be made immune to temperature variation and supply voltage changes,
whilst keeping a sufficiently good dynamic range.  Haven't decided on
the detector mixer device yet, but an NE612 or similar mixer will
probably do,  and the rest of the gain, at audio frequencies, will be
simple op-amp circuitry before feeding into a 12bit A/D converter and
possibly an audio amplifier.    Any residual temperature/gain variation
can be taken out with a thermistor and conditioning circuit applied to
the AGC.

IF filtering will be a 300Hz bandwidth mechanical filter at 455kHz.
Very extensive front end filtering is needed to remove the image
response which sits in the MF broadcast band at 910 - 1070kHz.   130dB
in total apportioned into two filter blocks either side of the front end
gain stage, Fc at 160kHz so that the 198kHz Droitwich signal is
attenuated by 20dB before it reaches the mixer.  Needless to say, all LO
frequencies will be derived from one master 5MHz source.
All in the future at the moment - the shack is still in bebuild.

Andy  G4JNT


> -----Original Message-----
> From: Alberto di Bene [mailto:dibene@usa.net]
> Sent: 2001-01-05 17:09
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: RE: LF Spurious signal on south coast
> 
> 
> Talbot Andrew wrote:
> 
> > [snip]
> >   Next project is a proper LF receiver with accurate and
> > repeatable signal measuring capability and built in A/D converter to
> > avoid having to use soundcards or external DSP for narrow 
> band working.
> > [snip]
> 
> Andy,
>            do you know these chips :  
> http://www.alesis-semi.com/index.html
> From a first look they look quite useful for such a project. 
> And they are
> inexpensive, too.
> Just a thought...
> 
> 73
> Alberto    I2PHD
> 
> 
> 


-- 
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution, 
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.