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:[email protected]]
Sent: 2001-01-05 17:09
To: [email protected]
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
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