Topband: Analog to Digital converter for 1.8MHz

[Steve Ireland]

At 08:38 AM 2/07/2004 -0700, you wrote:
>N6RK said:
>You certainly want to bandpass filter the input
>to get rid of AM BCB signals. 

VK6VZ replied:

Hi Rick

Thanks very much for the thoughts.

I am currently using an experimental direct conversion receiver, using a
four-section tuneable Cohn filter (in front of a 10dB RF amplifer, followed
by a Minicirkits SBL-3 mixer) designed by my friend VK6APH which has a
slot-like passband of about 10KHz before it needs retuning.   

For anyone wanting to know about Cohn filters, take a look at some of the
designs in the classic 'Solid State Design for the Radio Amateur' book, by
Wes Hayward, W7ZOI and Dough De Maw, W1FB


>Assuming a passband
>250 kHz wide and a stopband 500 kHz wide, you only
>need to sample at 1 MSample/s.  (Of course you still
>need an "analog bandwidth" of at least 1.8 MHz.)
>However, the dynamic range of 71.5 dB may not do
>so well in a contest with lots of strong signals.

Here, near Perth - the most isolated capital city in the world -  usually
the only strong signal my receiver has to deal with is my friend Mike VK6HD
(450 km away).  Thus I do not have the same problems in this part of the
world that you have in North America.  1.8MHz signals that approach 0.5uV
are all too rare here!

I am really enjoyiing using the direct conversion receiver, using a KK7P
dsp board and an old laptop to give switchable 2.0/0.5KHz bandwidths, with
binaural reception.  It sure sounds a lot nicer than my other superhet
radios and is as sensitive as them.  

Vy 73

Steve, VK6VZ


>
>Rick N6RK
>