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Date: 1 Aug 2001 08:40:35 +0100
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From: "Andrew Talbot" <ACTALBOT@dstl.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: LF Receivers
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The BFO at 456kHz is derived by extracting the 57th harmonic of the 10MHz
divided down to 8kHz.  This was actually extremely simple and the most
satisfying part of the breadboarding process to get going !.  The 8kHz
squarewave is transformed up in impedance and applied directly to a small
and cheap 455kHz ceramic filter.  The output level was 400mV pk-pk, which
after a 3dB pad, is now at exactly the right level to drive the NE612
detector.  Also, the 8kHz is further divided by 2 to give the 4kHz interrupt
to drive the PIC controlling the A/D so all the back end is coherent and
locked.  Had a separate 4kHz interrupt not been used, the PIC would have to
have been clocked at 10MHz and the 4kHz interrupt generated by a messy
software routine.     Had another BFO freq, such as 454kHz been neseccary, a
PLL would probably have been used rather than direct generation.

The DDS does have steps of 2.32... mHz but, although the value is inelegant
and messy to visualise, at least it is exactly known.   What I do in the
driver software is to first calculate the 32 bit word for the DDS, then back
calculate the exact frequency that is generated and display this, along with
the error from what was requested.   Any corrections are then made in the
plotting / signal monitoring software.   I see little point in choosing
'nice' frequency steps just because they have a few digits to express them
accurately.

To software,  steps of   (10^7) / (2^32) are just as understandable as steps
of 2.5mHz and a computer probably thinks it is quite a pleasent number !

I now have the manual tuning option added and working, so will call it a day
for breadboarding and do a bit of gardening before writing up the design.   

Andy  G4JNT


> 
> Is the BFO phase locked to "an exact integer kHz"? If so, the DDS LO
> will be the only source of "odd steps". Clocked at 10MHz, it 
> will produce an LO signal that is a multiple of 2.328306437... mHz
(assuming 32-bit
> phase accumulator (or do you use the 48-bit creatures?)). 
> This is a quite unpleasant number if the RX is to be used for *ultra*
narrowband work.
> 
> 


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