Re: Ideas for AM, NB FM Demod and CW filter board

Arv Evans

Agree with Allison.  If what you want is a FM monitor receiver, then design and build something
specifically for that purpose.  If you want an AM General Coverage receiver, then build one
specifically for that purpose.  As it stands now the BITX or uBITX is not a good choice for
either mode.  Lots of modifications would be required and would still result in a compromise
design for those modes, and could also degrade the excellent design and performance of your
BITX or uBITX as an SSB and CW transceiver.

There are several ICs that can be used as the back-end of a AM/FM receiver.  They come
in WBFM or NBFM models, and some are suitable for both wide or narrow FM.   It should be
easy to design and build a simple down-converter to go ahead of one of those chips and
follow it with an audio amplifier stage.  For FM you would probably want AFC, and lots of
gain followed by a limiter ahead of the FM detector.  For AM you will probably want to include
some sort of AGC. 

What does seem to be useful for AM and FM and related to the uBITX or BITX-40 would be to
use a Raduino board as the frequency and mode control for a new design for AM, FM, or both. 
The Si5351a is inexpensive and could be a good source of LO signals for the down-converter.
Software for the Arduino Nano could be written to specifically support this layout.

If your goal is VHF or UHF operation on SSB or CW, then a transverter might be the most
cost effective way to go.  Doing this would only require that the BITX or uBITX output be
attenuated enough to match input level requirements for the transverter. 

My 2-cents worth.


On Tue, May 29, 2018 at 12:10 PM ajparent1/KB1GMX <kb1gmx@...> wrote:
On Tue, May 29, 2018 at 10:26 am, Ashhar Farhan wrote:
Superregen at 45mhz as AM/FM detector... I can't go there.  ;)  I did it 40 years ago at 10.7mhz
I learned a lot.  I wanted a RX that could hear Aircraft (118 to 136mhz) and 2M fm (144-148).
Its a great way to go for low power consumption and poor selectivity, not a no tune design!

Took a bit of digging to find a data sheet. 
Yes, but try to buy them from non questionable sources.  It went EOL years ago so the only
parts are NOS (new old stock that have been sitting).  However something like that would
solve the RX problem and needs to work at the 12mhz IF and its upper frequency for the
TDA1220 is 30 mhz.  It was a 1994 part and I even have two of them.  That and two filters
plus three tuneable cans to add fun to ones life.   Look for the TDA-7000 (also EOL) as
that works easily at 45mhz for FM rx.

That type of part goes in and out of fashion rather quickly but there is always something
new to replace it.  

It still leaves generating AM and FM.  Double sideband AM is easy,  sum the 45mhz though
a 6DB pad and a DBM and you get perfect low level modulation.  Gain down stream
becomes critical as the carrier has to be 1/4 peak power or it sounds distorted.

For AM and power do a class E rig.  Using switching fets its fairly easy to do
Monoband AM at 50W or more (with switchmode class D modulator).s

FM in the current scheme means audio to digital and feed that to the SI5351.  I think 
a bigger MPU might be needed. The alternate is 12mhz crystal osc and FM it using
a varicap diode, not  a no tune approach. That also requires audio processor
(preemphasis and clipping/limiting).

Both AM and FM will easily pass through the 45mhz filter and its wide enough.

I cut my teeth on AM (marine radio pre VHF and aircraft) and FM (Land mobile
and Marine VHF) commercial stuff must be 50 years ago.  It was fun.  


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