[HomeBrew] The Homebrew Mitch Lee receiver is on line

[Philip B Atchley]

Hi All,
Well, today I decided that my version of the "Mitch Lee" receiver is far
enough along that it has been temporarily put on line in the listening
position to see what further changes may have to be made.
 
There were/are a few bugs that still need to be worked out.
1.  Got the low gain problems with the IF dual strips taken care of.  I
had a couple bad trimmer capacitors AND a surplus relay "gotcha".  It
turned out that the 5 surplus relays (read "pulls") are polarity
sensitive on
their coils.  There is only a very tiny + mark to indicate such on the
relay can.  reverse the Voltage and they won't pull in.  Of course,
"Murphy" had to play a role in this.  Out of 5 relays, 4 had the Voltage
to the coils reversed 8^(
 
2.  Never did get the tunable pre-amp to work.  I had that board in and
out at LEAST 10 times.  I've built a number of these pre-amps and never
had one not work.  Anyway, that board will remain out and I will use an
outboard pre-amp until I build a whole new board with new varactors,
chokes etc, then test it OUTSIDE of the receiver before installation.  I
had changed the design just a little to use parts on hand and I'm not
sure if it were the Varactors I used (supposed to be the right
capacitance range), the FET or what.  
 
Anyway, the set is working quite well.  In a daytime bandscan it heard
virtually ALL the daytimers except a couple I believe are off the air
right now (I haven't heard OVE for a couple days or so).  31 of them,
something my Boatanchor receiver with up-converter can't do!   It ALSO
heard two beacons that are NEVER heard here on ANY receiver without a
very sharp Audio DSP filter (better than 20 Hz selectivity) due to strong
carriers on frequency.  It pulled them out just fine!  

I have the BFO in this unit set for a very pleasant 300 Hz note.  It
shouldn't be too fatiguing during long listening sessions.  I can set the
Beatnote higher, but this seems about optimum.
 
I measured the selectivity of the set, using very crude methods. 
Basically, I tuned in a steady carrier from a local NDB (FCH), set the RF
gain for -10dB of audio and then tuned to either side of the carrier till
the level fell by the desired amount in dB, then read the dial of the
DDS.  Results are quite spectacular.  
 
If you notice, the skirt on the high side isn't as steep as it is on the
low side.  This is typical of simple crystal filters.  However, the
selectivity still approaches that of most DSP units and exceeds some of
the cheaper units!
 
Narrow IF:
 
Levels referenced to level -10dB.
              Bandwidth.
-6 dB     16 Hz wide  (-7 and +9 Hz)
-12 dB   27 Hz wide  (-12 and +15 Hz)
-18 dB   37 Hz wide  (-16 and +21 Hz)
-24 dB   48 Hz  wide (-21 and + 27 Hz)
 
Wide IF: 
 
Levels referenced to level -10dB.
-6 dB    47 Hz wide  (-13 and +34 Hz)
-12 dB  65 Hz wide  (-24 and +41 Hz)
-18 dB  88 Hz wide  (-37 and +51 Hz)
-24 dB  113 Hz wide (-52 and +61 Hz)      
 
73 from the "Beaconeers Lair".
Phil, KO6BB
 
Where DX begins at the noise floor!
Merced, Central California, 37.3N  120.48W  CM97sh
 

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