microFM - armstrong based
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01-02-2018, 07:10 AM
Post: #1
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microFM - armstrong based
here is armstrong based microFM
inspired by sinclair microFM some minor changes ...by Aurel |
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01-08-2018, 02:22 AM
(This post was last modified: 01-14-2018 05:23 PM by Aurel.)
Post: #2
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RE: microFM - armstrong based
And here is a prototype circuit...
I decide to build simplified version of Armstrong design. My version don't have voltage regulator and electronic switch so i have 5 transitors. In first place because i have lot of parts and i cannot wait to see how this design work... Important part of this receiver is 70kHz cut off low pass filter created with resistor 6k8 and capacitor 330pF which disable double tuning of stations as we know FM bandpass is +-75kHz each station ,so we have here little bit under that ,and on this way reception is more clean. parts: i replace 1n5 caps with 1n capacitors T1 - base cap replaced with 120pF T3,T4 collector caps replaced with 180pF for diodes i use small glass diodes with lightblue line which are 1SS.... type transistors BC549 replaced with BC183 for T1 i try to use BF199 but not work with 1,5V and require circa 2.7V to work, also i try BFR 91A and work but i must change emitter resistor to 560 ohm Anyway , it looks that work best with BF494 ![]() Also i dont use trimer capacitor (green), i have one but it looks that work without . For PCB i use my own drilled PCB with raster 14x4 soldering places. I have one AM/FM plastic variable capacitor so i use FM side and connect main pin with 5p6 cap,27pF cap is connected to collector of T1(BF 494). Coil - i made it with 0.9mm Cu wire so i wound 6 turns over 6.5 mm drill, then i strech coil to cca 40mm. testing: First time i connect battery(1.5V) to phone jack(5mm) then plug ordinary 32ohm phones and i dont heare nothing... I try to set working point of T1 with trim-pot (100k) but still nothing then i look again into excellent John site (http://members.iinet.net.au/~cool386/sspcrx/sspcrx.html ) and i see that he redraw schematic and i figured that his limiter diode is polarized in a different way then is on original schematic. Cathode of this diode must be on transitor T4 base NOT on GND(-). So i do that and i get small noise in my phones which was good sign! Then i again set trim-pot very slowly and i finally get typical noise. I move volume potentiometer to maximum and and move variable cap and i receive local FM station very very loud and clear and i was become surprised about sound quality. Receiver is very sensitive to direction because there is no antenna - antenna is in fact built in coil L1. Just for testing i add small capacitor of 2pF and connect 0.5m insulated wire and i can say that receiver can pick up more FM stations on that way. John stated that sound is very deep but i cannot say that maybe because i use 1nF instead of 1n5 or becase i use BC183 , i am not sure . As conclusion I must say that i am surprised how receiver work very well with very good sound quality. I think that sound is better than with TDA7000 based chips. |
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01-15-2018, 04:15 AM
Post: #3
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RE: microFM - armstrong based
Here is PCB layout for microFM2
created on soldering point raster 14x4. |
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