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 Post subject: Pulse Counting FM2 receiver
PostPosted: Jan Thu 11, 2018 7:08 pm 
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Joined: Feb Sun 16, 2014 10:03 am
Posts: 13
Hi to all!
I decide to build my version of microFM2 receiver:

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(deviation) 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 Smile
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.

Schematic and real circuit you can see here:
http://euradioboard.createmybb3.com/att ... php?aid=19
http://euradioboard.createmybb3.com/att ... php?aid=21

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Last edited by aurel on Jan Sat 13, 2018 6:07 pm, edited 1 time in total.

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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Fri 12, 2018 4:43 am 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 536
Location: Equinunk PA 18417
Welcome and thankyou! Schematics available only to members of euradioboard :(

Fortunately the first link goes to an interesting and long page.

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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Fri 12, 2018 4:44 pm 
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Joined: Feb Sun 16, 2014 10:03 am
Posts: 13
Sorry Rich
I try to upload images here in attachment but sometings go wrong
so i will try again..
it looks that images are too big so i give you links to images...

http://aurelsoft.ucoz.com/FMpulse2_a.gif
http://aurelsoft.ucoz.com/fm2sch.png

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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Fri 12, 2018 8:26 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 138
aurel wrote:
as we know FM bandwith is +-75kHz each station ,so we have here little
bit under that ,and on this way reception is more clean.


The +-75 kHz figure is not the bandwidth, but the maximum allowable Deviation.

Modulation Index = Deviation/Modulation Frequency

If the maximum allowable modulation frequency is 15 kHz, then the Modulation Index = 75/15 = 5.

From the Bessel chart below it may be seen that an FM spectrum with a modulation index of 5 has 8 significant sidebands or 8 X 15 kHz of bandwidth on each side of the carrier for a total of 240 kHZ total bandwidth which actually exceeds the North American allotted 200 kHz bandwidth per station.

Other single chip FM receivers which have a typically 70 KHz IF frequency, overcome this limitation by using a sliding IF center frequency which dynamically accommodates the larger required bandwidth should the need arise.


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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Fri 12, 2018 9:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1747
Location: Monroe, NC 28112 USA
Have you seen the latest article on the subject in the BVWS Bulletin? Quite extensive....

https://www.bvws.org.uk/ Link is not the article... Don't think it is on-line...

Robert


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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Fri 12, 2018 11:54 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 138
Robert Lozier wrote:
Have you seen the latest article on the subject in the BVWS Bulletin? Quite extensive....

https://www.bvws.org.uk/ Link is not the article... Don't think it is on-line...

Robert


The author of the article in the magazine link is David Smith.

A Google search revealed this site.

http://members.iinet.net.au/~cool386/sspcrx/sspcrx.html


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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Sat 13, 2018 3:37 pm 
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Joined: Feb Sun 16, 2014 10:03 am
Posts: 13
ok deviation or bandpass is in Europe 200kHz
and this is not important for this topic than important is that
receiver work and work very well for circuit on only 1.5V battery
John elaboration is excellent and worth each word.
So I can recommend to anyone who wish to build simple and useful
radio to build this one.

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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Sat 13, 2018 3:56 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 138
aurel wrote:
ok deviation or bandpass is in Europe 200kHz
and this is not important for this topic


Deviation and bandwidth are not the same thing.

aurel wrote:
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 bandwith is +-75kHz each station ,so we have here little
bit under that ,and on this way reception is more clean.


In the West European countries and the United States of America, the maximum deviation is 75 kHz. In the ex-USSR and in some other European countries, it is 50 kHz.

Although probably not important for a tiny receiver using an earbud, bandwidth is important for a desktop receiver because it determines the maximum fidelity from the received signal. The decision to use a 70 kHz bandwidth because the deviation is 75 kHz is not a scientific decision because there is not a direct relationship between deviation and bandwidth.

An FM receiver with an IF bandwidth less than 200 kHz will be sacrificing some fidelity unless other electronic means are taken to compensate.

Some of the examples from other authors in your original post are using an IF bandwidth of 200 kHz, not 70 kHz.

These standards were introduced in the 1940's and later introductions like stereo and subcarriers were slotted in to the 200 kHz bandwidth in portions of the received spectrum to minimize any possibility of interference between the new additions and the received signal.

For interest, if you want to build your own FM radio station in any country you have to follow the rules below.

https://www.google.ca/url?sa=t&rct=j&q= ... CQ9GAHm4Fa


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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Sat 13, 2018 6:05 pm 
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Joined: Feb Sun 16, 2014 10:03 am
Posts: 13
I don't have in plan to argue with you because you
insist on one wrong word in my first post.
Is that ok?

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 Post subject: Re: Pulse Counting FM2 receiver
PostPosted: Jan Sat 13, 2018 6:31 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 138
aurel wrote:
I don't have in plan to argue with you because you
insist on one wrong word in my first post.
Is that ok?


Not a wrong word, but a completely wrong concept, which if corrected I would hope that anyone building the receiver (including yourself) would benefit in being aware of, allowing them to tailor their receivers to improve its performance.


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