455 KHz tunable BFO circuit

[Richard9025]

Hi !
I want to make a 455khz tunable bfo without any transformers , based on the model on this page http://spazioinwind.libero.it/ik1zyw/hardware/osc455.html
I made a schematic based on what he says :

See if I correctly made the 12v DC to 5v DC circuit (dotted line) .
Also , if I correctly connected the pins of 74hc14 chip and the ceramic filter .
CFWLA450KHFA-B0 datasheet : http://www.murata.com/products/productdetail?partno=CFWLA450KHFA-B0

 

[duke37]

The power supply looks OK.
The hex invertor is wrong. Unused inputs need to be connected to something, You have used ground which is good. This gives a high output which you have shorted to ground. this will overdissipate the chip.
C3 is part of the oscillation circuit and must be much smaller.

 

[CDRIVE]

The datasheets for the 7805 and the 78L05 recommend 0.1μF and 0.01μF respectively for the output capacitor. I have gone as high as 10μF on the 7805. Personally I think 47μF is a bit much for that little L05. I could be wrong and it wouldn't be the first time.

Chris

 

[Audioguru]

Each output of the 74HC14 outputs that are shorted to ground will try to draw 60mA so the 5 of then will try to draw 300mA! But the maximum allowed output current is only 25mA for each output and the power pins on the IC have a maximum allowed current of only 50mA.

Simply connect the unused inputs to ground and connect nothing to the outputs that will be at a logic high.

 

[Richard9025]

V2

What value should I use for C3 ?

 

[duke37]

For a 4093 oscillator without the filter, the frequency = 2.7/(R*C)
So if the resistor is 3k, then C = 2.7/3k/470k = 19nF ?
Do you have a copy if the correct circuit?
Edit
1.9nF?

 

[Richard9025]

Do you have a copy if the correct circuit?

if that "i" in "if" should be an "o" : "Do you have a copy OF the correct circuit" then no
the schematic I made is based on the instructions found on this site : http://spazioinwind.libero.it/ik1zyw/hardware/osc455.html
a photo of his schematic (please visit that site !)

Also see that the ceramic filter is connected to +5V in that schematic but my ceramic filter only has 1 input 1 output and 3 ground pins .
My schematic is what I understand from what he says (I might be completely wrong)

 

[duke37]

The link suggests that a subharmonic is generated and then the wanted frequency is selected by the filter. This could generate a good sine wave but does not help the frequency stability.
A better option may be to put the filter into a feedback circuit.

 

[Richard9025]

Hmm , At least the schematic is ok ? (connections)
Now I'm searching a schematic for a tunable 455khz bfo without any transformers (ceramic resonator/filter based)

 

[duke37]

A ceramic resonator will not allow you to move the frequency more than a little.
What shift do you need?
What is wrong with an inductor?

 

[Richard9025]

What is wrong with an inductor?

I can't find any

What shift do you need?

better than ±5khz

 

[CDRIVE]

Inductors are the easiest of all component to make on your own.

Chris

 

[Richard9025]

I found all the components in the local electronics shop
I'll go tommorow to them , buy the components and build the circuit , see if it works
thank you for correcting me on the first schematic snippet !

 

[Richard9025]

Got all the components except the ceramic filter , they were out of stock
Can I replace the ceramic filter with a ceramic resonator ?
Where can I find a ceramic filter ? (ex: cordless phone)

 

[CDRIVE]

Did you look here?
http://www.ebay.com/sch/i.html?_fro...ter.TRS0&_nkw=455+KHz+ceramic+filter&_sacat=0

Chris

 

[Richard9025]

Umm , I can't just order one thing from ebay , the shipping is twice the part .
I buy parts from my local electronics stores (no shipping , very cheap) (adelectrocom , syscomelectro , conexelectronic , cometelectronics , vitacom , etc etc)
I'll just wait until the ceramic filter is back on stock .
I just got a vintage portable transistor romanian radio that has a little problem , I'll make a new thread on this forum.

 

[CDRIVE]

I rarely order any electronics on ebay that requires shipping costs. Machinery and heavy tools yes but they're not coming from China.

Chris