Replacement of two components

[shaiqbashir]

Hi Guys!

Well! i have a circuit in which the following two components are used. This circuit is of a transmitter:

1)Varicap Diode
2)Ceramic Filter (crystal)

one problem is this that the above two components, i just donot know about them. Can u plz suggest me any alternative, that i can put in my circuit. Alternative should be a simpler one. Lets say, i read somewhere that the Varicap diode can be replaced by an ordinary diode like LED. is it true.

Here's my circuit:
Thanks in advance

View attachment 37295

 

[audioguru2]

Hi Shaiq,
The BB109 varicap diode is obsolete, so I couldn't find its details. It changes its capacitance when its reverse voltage is changed. There are many other more modern varicaps.
I wouldn't use an LED because its max reverse voltage rating is only 5V, and its capacitance usually isn't documented. Other people have used the reverse-biased collector-base diode of a 2N4401 transistor, or paralleled transistor diodes as a varicap.

A 10.7Mhz ceramic filter is used in the IF section in many modern FM radios. Most electronics parts suppliers have them, or take one out of a radio.

That's a unique circuit. It oscillates at 10.7Mhz, then uses its 9th harmonic for its 96.3Mhz carrier wave. The varicap frequency-modulates its 10.7MHz frequency only a small amount, and its 96.3MHz carrier is modulated with 9 times more frequency deviation.
If many people built and used it, then 96.3Mhz would get jammed! ;D

 

[SM2GXN]

Yes BB109 is obsolete but close replacement is BB143 and data sheet can be found at http://www.datasheetarchive.com/
Guess you can't move the resonant frequency of the 10,7Mhz ceramic resonator that much to get a decent deviation though it is 9 times, I might be wrong.
I never thought about this but the Vr to capacitance curve for a varicap got an linear region if you go outside a little would it be an total disaster audioguru?
If you use another replacement can you just simply bias the varicap (through an inductor) so you are about the 9th harmonic of the ceramic filter and then modulate with a proper voltage from the inverter?
Or am I way out....

Bjorn

 

[audioguru2]

Guess you can't move the resonant frequency of the 10,7Mhz ceramic resonator that much to get a decent deviation though it is 9 times, I might be wrong.

Full deviation for an FM station is plus and minus 75kHz, so 1/9th is only 8.3kHz which is possible by shifting the wideband ceramic filter's frequency.

I never thought about this but the Vr to capacitance curve for a varicap got an linear region if you go outside a little would it be an total disaster audioguru?

The varicap operates at about 4.5V in this circuit, right where the BB143 that you recommend has a sharp bend in its capacitance curve. It would be very distorted. A link in Google says a BB809 is equal to a BB109. Its capacitance is very linear and it is about 10 times that of your BB143.

If you use another replacement can you just simply bias the varicap (through an inductor) so you are about the 9th harmonic of the ceramic filter and then modulate with a proper voltage from the inverter?
Or am I way out....

No, the varicap varies the 10.7MHz frequency of the ceramic filter that is the tuned circuit of the N2 oscillator. N3 and N4 to N6 square the 10.7MHz to produce harmonics.

 

[SM2GXN]

Hi!

The replacement for the BB109 was taken from VRT database hm... I can see that the range is quite different compared to BB809 and as you said a sharp bend  4-6 volts. Could have fooled shaiqbashir with my recommended replacement (thank you audioguru) causing decreased deviation.
I read that ceramic resonators can be pulled over a wider range compared to a crystal, disadvantage is that ceramic resonators are not as stable as a quart crystal, I learned something new
Is it possible to put a coupling capacitor right after the 100k resistor and then put a variable voltage to the varicap through an inductor and thus make the carrier freqency to be tuned in a small range without going to close to the unlinear portion causing distortion?
Thank you audioguru for making good and "easy to read posts"

Bjorn

 

[audioguru2]

I read that ceramic resonators can be pulled over a wider range compared to a crystal, disadvantage is that ceramic resonators are not as stable as a quart crystal, I learned something new

Yeah, unlike an accurate single frequency crystal, a ceramic filter for an FM IF is wideband, about 180kHz.

Is it possible to put a coupling capacitor right after the 100k resistor and then put a variable voltage to the varicap through an inductor and thus make the carrier freqency to be tuned in a small range without going to close to the unlinear portion causing distortion?

You don't need an inductor, a 100k resistor will provide plenty of isolation. Your BB143 substitute has a very low capacitance and probably won't work in this circuit. A BB809 has much more capacitance and a much wider range and will allow the project's output frequency to be tuned maybe a couple of FM station frequencies with a 2V to 29V control voltage.

Thank you audioguru for making good and "easy to read posts"

Glad to help you, Bjorn and thanks.