what the role of Signal diode 1N914 in this schematic? Is it common Germanium diode?

[Willen]

Look at D1, I've never seen such kind of diode in oscillator of FM Tx. For why it is used here? Is it equivalent of 1N4148 germenium diode or like 1n5711 t?
Thanks

 

[ChrisP58]

The 1N914 is a common Silicone diode. It is in very similar to the 1N4148

https://www.electro-tech-online.com/attachments/1n4148-pdf.65829/

 

[audioguru]

The 1N914 is a common Silicone diode. It is in very similar to the 1N4148

No.
Silicone is synthetic rubber. It is a SILICON diode.
The 1N914 has a maximum continuous current rating of 150mA and 300mA for the 1N4148.
The 1N5711 is a Schottky diode with a maximum continuous current rating of only 15mA.

Geranium diodes are very old, 1N34A was common 50 years ago as an AM detector.

In this horrible FM transmitter, the diode is an IDIOT DIODE. It shorts the battery (to protect the circuit) if it is connected backwards by an idiot. I think the diode should be rated for 1A or more (1N4002).

 

[unclejed613]

the 1N914 the OP is referring to is D1, not D2. D1 biases the base of Q3 at 0.7V. a germanium diode here would not work, since it's forward voltage would only be around 0.3V. it's a rather ingenious method of providing bias since it eliminates the typical resistor divider that would load down the tuned circuit (C6-L1)

 

[WTP Pepper]

D1 is used to clamp the drive to the buffer transistor to prevent overdrive that could either pop the transistor or cause it to overdrive and produce unwanted harmonics.
In my experience 1N914 & 1N4148 are often interchanged as they are both small signal diodes and are rarely implemented in a circuit where its forward current becomes an issue. i.e as a power rectifier.

I personally think it's not a bad design. Plenty of filtering design to take into account unwanted harmonics. It's drawn a bit weird regarding the PNP transistor and the ground point travelling around the circuit after C10 & Q4 onwards.

Audioguru - He specifically refers to D1 and not D2* which is an idiot diode.

 

[audioguru]

D1 is used to clamp the drive to the buffer transistor to prevent overdrive that could either pop the transistor or cause it to overdrive and produce unwanted harmonics.

D1 simply biases Q3. It doesn't clamp anything.

I personally think it's not a bad design. Plenty of filtering design to take into account unwanted harmonics.

There is no audio filtering. Noise on the supply goes directly to the microphone through R1 and to the preamp transistor Q1 through R2.
R1 and R2 should have a series resistor from +12V and a filter capacitor to ground.

The audio will sound muffled like your stereo with its treble tone control turned all the way down because the circuit is missing pre-emphasis like all FM stations have (then the de-emphasis in all FM radios cuts the treble sounds down to normal and also cuts hiss).

I don't see an RF oscillator. Do you?
Maybe L1 is nearby L2?

 

[ccurtis]

the 1N914 the OP is referring to is D1, not D2. D1 biases the base of Q3 at 0.7V. a germanium diode here would not work, since it's forward voltage would only be around 0.3V. it's a rather ingenious method of providing bias since it eliminates the typical resistor divider that would load down the tuned circuit (C6-L1)

Agreed, and I only want to add that the forward voltage of D1 is significantly greater than an ordinary silicon PN junction because the 1N914 has metal doping, also making it a faster diode. Thus, in this application, D1 forces just enough voltage to the base of Q3 to assure that Q3 remains in its conduction region, under a wide range of forward current through D1.

 

[panic mode]

I don't see an RF oscillator. Do you?

Isn't Q2 RF oscillator, C7 feedback and L1-C6 tuned circuit?
This is same (although using PNP transistor) as most oscillators in hobby FM transmitters.

 

[audioguru]

Isn't Q2 RF oscillator, C7 feedback and L1-C6 tuned circuit?
This is same (although using PNP transistor) as most oscillators in hobby FM transmitters.

I see the simple oscillator when I turn the upside-down PNP transistor right-side up.

 

[DJAE]

Look at D1, I've never seen such kind of diode in oscillator of FM Tx. For why it is used here? Is it equivalent of 1N4148 germenium diode or like 1n5711 t?
Thanks

I believe it biases the base on the of the final transistor and prevents instability due to temerature fluctuation (unlike a resistive divider). I'm not 100% though. It also looks as though it may form capacitive feedback from emitter to base along with C5 (being reverse biased it will represent a capacitance to the discharging coil).

I could be (probably am) wrong though.

 

[WTP Pepper]

D1 simply biases Q3. It doesn't clamp anything.

OK. Closer looking you are correct.

 

[panic mode]

this circuit is not drawn in a conventional way and that makes it a bit awkward to follow...

 

[audioguru]

this circuit is not drawn in a conventional and that makes it a bit awkward to follow...

Yes.
Many parts are upside down. I tried to fix it but I gave up because too many parts are upside down.

Maybe the person who drew the schematic is a weirdo.

 

[unclejed613]

the ground run goes around the outside of the schematic. looks like whoever drew it was trying to keep the components laid out as they are laid out on the PCB. i've seen schematics where the whole thing literally is "upside down" with ground at the top and the +Vcc rail at the bottom. in the US generally the input side of a circuit is at the left, and outputs at the right. some parts of europe, however seem to put the input on the right, and output on the left. with a lot of experience reading schematics, following these sometimes "odd" conventions isn't as hard as it looks.