another FM transmitter project

[Willen]

Hi AG,
Once you had fixed an opamp based mixers and pre-emphasis. This one- www.electro-tech-online.com/attachments/fm-preamp-png.89033/

This was modified for a PLL FM Tx. And you also edited the audio input section of the PLL FM Tx, which is attached here (attached at bottom).

Today I am staring the whole circuit to design its PCB layout. I am stopped to the audio input section where the designer (Dick Cappels) added two 1N4148 back to back to the ground to limit the audio to 1.2V peak to peak.

And, here in the Opamp pre-emphasis, its output peak to peak is maybe around 4.5V (half of 9V supply). If it is then the audio would be clipped VERY highly in the two back to back diode and cause HUGE distortion. I don't know if the output of Opamp is being attenuated by a 10k resistor to Gnd. It's not mean that you modified input section incorrectly but I want to ask how two 1N4148 are not clipping the audio badly?

the two diode also used there to save the PLL chip from static charge, so it is important to be there.

Thank you!

 

[audioguru]

An opamp can easily drive a load of 10k or even 2k with no attenuation. With a 9V supply, the maximum output from a TL072 and most other opamps is about 3.2V peak.

The clipping diodes in Dick's FM transmitter prevent over-deviation of the FM carrier so that it does not cause interference to adjacent FM stations and does not cause HUGE distortion in FM radios.

The output from an electret mic when talking loudly about 10cm away is 10mV RMS which is 14.4mV peak. If the gain of the preamp is 27 times then the peak from the preamp is 0.39V which does not cause the clipping diodes to conduct.
In Europe and Asia, the 50us pre-emphasis boosts 15kHz 12 times so if 15kHz is at full blast (it never is full blast) then its peak from the preamp will be 4.7V. If 15kHz is too loud then lower frequencies also will be too loud so simply turn down their levels.

 

[Willen]

Your circuit will work fine because the varactor diode is effectively parallel with the capacitor in the tuned LC circuit.
But the frequency will change as the battery voltage runs down.

An ordinary silicon diode or rectifier or the collector-base diode of a transistor are also varactor diodes but with a small range of capacitance.

Hi audioguru,

Once you had posted this schematic (re-drawn by me) as an example of voltage tuning oscillator using a general 1N4007 diode as a varactor. It looks pretty cool and simulations worked nice. With almost 135nH inductor, I got around 97MHz to 104MHz frequency with 0V to 5V tuning voltage.

Question is: in the Mod 4 there's 100nH coil with around 15pf capacitor as tank circuit which is oscillating around FM band. Here in the attached circuit, there is an inductors almost same as MOD 4 but has its unusual capacitor- 100pf series with 10pf (generated across the Diode) grounded (seems bigger capacitor than MOD4). How I can visualize the working?

 

[audioguru]

The sum of capacitors in series is less than the one with the smallest capacitance.
The 100pF is a coupling capacitor that couples the capacitance of the diode to the inductor and collector. If the diode's capacitance is 10pF then the total capacitance of the series capacitance is 4.76pF plus: stray wiring capacitance, transistor capacitance and antenna capacitance.

The frequency will change if the +5V changes, if the temperature changes and if something changes the capacitance of the antenna by moving toward or away from it.

The antenna has an impedance of about 75 ohms to ground. When simulating it you need an output coupling capacitor to feed the 75 ohm resistor connected to ground. In my simulations the capacitor-coupled antenna impedance to ground prevents oscillation.

 

[Willen]

The sum of capacitors in series is less than the one with the smallest capacitance.
The 100pF is a coupling capacitor that couples the capacitance of the diode to the inductor and collector. If the diode's capacitance is 10pF then the total capacitance of the series capacitance is 4.76pF plus: stray wiring capacitance, transistor capacitance and antenna capacitance.

The frequency will change if the +5V changes, if the temperature changes and if something changes the capacitance of the antenna by moving toward or away from it.

The antenna has an impedance of about 75 ohms to ground. When simulating it you need an output coupling capacitor to feed the 75 ohm resistor connected to ground. In my simulations the capacitor-coupled antenna impedance to ground prevents oscillation.

Oh it seems pretty simple then. In the simulation, for slower circuits like audio amplifiers and regulators, I just write 10m to 50m as simulation command. For the FM transmitter it needs VHF so I always copy your command (4050n 4000n 50p) for fast. I never seen the last '50p' in others' simulation. I think it's time step but I don't know what it is.

 

[audioguru]

I tried changing the "timestep" to 10p and to 10n and it made no difference.

 

[Willen]

I tried changing the "timestep" to 10p and to 10n and it made no difference.

And if we deleted it?

 

[audioguru]

I took an amplifier circuit and sim'd it in LTspiceIV. Its Max timestep was 100u. I deleted it and it made no difference.

 

[Willen]

I took an amplifier circuit and sim'd it in LTspiceIV. Its Max timestep was 100u. I deleted it and it made no difference.

Hi, My main problem is I am not understanding about the term 'timestep'. What it is exactly?

 

[audioguru]

I didn't look but the "help" section on LTspice might define "Timestep" and how to type it.