RC-(remote controlled) Car

[audioguru2]

Your varactor diodes have +9V on their anodes and ground on their cathodes. They are forward biased. Turn them around.

 

[faizanbrohi]

Hello i am back with some modifications . here is a v1.1 of the oscillator TX-500 . i have added a 1nF capacitor for better stability . the rest is that varactor diodes have changed their posistion. Now i will build the power meter... Asap

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[audioguru2]

The 1nF capacitor you added reduces modulating frequencies above about 500Hz, and about 5kHz will be down -3dB. Your DTMF decoder might not work properly with the higher tones rolled-off. The varactor diodes look correct now.

 

[faizanbrohi]

So if i remove it , it will be ok for DTMF and if i remove it . it will work for voice.
now i will post the power meter shortly. one more thing i am putting a 12V voltage regulator for both the VHF power meter and as well as the VHF Amplifer. is it ok.

 

[audioguru2]

An ordinary 12V regulator needs a minimum of about 14.5V on its input to work properly. Your 12V battery pack won't have enough voltage.
Low dropout regulators are made for 5V and if somebody makes one for 12V then it would need a minimum input voltage of about 12.5V.

 

[faizanbrohi]

WHy not use a step up boost voltage regulator , like the ti's UC2577 , but it has a bulky circuit and high amperage . i think a small step up boost voltage regulator with 0.5 A and 12 V regulation voltage would be ok .
But then again why bother , and just connect the 12 Volt battery pack directly  ;D

 

[audioguru2]

The VHF amplifier and the power meter don't need their supply to be regulated. The output power will become reduced a little as the battery runs down.

 

[faizanbrohi]

Well here is the schematic of the Power meter . i am feeling a little sleepy , will make assemble and develop the board for the oscillator and power meter , until then
astalavista baby !!!

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[faizanbrohi]

Well The PCb Layouts for the power meter are made , now its time to prepare the board and practically test it . will update soon

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[faizanbrohi]

Hello again , i have a question how can i implement a crystal oscillator i found on the net in my modulator and replace the LC circuit  . the desciption and circuits of fundamental crystal oscillators and overtone crystals are given here http://hem.passagen.se/communication/txo.html

One more thing how can i use it as a refrence oscillator

 

[audioguru2]

Hi Faizan,
If you use a crystal oscillator then you won't be able to frequency modulate it.
You can't get crystals with a high enough frequency anyway.

Old FM transmitters used an 11MHz crystal oscillator that was frequency modulated a little then it was followed by many frequency multiplying stages to reach 99MHz with a wide frequency deviation.
Modern FM transmitters use a low frequency crystal oscillator in a phase-locked-loop frequency synthesizer circuit then the phase-locked-loop's oscillator is frequency modulated.

 

[faizanbrohi]

yeah i get it , this means the TX-500 will be rock solid transmitter if it is PLL Controlled . and one more thing the IC i have for the radio is a KA22425 and also it's equivalent CXA1618M , well the PDf is attached .  i know it is not PLL Controlled but why it is stable . i also used it to check my walkie talkie and what do you know , it is quite stable also , it was also quite stable with my FM transmitters , i previously build . it also has a LED indicator to indicate if the channel is selected

KA2245d.pdf

CXA161.pdf

 

[audioguru2]

Those "all-in-one" radio ICs are not very good. I have a similar one in my cheap clock radio and the Sony one is in my inexpensive Sony AM-FM Walkman. My stereo and car radio tuners are much better.

Those cheap radios are "stable" because they have AFC (automatic frequency control) to coverup frequency drifting.

 

[faizanbrohi]

But if you can implement AFC on the reciever side , why not in the transmitter side . But i think for my project that radio is sufficient ,

 

[faizanbrohi]

hello again , after a long time i am back , and this time i have found two new crystal controlled IC's that suffice my needs , MC2833 FM Transmitter and MC13136/35 and MC3362 FM Reciever IC's . These IC's are crystal controlled and i think they will be fine for my Radio controlled Car. But Will it Work good for DTMF . It think it might. Although motorola does not manufacture these IC's , but these IC's Are available here in Pakistan at a very Cheap Price of Less than a Dollar . The Main Pain in the Butt is finding the crystals and the Inductors Just right and since the Transmitter is low power i have to build a Power Amplifier of 1W/0.5W for it. The Schematics and pinouts are attached.

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[faizanbrohi]

Does anyone knows dbm to mW conversion . the output power of The Transmitter is +10 dbM , how much will be it in mWatts .  ???

 

[MadFalcon]

decibal gain doubles your wattage for ever +3 dB. So whatever your input wattage before the decibal gain, double that approximately 3 times. For example if your input wattage before the gain is 10mW, then +9dBm would be 80mW. (((10mw*2)*2)*2) = Final mW. Hope that helps.

 

[AN920]

Into a 50 Ohm load
0dBm =  1mW
10dBm = 10mW
20dBm = 100mW