FM Transmitter Circuit VHF - Crystal Controlled. Alignment

Alignment of the BBG Transmitter Circuit

BBG FM (NBFM ) VHF
Crystal Controlled, Xtal Locked 1/2 WattTransmitter Circuit - Alignment
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The Difference Between A FM VFO Voice Transmitter And A FM Crystal Controlled Voice Transmitter Is About As Different As Night And Day. A simple VFO ( Variable Frequency Oscillator ) is used in simple one or two stage voice FM transmitters and the voltage level of the audio determines the frequency shift of the oscillator, the more or stronger the audio, the wider, the FM oscillator expands. The limits on a modulated FM wideband signal is plus or minus 75 KHz so the audio signal is usually kept at a level to where it bumps its oscillator back and forth no more than about 150 KHz. This is called the FMWB spectrum ( FM Wideband ).

I Had An Email While Writing This Page from someone asking me if he could modify the BBG Transmitter to the 88 / 108 FM Band. The BBG was designed and operates in the upper VHF frequency range from around 150 to 174 MHz, in the NBFM ( Narrow Band Frequency Modulation ). The 88 to 108 frequency band is designed for wideband modulation and all receivers in that spectrum are designed to accept and demodulate those signals from 150 KHz down to an audible frequency that you can hear and understand which includes voice, music, background and etc. In other words a wideband signal is designed from the transmitter standpoint to transmit all of those frequencies and the receiver is designed to decode ( Demodulate ) all of that audio for clear reception.
On The Other Hand, a transmitter such as the BBG has been designed to operate in the NBFM mode ( Narrow Band Frequency Modulation ), which only transmits speech transmissions of around 300 to 3000 Hz, similar to the telephone voice standard, with a maximum spread of it's oscillator of a plus or minus 5KHz. To put it in another term, rather than spreading its oscillator 150 Khz wide, it only spreads it a maximum of around 10KHz. Again, the receivers used to monitor these types of transmissions ( NBFM ) are totally different and catered to receive and demodulate these types of transmitters.

Xtal Locked FM Crystal Controlled Transmitter Circuit - BBG
Alignment Of The BBG. Unlike a simple FM VFO Transmitter where all you have to do is to tune your FM radio onto a station and then tune a single capacitor until the transmitter is picked up, the BBG requires precise alignment.

If You Are Skilled At Alignment And Have The Necessary Instruments, you are set, and already know what to do. More than likely, 90% of those looking at the board above either don't have the instruments to correctly align this unit, nor have a clue where to start. There is a link to another page near the end of this page where I have put together a simple Absolute FSM (Field Strength Meter) and instructions for anyone that does not have access to the instruments required for alignment, that will allow you to build your own.

A Little Background On The Instrument I Have Used For Over 25 Years In The Alignment Of The BBG. I have always use a Jerrold Model 704B Absolute Field Strength. My antenna is a 1/2 Wave Dipole ( Set Of Old Rabbit Ears adjusted to the frequency of the transmitter I am about to align ) mounted vertically on a homemade wooden stand. The input is fed to the 72 Ohm input you see on the instrument.

Jerrold Model 704B Field Strength Meter
From There, The Instrument Is Turned On, the frequency is dialed in at the top left, and the attenuators are set so that I don't bang the needle, and only for a minimum reading.

C4 is adjusted 1st. This kickstarts the oscillator into operation and is adjusted back and forth until the center of the adjustment is found, based on the swing of the needle on the Jerrold. At this point the attenuator is decreased for a higher reading on the scale.
C9 is adjusted next. This portion of the circuit amplifies the crystal frequency, and as you can see above the crystal is a 16MHz. Again, the capacitor is turned back and forth until the needle of the instrument peaks.
C11 is adjusted next. This capacitor feeds the base of Q5. Again, the needle of the instrument is watched and the capacitor adjusted back and forth for a peak needle - and the instrument attenuator is again decreased, or increased to keep the needle about midrange on it's scale.
C13 and C15 are adjusted next. Jog them back and forth for a peak signal and attenuate the instrument to center scale.
C16 is adjusted next. Before you adjust C16, increase the attenuator since more than likely, the needle will go to full scale.
Last, adjust C18. This will match your antenna.

FINAL STEP: Repeat the alignment steps above. Once you are confident that each capacitor has been adjusted to maximum, and at it's center:
Turn On Your Scanner, and punch in the frequency of your transmitter. This particular unit has a 16 MHz fundamental crystal so it's final output would be 160.00 MHz. Set your scanner to 160.00 MHz. The last step is to start talking to yourself and slowly adjust C4. This will warp the crystal down and set its correct deviation. When your voice comes out crystal clear and you hear feedback over your scanner you have correctly aligned the unit. Now, just take a birthday candle, light it and drip just a "Tad" of hot wax over each trimmer capacitor. That will keep them from shifting during normal usage and moving around.





FM Transmitter Circuit Alignment Unit