3B1 19  Understand the block diagram of an s.s.b transmitter employing mixers to generate the final frequency.

Fig. 1

Note: The diagram shows the Balanced Modulator and the Filter as two separate items but sometimes they are combined into a Balanced Modulator with Filter in one block                                                                                                                                                                                                           .

A general over view

When considering a transmitter it is probably best to think what it is trying to do. So what is it trying to do ?

• Create an RF signal inside the amateur band

• For the signal to be nice and clean

• For the signal to be stable

• For the signal to have minimum band width for the type of transmission

• For the signal to have the correct power output

• for the transmitter to have minimum output on other frequencies

Understand the block diagram of an FM transmitter employing either frequency multipliers or mixers to generate the final frequency.

Fig. 2

Note A   In the diagram the Microphone feed the Audio Amplifier which when feeding the Oscillator directly is feeding it with  Frequency Modulation caused by the Microphone.

Note B  In the diagram an alternative is for the Audio Amplifier to feed Phase Modulation directly to the Buffer Amplifier. Phase modulation works by modulating, or changing the rate at which the audio sine wave  moves relative to an unmodulated wave.

frequency multipliers

This FM version is using frequency multipliers. The Audio signal is acting directly onto the frequency oscillator which then passes the signal to a buffer amplifier to make it suitable to go into the frequency multiplier stage. When is has been multiplied up sufficiently to the output frequency it is filtered and the driver prepares the correct level of signal for the RF amplifier, which is followed by filtering to present the signal to the aerial.

However this arrangement has for the most part been overtaken by the general transmitter diagram fig 1 which can also be used for FM - when suitable modulators are employed.

So what else is there to know ?

This general over view, above,  has shown you what happens to the signal from mic to aerial. The other sections, which follow below will give a more detailed account as to the activity of individual parts.

Fig. 1

When any piece of equipment, using an oscillator warms up, the frequency of oscillation changes. In the design stage of the equipment such variation will have been taken into account.

Should the frequency change after the warm up period then the effect of the change in frequency is called "drift".

You can sometime hear this drift of signals on the HF bands when older equipment is being used and to keep track of the frequency you would need to re-tune using the RIT. Then if you were to work the station your TX would go out on the original frequency and you would be Receiving on different frequencies as controlled by the RIT.

Should you own old equipment then you must keep the amount of the "drift" to a minimum less than 3kHz else you could move in frequency out of the pass band of the station that is tuned to your signal. This drift would cause interference to other stations using nearby frequencies and could result in out of band transmissions - hence the reason to minimize drift.

There could be calculations in relation to this question such as how much drift could be allowed to occur before reaching a band edge. Drift is considered in ppm which is Part Per Million. so at 145MHz a drift of 40 ppm would be 145 x 40 = 5800Hz or 5.8kHz so with the band edge at 146MHz you could only go upto  146,000,000 - 5,800 = 145.994MHz.

Understanding ppm and that MHz is a million Hz make the calculation quite easy when you bring everything to Hz and then convert back to MHz etc

The start of this process is with the crystal oscillators. These have to create :-

• Stable

• Accurate

• Known

frequency suitable for the part of the circuit in which they are employed.

The reason for the stability is so that they do not "drift" off frequency and possibly put the transmissions outside the amateur bands but almost certainly a drift in frequency would take you into another QSO!! (Drift is a slight and gradual, yet unwanted change or frequency.)

The reason for the accuracy is that you must know for absolute certainty that you are inside the amateur bands. It is desirable to ensure that you use the appropriate frequency for the mode you are using - SSB, FM, CW or data ETC.

There are several ways to achieve the above:-

Crystal oscillator

The old way was to use a crystal oscillator and to change the crystal to change frequency, very stable and the frequencies are accurately known but limited to the number of crystals you have and they were and still are expensive!

VARIABLE Frequency Oscillator

Another old way was to use a VARIABLE Frequency Oscillator. This gave you an infinite number of frequencies in a section of the amateur band.

Changing bands involved using a harmonic (multiple) of the same oscillator and retuning later stages of the transmitter to suit. The draw back was that such units were affected by heat, impact, when, say, you tapped your rig (microphonics) and thus did drift off frequency, and could give additional modulation to the wanted mode.

Frequency synthesiser

The modern way is to use a crystal oscillator which gives a stable frequency and link this to a frequency synthesiser which then gives you an accurate range of frequencies.

In all the methods mentioned above a stable voltage is needed which is in addition to the voltage power source for the power amplifier -again this is to ensure stability.

The synthesiser has two variants the PLL or Phase Locked Loop and the DDS or Direct Digital Synthesis (the DDS is not covered here as not part of the syllabus).

HOWEVER there is one draw back in using a synthesiser rather than a crystal oscillator or VFO and that is what is called "noise level" - this is the back ground level of noise present in higher quantities than crystal or VFO oscillators.

WHY 2 Crystal oscillators?

These are in two different parts of the circuits and thus are performing quite separate operations (Note: some designs can use more than two crystal oscillators).

So what happens from microphone to aerial ?

The audio amplifier is linked to the modulator. Once the appropriate modulation has been applied, we now have a modulated signal which may be appropriately filtered but it is not on the correct frequency. This signal is then mixed with the output from the synthesiser to produce the desired output frequency, this then passes into the RF power amplifier and then is filtered again before it goes into the aerial.

So the transmitter has achieved

• Create an RF signal inside the amateur band - All oscillators correctly chosen and working properly.

• For the signal to be nice and clean - Appropriate design and filtering.

• For the signal to be stable - crystal oscillator.

• For the signal to have minimum band width for the type of transmission - filters and the use of linear amplification for SSB or other amplitude modulation modes.

• For the signal to have the correct power output - correct use of RF power amplifier

• for the signal to have minimum output on other frequencies - Band pass and harmonic filtering.