Amp Solid State Relay
ence frequencies which is applied to the programmable frequency divider, !C4, via Q28 and Q29,
So IC4 is the thumbw programmed by leel switches to divide the relevant difference frequency from Q27 to provide a 10-kHz output which is applied to the phase comparator, IC5.
Note, by the way, that the difference between the transmit and receive offset frequencies is 10.7 MHz, which is the required intermediate frequency.
So far so good But now we have to backtrack a fittle There is a problem in that IC4 cannot precisely divide frequencies that are not an exact multiple of 10 kHz. Therefore, that example of 595 kHz (the lowest difference frequency) is not valid And in fact those offsei os-i illator frequencies given above are not quite correct.
Because of the provision for 5-kHz channel spacing, the offset oscillator crystals are in fact 1666 Hz too hi^h When the third harmonic of each crystal is considered, it will be 5 kHz high So in normal operation, the crystals are pulled low by LI4 and L15 for X4, and L16 and L17 for X3, So the normal offset transmit frequency is 47.i) MHz (143,4-MHz 3rd harmonic) and the offset receive frequency is 44.2333 MHz (3rd harmonic is 132.7 MHz)
When these offset frequencies are subtracted from the vco. the range of difference frequencies will be600 kHz to 4.6 MHz. And note that 600 kHz is an exact multiple of 10 kHz,
When the + 5-kHz facility is switched on, L15 and Li 7 are switched out of circuit by diodes D24 and D25 so that now the crystals do run 1666 Hz hi^h and so the vco frequency is shifted up by 5 kHz,
Note that when the 10-
kHz outputs of IC6 and IC4 (the programmable divider) are locked together, ICS turns on Q30r This turns on Q18 and Q19 and thus allows the transmitter to operate. Thus the transmitter is prevented from producing signals which are outside the 144-to148-MHz band.
But what about that + 5kHz offset we have just discussed? When that is applied, it would be possible for the vco to operate at 148005 MHz and still produce a lock condition. The circuit design takes care of this possibility, too, since the thumbwheels are wired to only permit a maximum vco frequency of 147.99 MHz. When the 5 kHz is added, this gives a maximum vco frequency of 147.995 MHz, which is still inside the band limits.
Strictly speaking then, this means that only 399 channels are available with 10kHz spacing and 798 channels with 5-kHz spacing (144.005 to 147.995 MHz),
± 600-kHz Offset
Yet another factor has to be taken care of by the frequency-synthesizer circuitry. For repeater operation, the transmitter frequency usually has to be off set by minus 600 kHz from the receive frequency. Less often, it may have to be changed by plus 600 kHz, This condition coutd be met by adding more crystals to the offset oscillator circuitry, but tn this circuit it has been achieved digitally.
As well as avoiding the expense of extra crystals, the digital method of offset does not require any alignment. IC2 and IC3 are digital adders They add a code of 60 or 120 to the code applied by IC4. In the normal simplex mode, the addition of the (>0 code is the standard, f or -600-kHz repeater operation, this code is removed (controlled by D18 and IC2)
For +60frkHz operation, IC2 and IC3 are brought into play by D29 and D27 to add a code of 120 to IC4.
A neat advantage ot this scheme is that it allows the 'anti-repeater' operation whereby the receiver only can be shifted by ±600 kHz. This is achieved by the pushbutton in conjunction with Q23r Q24, and associated diodes. The advantage of the anti-repeater function is that it allows the operator to listen directly to his contact instead of via the repeater.
Note that when the 600kHz offset facility is in use, the out-of-band protection circuitry does not prevent transmission outside the band limits In this case it is up to the operator to make sure he or she does not transgress.
A +10-V regulated supply derived from Q1, Q2, and D2 supplies power to the vco, offset oscillator, frequency-synthesizer circuitry, and mix-down amplifier (Q28 and Q29), The + 10A regulated rail is also switched to various other sections of the circuit by Q4 and Q5, depending on whether the transceiver is in the receive or transmit mode,
When in the receive mode, the press-to-taIk switch is open and D3, D4, and D5 cannot conduct. Therefore, Q4 supplies the + 10-V receive rail. When the PTT switch is closed for transmit mode, D3 and D4 conduct, turning off Q4 and turning on Q5 to supply the + 10-V transmit rail D5 also conducts, turning on Q3 to supply the +12-V transmit rail.
The final two stages of the rf power amplifier Q21 and Q22, are powered directly from the 13 B-V (battery) supply as is the audio amplifier. This is OK since Q21 and Q22 are normally biased off and can only operate when Q19 and Q20 are turned on by the +12-V transmit rail.
In Part II of this article, the construction and alignment of the DSE Commander will be detailed ■
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