Synthesizer; Reference Frequency Generation; First Vco Frequency Generation - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: RF Board
The reference oscillator generates the 16.8 MHz signal that serves as the reference for all radio
frequency accuracy. It uses a proprietary temperature compensation circuit to keep the radio within
its specified frequency tolerance.
The receiver back-end uses the ABACUS II IC (U301) to demodulate all the way to baseband,
starting from the first IF.
3.1.2

Synthesizer

This section discusses the synthesizer components and detailed theory of operation.

3.1.2.1 Reference Frequency Generation

The reference oscillator (U600) generates a 16.8 MHz reference signal that is tuned onto frequency
via a DC-fed varactor input. The digital/analog IC (U502), which is on the command board and is
under the control of the serial data bus, generates the DC voltage to the varactor. The reference
signal from U600-3 is capacitively coupled into the prescaler (U601-21), where it is divided by 8. The
resulting 2.1 MHz signal is routed to the synthesizer IC (U602).
The 2.1 MHz signal is divided by 7, with the result, a 300 kHz signal, serving the following purposes:
• Input to the prescaler IC for second VCO reference
• A source for the negative voltage generator
• Input to the programmable reference divider

3.1.2.2 First VCO Frequency Generation

For reasons of clarity and simplicity, 800 MHz is used as the example product in all synthesizer text.
In the 800 MHz models, the feedback is taken before the doubler circuit of the VCO. Band-to-band
and kit-to-kit variations are noted in the text as required.
The first VCO in ASTRO radios is a thick-film hybrid transmission line resonator. Its frequency is
controlled by a DC-fed varactor bank.
A transmission line feedback path from J601-1 to C604 couples the output frequency back to the
prescaler. The signal from the prescaler output (U601, pin 40) is routed to the synthesizer input
(U602, pin 27), where it is divided by the A&B counters of the loop divider. The loop equations
required for calculating the counter values are as follows:
NOTE: These are examples — the prescaler modulus and the reference frequency are
programmable and vary from band-to-band. The examples that follow are for 800 MHz and
assume:
P / P + 1= 255 / 256 and F
EQUATION: N = F
EXAMPLE: N = (F
EQUATION: A = (fractional remainder of N / P) (P)
EXAMPLE: A = N / P = (72,000 / 255) = 252.8627; .8627 x 255
or A = 220
EQUATlON: B = [N - {A x (P + 1)}] / P
EXAMPLE: B = [64,480 - {220 x (255+1)}] / 255 or B = 32
Plug in the calculated numbers to test the value of N with the following equation:
EQUATION: N = B (P) + A (P + 1)
EXAMPLE: N = (32) (255) + (220) (256) or N = 64,480
6881076C25-E
= 6.25 kHz. For UHF and VHF, P / P + 1= 127 / 128 and F
r
/ F
vco r
/ F ) = (403 MHz / 6.25 kHz) or N = 64,480
vco r
3-3
= 5 kHz.
r
September 5, 2008