Operational Description

FCC ID: AJK8222168

Operational Description

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$1.1 OPERATIONAL DESCRIPTION Major circuit schematics are attached to this report as Exhibit E. 1.1.1 GENERAL DESCRIPTION OF CIRCUITRY The VHF-2100E consists of 4 sub-assemblies; VHF RF Assembly, Digital/Power Supply Assembly, Rear Interconnect Assembly, and Front Panel LED Assembly. The VHF-2100E power amplifier linearly amplifies the transmitter synthesizer output to a 25 watt (minimum) carrier power level for AM voice or analog data operation. The power output in VDL Mode 2 is a minimum of 18 Watts. Voice, analog data, or digital data is impressed on the input RF from the synthesizer in a closed feedback loop vector (I-Q) modulator system, used as a variable RF attenuator and phase modulator. A synchronous detector sampling the RF power output provides amplitude and phase (I-Q) feedback information to the vector modulator to linearize the amplitude and phase characteristics of the power amplifier. A low pass filter at the output attenuates the transmitter harmonics further. The worst case harmonic found during testing was 76 dB below the carrier level. The VHF-2100E receiver utilizes a single conversion design. The receiver synthesizer injection is applied to the first mixer, generating a 28.900 MHz first IF frequency (high side injection) which is amplified and then detected by use of Digital Signal Processing technology. For voice operation, a noise quieting squelch and carrier squelch, both implemented in DSP, are provided. In data modes, no audio is presented to the operators. For receive operation, the VHF-2100E frequency synthesizer is a single loop, TXCO reference controlled design and provides the receive LO for any 25 kHz or 8.33 kHz channel selected in the 118.000 to 152.000 MHz frequency range. A second, dedicated frequency synthesizer is used for transmit operation. It is a single loop, TXCO reference controlled design and provides the direct transmit frequency RF drive for any 25 kHz or 8.33 kHz channel selected in the 118.000 to 152 MHz frequency range to the vector modulator and RF power amplifier. Both receive and transmit frequency synthesizers use a common 23.1 MHz +/- 5 PPM temperature compensated crystal oscillator (TCXO) as the reference. Both synthesizers use advanced DSP technology, sigma-delta modulated fractional-N synthesis, to improve spurious performance and reduce phase noise while providing small channel spacing. 1.1.2 NECESSARY BANDWIDTH CALCULATIONS Because of the high linearity of the VHF-2100E transmitter, the required bandwidth for transmissions follows the mathematical rules for the mode of communications in use. For DSB AM transmissions, the bandwidth for 99% power is twice the modulating frequency. Two modes of AM are used, 5K00A3E and 10K0A3D. In both cases, the 99% power bandwidth is twice the modulating frequency. The tests are performed with a modulating frequency of 2.5 kHz, yielding an occupied bandwidth of 5 kHz. In DO-281 and DO-271 data modes, 14K0G1DE and 14K0G7WET, system MOPS describes a power profile both in channel and in adjacent channels. The VHF-2100E out-performs the MOPS by typically 6 dB, and often as much as 10 dB. 1.1.3 CIRCUITRY FOR SUPPRESSION OF SPURIOUS RADIATION The low-pass filter suppresses transmitter spurious radiator. This is located on the RF Assembly and shown on schematic 828-1436-002. An additional low pass filter, common to both transmit and receive functions is in the common antenna cable from the unit. This$

filter provides additional filtering for transmitter harmonics in the 1.5 GHz GPS band.
Non-harmonic spurious radiation is reduced by the advanced design of the phase locked
loop frequency synthesizers and careful circuit board design.
1.1.4 CIRCUITRY FOR LIMITING MODULATION
Modulation limiting is achieved in the audio compressor that is implemented in software
on the Signal Processor. As the compressor characteristic curves in this report show, this
compressor circuit operates as a linear amplifier until the input signal reaches such level
that the software begins to reduce the gain by mathematically scaling the internal digitally
processed signal. The large dynamic range above the compression knee minimizes the
possibility of over-modulation. This guarantees that the RF output cannot go to zero,
even during severe overdrive of the audio input to the compressor.
1.1.5 CIRCUITRY FOR LIMITING POWER
The carrier power output is controlled by adjusting the DC component of the audio signal
applied to the modulator controller, U800 on the RF assembly, 828-1436-002. This DC
level is a service adjustment, not accessible to the operator.
A software controlled reduction of peak power occurs during times of low power supply
voltage (less than 22.0 VDC) to prevent distortion of the positive modulation peaks. This
is also a service adjustment and not accessible to the operator.
Separate modulation adjustments are provided for 25 KHz channel and 8.33 KHz
channel operation. The 25 KHz channel and 8.33 channel modulation levels are set in
software by values written into non-volatile memory. This is a service adjustment and not
accessible to the operator.
1.2 DIGITAL MODULATION TECHNIQUES
The Modulation format of the VHF-2100E Mode 2 is emission designator 14K0G1DE and
14K0G7WET for Mode 3. These are digital 31.5 kB/sec (10.5 kSym/sec) D8PSK
waveforms using raised cosine shaping with excess bandwidth factor of 0.6.
The modulation waveform is generated in software by a Digital Signal Processor in
complex form (I,Q) with 12 bit precision. 12 bit resolution D/A converter outputs at an 84
k/sample rate is filtered by a 2 section linear phase type low pass filter with –3 dB cutoff
frequency of 8.5 kHz. The amplitude and phase response of the low pass filters is
shown in Table 1.
Table 1

Frequency (Hz) Amplitude (dBr) Phase (Degrees)

100 0 -32

1000 0 -35

2000 -0.2 -40

3000 -0.3 -43

4000 -0.5 -46

5000 -1.5 -48

8500 -3 -50

10000 -4 -55

20000 -10 -70

100000 -30 -80

The filtered I, Q outputs are applied to a Vector (I,Q) modulator which creates the
composite D8PSK waveform directly at the desired transmit frequency. Figure 1 shows a
typical I channel baseband modulation signal during transmission of a VDL Mode 2 or
Mode 3 message. The Q channel will be similar. Figure 2 shows the transmitted I and Q
constellation showing the composite phase and amplitude characteristics produced by
the I and Q signals. The constellation data points are shown in contrasting color.

Figure 1

2
1.453733

1

RRxmt 0
i

1

1.453733 2
100 200 300 400 500 600
90 i 600
I channel

Figure 2

Transmitted Signal

Im( xmti)
Quadrature

Im( Circj)

Re( xmti) , Re( Circj)
Inphase

Document Created: 2006-02-16 15:57:54
Document Modified: 2006-02-16 15:57:54