6 Theory of Operation
The C-2002 is a Digital Signal Processor (DSP) based product. Because of this, many of the signals that once could be probed on older products, are handled within the DSP itself. This would include DTMF generators and decoders, notch filters, tone generators and decoders, and all of the audio summing. A great deal of the gain controls are included within the software of the DSP. There are four analog audio sections on the C-2002, they are:
RX Input: | Audio received on the line from an external source. |
RX Output: | RX Input audio that has been sampled and processed by the DSP, then sent to a local speaker. |
TX Input: | Audio from a local microphone, handset or auxiliary audio source. |
TX Output: | TX Input audio that has been sampled and processed by the DSP, or internally generated |
| Tones/DTMF that is summed with TX Input audio. The result is routed to TX line out. |
The following sections discuss the portions of the C-2002 that can be tested using a common oscilloscope and voltage meter by a qualified service technician.
6.1 Microphone Input Paths
Audio can be input from four locations. These are the desk microphone jack (J24, RJ-11 modular), handset jack (J21, RJ-12 modular), panel mic (built into keypad PCB), and the auxiliary connector (J36) on the rear of the unit. For each of these inputs, a single inverting Op-Amp gain stage exists to bring the maximum expected level of the audio up to approximately 3.8Vp-p. This is the optimum level for the analog to digital converters (ADC). The trim potentiometer R196 set the audio levels from the desk microphone jack, there is no Handset microphone gain adjustment. All audio inputs, mentioned previously, share one half of the ADC (U12) with a CMOS switch (U34-B) used to route audio from the appropriate source to the actual input pin of the ADC. The auxiliary input (J36-4) is used to include an additional audio source into the transmit path. It has an integrated PTT (J36-2) input that when pulled low will select audio from this source for transmission. Audio or tone levels injected at the Auxiliary input should be on the order of 1.5 Vp-p, as the fixed gain ratio of R190/R189 = 2 and the target ADC level is approximately 3.5vp-p. There are no hardware adjustments available for the Panel microphone.
6.2 Audio Output Paths
SPEAKER AUDIO:
Speaker audio originates, as all audio output signals do, from the DSP. The digitized audio is clocked to DAC U10, pin 1. The analog audio is sourced on U10 pin 8, passes through an analog filter and inverting gain stage U14-A, before arriving at the speaker audio driver U24. The two Watt speaker driver audio is passed to the speaker connector J5. Speaker audio levels are software controlled.
TRANSMIT AUDIO LINE (ONE/TWO):
Transmit audio, still digitized, is clocked to U9 pin 1 for both transmit channels. Analog transmit audio can be seen on U9 pins (8/5). Transmit audio passes through an analog filter before reaching the output gain stage. TX output levels can be adjusted for lines (1/2) by potentiometers (R125/R143). Each Transit audio path is 600 Ohm transformer coupled (T1/T4) and engaged during PTT by a relay contact closure (K4/K6). Transmit audio can be found on connectors (J11/J27) pins 4(TX+) and 5(TX-).
HANDSET SIDETONE:
Handset sidetone is taken from U10 pin 5 and filtered. The output goes to an inverting op-amp stage and is coupled through a 0.1uF cap and 150 ohm resistor. The sidetone level can be measured at TP10.
For complete schematic details, refer to Section 10 the Specifications section.
6.3 Line Receive Paths Line (One/Two)
Receive audio input can be found on the Line input connectors (J11/J27) pins 3(RX+) and 6(RX-). The receive audio will pass through lightning protection and filtering before reaching a 10K transformer (T2/T3). The receive audio path can be loaded to 600 Ohms by placing jumper (J17/J25) in the “A” position. Following the transformer stage is a gain stage. Potentiometers (R153/R161) can be used to set receive audio levels at the ADC. This level can be measured at test points (TP11/TP12). Finally, receive audio is sampled at U13 (pin 8/pin5).
