Exciter T500.
The circuits needs some modifications to correctly modulate the MMDVM signal.
The 3 Khz low pass filter just before the modulator ( U7B) needs to be modified for 5 Khz audio roll off . Replace C57 100nf with a 39nF.
Put a jumper between JP3 pin 3 and the right side of R101 ( towards the front Panel )
The
modulating signal from the INADVM can be fed from the rear DB25
J1 at pin 19 . Gnd from J1 pin 13 or 25 . The PTT ( Low active)
is hooked to J1 Pin 3 .
Remove ALL the Following Jumpers : JP3, JP7, JP8, JP9, JP11,
Adjust RV1 and RV3 Full CCW
Adjust the INADVM TX level to full CW ,
Using MMDDVMCal program from G4KLX github https://github.com/g4klx
Start MMDVMCal ( ex : ./MMDVMCal /dev/ttyACM0 )
Select the DMR modulation ( key D)
Adjust the TX level to 84 with the T Key ( Shift t)
Generate a PTT command ( Space Bar)
Adjust RV2 ( Mod Balance ) for a Carrier NULL .
In the file MMDVM.ini [modem] adjust the TX level to 80 and TxInvert to 0
Receiver R500
The receiver is adjusted using the procedure in the manufacturer's manual.
Remove jumper Jp4, Jp8 Jp9 and put a jumper on Jp6 and Jp7 between pins 2 and 3
Use the Discriminator output from the receiver . The connector P1 ( DB25 ) in the back of the receiver has pin 18 hooked to the Discriminator audio . Pin 5 is Audio GND .
Signal RSSI can be routed to the back DB25 by putting a jumper between JP4-2 and the junction of R22 and C40. The RSSI signal is also available on the front DB9. Use a ohmmeter between Front Panel Db9 pin 7 to identify the juction point.
Receiver
Adjustment
Hook Trace B of a scope to receiver COS- pin 16 2 Vdc /div .
Hook Trace A of a dual trace scope to DISCR ( J1 pin 3 ) and GND Adjust the scope trace A at 0,2 V/Div DC coupling. Adjust the Discriminator output RV1 for minimum distortion and 0.4V P toP with a generator modulated at 2,7 Khz Deviation.
Hook Trace A of a dual trace scope to INADVM U1B pin 6 or 7.
Adjust the scope trace A at 0,5V/Div DC coupling . Position the GND level at -1.6 Vdc.
Scope Horz TB to 1 mSec/div trigger Normal to trace B Dc coupling
Adjust the Rxlevel to 50 RXInvert to 1 and debug to 1 in the [modem] section of MMDVM.ini
Start MMDVMHost
Adjust INADVM RX pot for 2 V P to P at U1B pin 6 and 7.. Jumper J1 at 3.3 V (A) when transmitting a DMR signal ( MD380 ) . Adjust the receiver TCXO for trace A between +1 and -1 Volts . If the trace can not be centered between +1 and -1 Volts readjust slightly Discriminator coil L10 and readjust TCXO
Transmit to the receiver using a DMR radio . Look for a setting of 750 +- 50 for the Treshold reading ( The fourth number in the line of MMDVMHost screen...
(Ex.)Debug: DMRSlotRX: voice terminator found slot/pos/centre/threshold 1 449 -548 772
Adjust INADVM RX Level pot R12 for a reading of 750 +- 50 for the treshold reading ….
Re trim RV1 TCXO and L10 of RX and R12 of INADVM for MMDVMHost treshold reading at 750 and minimum BER.
The BER for long transmissions should be very near 0.0 % …..