Tuneup instructions for the WA4DSY 56KB RF modem


The modem has a built in sweep generator and several test points to facilitate adjustment of the various inductors using only a 30 mhz dual trace oscilloscope and a voltmeter. After the transmitter is properly adjusted it may be used as a signal generator to align the receiver. The modems carrier frequency is synthesized. Eight independent transmit and receive frequencies are stored in memory addresses in the EPROM. The memory address is selected either with switches 8, 9 and 10 on DIP switch S4 (3 bit binary code) or by rotating switch S6 to one of 8 positions. The frequency data are read into the synthesizer chip only once when the power is first turned on. Changing the frequency selection switches while the power is on has no effect. Two of the frequencies, 28.000 mhz and 30.000 mhz , are intended for tune up use only. DIP switch S4 (7 or 10 switches depending on modem configuration) has 2 switches to control the test modes. Switch 5 (KEY TX) puts the modem in transmit mode and disables the watchdog timer. The modem will assert PTT and transmit scrambled mark bits when switch 5 is on. Switch 6 (TUNE-UP) causes the sweep generator logic to be loaded into the Xilinx FPGA chip when the modem is first powered up. Changing switch 6 while power is on has no effect. The built in digital sweep generator sweeps a 200 kHz range from 338 kHz to 538 kHz at a 243 HZ rate. The output drives the transmitter. It also applies a 243 HZ square wave to the RX and TX clock test points for scope sync. The rising edge of the square wave marks the center frequency (448 kHz) of the first IF filter. The scope can be calibrated to emulate a 20 kHz per horizontal division spectrum analyzer by adjusting the sweep speed and horz. position so the square wave is exactly 10 divisions wide. The other scope channel can be used to probe various test points to observe the amplitude vs frequency response. All the tune-up procedures assume that one scope channel is connected to TP-7 (RX CLK) and the scope is triggered from that channel only. The tune-up procedure is straight froward. First the transmit and receive VCOs are are adjusted so they will tune 28 to 30 mhz. Next, the transmitter is aligned. Then the transmitters signal is used to align the receiver. The transmitter signal must be attenuated by 50 DB to avoid receiver overload. See the schematic below.

50 DB attenuator

Detailed Tuneup Instructions

Equipment Required

Table 1 Switch S4 functions

1Interface: Off=RS432 On=CMOS
2Duplex: Off=Half On=Full
3On=Repeater mode
4On=Disable Scrambler
5On=Key Transmitter
6On=Tuneup Mode
7On=Disable Watchdog Timer
8Frequency Select 2 *
9Frequency Select 1 *
10Frequency Select 0 *
Switches 8, 9 and 10 may not be present if the modem is equipped with a front panel rotary switch.

Table 2 frequency selection


VCO Tuneup

The goal is to adjust the VCO coils so the modem will tune from 28 to 30 mhz. The transmit VCO coil (L10) is under the shield assembly. The receive VCO coil (L15) is near pin 24 on U12. Please follow the step by step instructions below.
  1. Set the modem for "Tune-up mode" ie: switch 6 ON, switches 1,2,3,4,5,7 OFF.
  2. Set the frequency to 30 mhz. See table 2.
  3. Turn on the modem power switch. Both the RDY and PTT LEDs should light.
  4. Connect the minus voltmeter lead to the modems "GND TERMINAL" and the plus lead to test point 1, "TX VCO".
  5. Adjust coil L10 (under shield cover) for a meter reading of 3.6 volts.
  6. Connect the plus meter lead to test point 2, "
  7. Adjust coil L15 for a meter reading of 3.8 volts.
  8. Turn OFF the modem power and set switch 10 OFF (8 and 9 on) or set switch S6 to position 6. This sets the modem to 28 mhz.
  9. Measure the voltage at test point 1, 'TX VCO". it should be between 1 and 2 volts.
  10. Measure the voltage at test point 2, RX VCO". It should be between 0.2 and 0.8 volts. The VCO may be out of lock if the voltage is outside this range.
note: If the modem is to be used on only one frequency you can
adjust the VCO coils L10 and L15 for 2.0 volts on TP-1
and TP-2 when the desired operating frequency is selected.

Transmitter Adjustments

Before starting be sure to set the TX LEVEL control (R20) to midrange and install a 50 ohm load on the TX Output connector. The 28 to 30 mhz transmit bandpass filter (L7 and L8) should be adjusted for maximum output at test point 9 with the modem transmitting close to 29 mhz (29.05 for example). This filter is 2 mhz wide and should be checked at 28, 29 and 30 mhz to make sure the whole band is passed at approximately the same level. The level should be between 500 and 800 mv peak to peak. The 448 kHz transmitter filter (L3, L4, L5) should be adjusted until the frequency response envelope is centered on the rising square wave edge (448 kHz) and is as flat across the top as possible. See figure 1 below.

Figure 1.

The amplitude should start to roll off at plus and minus 40 kHz (2 divisions) from center. Inductor L4 will have the largest effect. All 3 Inductors interact and proper alignment may require several passes through L3, L4, and L5. Fortunately, modem performance is not affected much by moderate filter adjustment errors. You can observe the transmitter passband response at either test point 8 or 9. The signal at test point 8 is only about 50 millivolts peak to peak at 448 kHz. You may need a 1:1 scope probe to get enough amplitude to see the response shape. The signal at test point 9 is about 600 millivolts (with a 50 ohm load on the modem) at the modems transmit IF frequency (28-30 mhz). The response at TP-9 includes the effects of all the filters in the transmit chain, including F1 and F2. If the response shape at TP-9 is incorrect you should check TP-8 to rule out problems with F1 or F2. If L21 or L22 are defective or the incorrect value the frequency response can be adversly affected. You may probe any point in the transmit chain to observe the frequency response and isolate trouble.

Receiver Alignment

Once the transmitter is properly adjusted it can be used as a signal generator for receiver alignment. A 50 db attenuator must be used between the transmitter output and the receiver input. If an attenuator is not available you can build one with two 51 ohm resistors and one 8200 ohm resistor. A less satisfactory method is to place a 15K ohm resistor between the TX output jack and the RX input jack. This slightly over drives the receiver and results in slightly distorted bandpass envelopes on the scope. Use this method only if a 50 db attenuator is unavailable. In the production environment a reference modem should be built and used as a signal source. Prepare the modem for receiver tune up by selecting the desired operating frequency. Be sure to cycle the power to make the frequency change effective. With the attenuator connected between TX output and RX input and a scope probe connected to TP-5 (IF), inductors L13 and L14 should be adjusted for maximum signal level. L18, L19, and L20 can now be adjusted to center the frequency response envelope with the best flatness. The frequency response should start to roll off at plus and minus 1.5 divisions (30 kHz). See figure 2. As with the transmit filter, these three inductors will interact and several passes may be required to obtain the best frequency response shape.

Figure 2.

After tuning L18, L19 and L20, the modem should be turned off . Put the modem into "KEY TX" mode by turning off switch 6 on S4 and turning on switch 5. You must also select a simplex frequency (TX and RX frequency the same) to allow the receiver to hear the transmitter. Turn the power on. All the front panel LEDs should be lighted. Adjust inductor L11 for a 2.5 volt reading at test point 3 (EYE). You may now look at test point 3 with the scope to verify that the "eye pattern" looks clean. See figure 3. You may also use a signal generator or a reference modem transmitting on the desired receive frequency for the signal source.

Figure 3.