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
- Dual trace scope, 30mhz minimum bandwidth.
- DC Voltmeter, high impedance.
- Tuning tools for Toko coils. (improper tools may break the slugs)
Table 1 Switch S4 functions
|1||Interface: Off=RS432 On=CMOS
|2||Duplex: Off=Half On=Full
|7||On=Disable Watchdog Timer
|8||Frequency Select 2 *
|9||Frequency Select 1 *
|10||Frequency 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
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.
note: If the modem is to be used on only one frequency you can
- Set the modem for "Tune-up mode" ie: switch 6 ON, switches 1,2,3,4,5,7 OFF.
- Set the frequency to 30 mhz. See table 2.
- Turn on the modem power switch. Both the RDY and PTT LEDs should light.
- Connect the minus voltmeter lead to the modems "GND TERMINAL"
and the plus lead to test point 1, "TX VCO".
- Adjust coil L10 (under shield cover) for a meter reading of 3.6 volts.
- Connect the plus meter lead to test point 2, "
- Adjust coil L15 for a meter reading of 3.8 volts.
- 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.
- Measure the voltage at test point 1, 'TX VCO".
it should be between 1 and 2 volts.
- 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
adjust the VCO coils L10 and L15 for 2.0 volts on TP-1
and TP-2 when the desired operating frequency is selected.
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.
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.
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
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
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.