Board construction
The TX1 main board is a small (94 mm x 50 mm) double side PCB. All components are of SMD type and are assembled on the top layer. The bottom plane provides low impedance RF and DC path as well as good heat spreading. The TX board is installed on bottom of the aluminum enclosure. The JUMA-TX1 can be built without winding any coils or transformers!

Features
The board contains the RF path from DDS board to the antenna. There are three amplifier stages which provide the required gain. CW keying is handled by the first amplifier. Driver and the final stages are using low cost SMD MOSFETs intended for switching applications. The board includes a semi break T/R switch (VOX) and low-pass filters for the 80 m and 40 m bands. Although the TX is a CW transmitter the MOSFETs are biased for linear operation to achieve the required gain and to maintain an excellent keying performance. Nominal output power is 5W with a 13.8V supply voltage and typical current is less than 1.2 A.

CW keying amplifier
The RF input signal, 0.25Vp-p, coming from the DDS board is fed to the keying amplifier IC1. It is a tiny single gate CMOS inverting buffer which is biased for linear operation. The gain is adjustable by the trimmer R2. The keying is made by switching the supply voltage of this amplifier. Key-down signal is pulling down the input of the Schmitt trigger inverter IC2. This drives the "click" filter R17/C27 which provides good clickless and prompt keying. Filtered keying DC signal is buffered by transistor TR4 which provides the supply to the keying amplifier IC1.

Semi break T/R switch
The key down signal is charging the C28 which drives the Schmitt trigger input of IC3. The output of the IC3 controls the antenna relay and the bias voltages. The semi break (VOX) release time can be adjusted by changing the value of capacitor C28 and/or resistor R18. The STBY signal is used to switch the TX to the standby state.

Driver and final stages
D-pak type SMD MOSFETs are used in the RF amplifier. TR1 is the driver amplifier. Input impedance is damped down to 50 ohm with R3 to eliminate the effect of the input and reverse capacitance of the MOSFET. The driver MOSFET operates in class A. The bias current is adjusted with the multi turn trimmer resistor R4. Source resistor R5 and the capacitor C6 are used to align a flat frequency response from 3 MHz to 8 MHz. A commercial SMD common mode signal filter is used as a wide band RF transformer (T1). The filter is made by TDK and has a turn ratio of 1:1. It has an excellent RF performance at the selected impedance level.

The secondary winding of the transformer T1 is differentially driving the two final stage SMD MOSFETs (TR2 and TR3) which are in a push-pull configuration. This stage is operating in class AB. The bias currents are adjusted with the multi-turn trimmers R6 and R9.

The output transformer T2 is also a commercial SMD common mode signal filter from TDK. This type has four windings. The drain-drain to antenna turns ratio is 1:1 which theoretically provides a maximum of 7 watts with a 13.8 volt supply. See the circuit diagram for connections of the windings. According to the measurements the TDK signal filter shows a very good performance as an output transformer and it can handle the 5 watt power level without over heating.

Low-pass filters
The low pass filters for the 80 m and 40 m amateur band are made of commercial SMD inductors of size 2220. They can handle the 5 watt level just fine. The changing of the filters is controlled automatically by the software in the DDS board. When the frequency is higher than 4 MHz the 40 m low-pass filter is selected. MOSFET TR7 is taking care of this filter change.

Relative output indication
Diode D4 and related components are used for relative power detection. The recovered DC signal is scaled to the DDS board analog input range of 0…0.234 V. The relative output is shown graphically as a bar in the LCD module.

Voltage regulator
The keying circuits, T/R switch and the bias supply voltage is +5.7 V, which is coming from the regulator REG1. The diode (D5) is used as a negative temperature compensation for the bias currents of the RF amplifier MOSFETs.

Failure protection
Over current protection is handled by the small size, on board fuse. In case of a wrong power supply polarity, the fuse will also blow with the aid of the reverse diodes in the MOSFETs TR1…TR3.

Special features

• During reception the antenna signal is connected to a RX via the relay contacts RL1. The antenna signal path is going through a low-pass filter to improve spurious response in a RX. This means that the TX frequency must be set to the same band as the RX. The 80m/40m crossover frequency is 4 MHz.
• JUMA-TX1 has no built in side tone generator, but the T/R switch is designed so that the transmitted CW signal can be monitored at a convenient level from RX. In STBY state you can key down and spot your frequency with a RX.
• During transmission the +5.7 volt DC is connected to the antenna output via resistors R10 and R11. This is the "JUMA protocol" which is used to activate an optional, external PA. This output signal will not harm anything, because it can supply only approx. 1 mA to the antenna output.
• The T/R switch can be disabled by turning the switch in the front panel to "STBY". You can then key down and spot your frequency with an RX or activate the DDS output for e.g. frequency measurement purposes.

Required equipment for adjustments
V/A/Ohm meter, 13.8V/1.5 A DC supply and a 50 ohm/5W dummy load. A home made dummy load can be used. Example: Three 120 ohm/2W resistors in parallel (do not use wire wound types).

Caution 1. The TX board must be installed on the bottom of the enclosure or on a similar aluminium plate to assure proper cooling.
Caution 3. Do not connect the power supply without checking the voltage and the polarity.

Before connecting the power supply
Turn the three multi turn bias trimmers R4, R6 and R9 so that the resistance from gates to GND of the MOSFETs TR1, TR2 and TR3 indicates minimum resistance. This ensures that the bias currents of the MOSFETs are zero in the beginning. This is important because otherwise the current can be very high and the fuse may blow.

Power supply check
Check and adjust the DC power supply voltage to 13.8V (±0.2V). Current limit, if available, is recommended to be adjusted to 1.5 A.

Bias adjustments
For bias adjustment cut out the RF drive from DDS board by selecting 0 Hz frequency or by disconnecting the RF-input (marked "RF IN" on the TX main board) by de-soldering the "hot" wire. Connect a current meter in series with the power supply + wire. Switch ON the power supply and JUMA-TX1 and turn the STBY switch to OPER position. Connect a CW key or a push button switch to the "key" connector and key down to activate the T/R switch. Now make a note of this stand-by current typically approx. 50 mA (approx. 15 mA more in 40 m band). First adjust the final stage bias by turning the trimmers R6 and R9 until each trimmer increases the current by 5 mA. Next adjust the driver bias by turning the trimmer R4 until the current increases 100 mA. The final current should be the stand-by current + 110 mA. Finally, reconnect the RF input coming from the DDS board.

Power output adjustment
Note1: A DC current meter in the power supply wire can be improper during RF power tests because the meter has internal resistance and causes a voltage drop with 1A current. Replace the meter with direct connection.

Note2: Do not use antenna as a load during adjustment because the impedance is unknown. Use only a 50 ohm dummy load.

Note3: Do not key down with carrier longer than 1 minute. This is importamt to avoid over heating.

Adjust the frequency to approx. 3.5 MHz. Key down and turn the RF gain trimmer R2 until the LCD relative output shows two bars below full scale in the LCD. Then you have 5 W output. Next adjust the frequency to 7 MHz and observe the 7 MHz low pass filter control functionality (audible click from the relay when passing 4 MHz). Check output power at 7 MHz. In the LCD relative power display, 3 bars difference between the bands is ok, that equals approx. 1 dB. If there is a large output power difference between the two bands, the gain can be compensated by changing the value of capacitor C6. The output power can be verified with an oscilloscope if available. It should show 45 Vp-p for 5 watts output. Typical power supply DC current is approx. 0.8 A at 3.5 MHz and approx. 1.2 A at 7.0 MHz.

Note4: When the TX is connected to an antenna, the relative RF output display can show higher or lower readings. This is normal because the relative output detector is a voltage detector and thus the antenna impedance affects the relative output reading.
 

Description of the signals (markings on the board, refer also to the circuit diagram)

RF IN (J1, J2)
RF input to the TX board. Nominal signal level 0.25 Vp-p. Input impedance approx. 1 kohms with a parallel capacitance of approx. 20pF.

KEY (J3, J4)
Active low CW key input. Can be driven by a conventional key, relay, open collector or open drain circuit. Open circuit voltage +5.7 V and active low sink current < 2 mA.

TX (J5)
TX transmit state output. High (+5.7 V) during transmit state. Note: TX signal will go high also in stand-by state when the "VOX" is activated by a key input.

STBY (J6, J7)
Stand-by select input. When this signal is grounded, the transmitter is in stand-by state and the T/R switch will not activate with a key signal but the DDS and RF driver will be activated whereupon you can spot the frequency.

7M (J8)
Band selection input. When high (3 V...15 V) selects the 40 m band low-pass filter. When low (< 1V) selects the 80 m band low-pass filter.

REL (J9)
Relative RF output DC signal. Output of 0.22 VDC corresponds to 5 W RF output with a nominal 50 ohm RF load. Output impedance 5 kohm.

+ (J10, J11)
Switched +13.8 VDC supply output for the DDS board

ANT (J13, J12)
Antenna connection, nominal 50 ohms.

+14 (J15, J14)
Power supply input. Voltage range +12 VDC...+15 VDC. Protected by a 2A fuse on the TX board.

RX (J17, J16)
Antenna output for RX. Please observe that the receive path goes via the PA low-pass filters.

ON/OFF (J18, J19)
Pads for power ON/OFF switch. Please observe that the power supply is directly connected to the driver and final stage MOSFETs.