Pozosta DF2FQ-T7F User Manual

Manual for the

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Amateur Radio Directory

70cm-FM/FSK-Transceiver

T7F

Holger Eckardt

DF2FQ

Kirchstockacherstr. 33

D-85662 Hohenbrunn

4199HE

Technical Data

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Amateur Radio Directory

General
Frequency range: 430 ... 440 MHz
Channel spacing: 12.5 and 25kHz
Receive-transmit delay time: <30ms
Temperature range: -5 ... +50° C
Power supply: 7 ... 14V, 60mA RX, max. 2.5A TX
Size: 145 x 75 x 22 mm

Receiver
Sensitivity: -118dBm for 20dB SINAD (CCITT) @1kHz
AF frequency response: 1Hz ... 7,000 Hz (-3dB)
AF total harmonic distortion: <1%
Intermodulation response: -54dB (3-tone test)
Adjacent channel response: <-56dB
Spurious response: <-60dB (1st image), <51dB (2nd image)

Transmitter
RF power: 1.5W at 7V, 6.5W at 12V
AF frequency response: 1Hz ... 15,000 Hz (-3dB)
AF distortion: <1%
Spurious transmission: -66dBc (1st harmonic), <-75dBc else
Spurious transients: <-40dBc on the adjacent channel

Circuit description

The circuit diagram is spread over four sheets. Figure 1 shows the synthesizer with modulation circuit,
figure 2 the receive section, figure 3 the transmitter and figure 4 the control circuit.

Synthesizer

Heart of the synthesizer is the VCO (voltage controlled oscillator) which supplies RX and TX as well.
A helix coil guarantees low oscillator noise and low sensitivity against microphony. Separate varicaps
are used for tuning and modulation. The VCO works on half the transmission frequency to decouple it
from interference by the power amplifier. A doubler follows the VCO, which again is followed by a
buffer amplifier. The attenuator between the stages gives some additional decoupling. In the collector
circuit of the buffer a notch filter is used to suppress the VCO frequency.

The synthesizer chip MB1504 controls the VCO. The current source of the internal phase detector is
much too weak for the fast switching time that is needed, so there is an push-pull amplifier placed on
the output of the phase discriminator. It drives the low-impedance loop filter.

The frequency response would be insufficient for packet radio if we would apply the modulation signal
only to the VCO. Below the cut-off frequency of the loop filter the deviation would decrease with 6dB
per octave. Since the cut-off frequency is 700Hz on 10Hz we wouldn’t have hardly any detectable
signal of the modulation. Therefore the reference oscillator is modulated as well. The frequency
response of this path is complementary to that of the VCO so both paths together give a perfectly flat
response.

The reference oscillator is also used to drive the second receiver mixer. Since the reference must be an
integer multiple of 25kHz the IF (intermediate frequency) becomes 450kHz instead of the conventional
455kHz. This has to be considered at the crystal filter.

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Receiver

Two helix filters act as pre-selector, one ahead and one behind the low noise amplifier T6. The dual­gate FET (field effect transistor) T5 acts as mixer. To obtain a good intermodulation response both
stages are driven with a relatively high supply current. Printed inductors match the high impedance
gates of the FET. The drain provides the IF signal. Due to the strong requirements for a flat group
delay a trimmer is used to optimize the matching of the crystal filter. The filter is followed by a buffer
amplifier and than by the IF circuit MC3371. Beside the 2nd mixer this IC contains a limiting amplifier,
the demodulator, a RSSI (radio signal strength indicator) circuit and an operational amplifier. The
latter is used as a 2nd order low-pass filter to suppress IF spurious on the output signal. The ceramic
filter for the 2nd IF is internally compensated for flat group delay response.

The RSSI output provides a current which is proportional to the logarithm of the RF (radio frequency)
input voltage. With a buffer amplifier this signal is good to drive a S-meter. It is also used to generate a
fast DCD (data carrier detect) signal which is advantageous in particular when operating over
multimode digipeaters. Within the dynamic range of the RSSI the potentiometer R53 determines the
trigger threshold.

Transmitter

The driver T7 boosts the VCO signal up to 30mW. This is sufficient to drive the PA (power amplifier)
module which at 12V supply voltage delivers an output power of 7W. Behind the low pass filter and
the pin diode switch a power of 6W or more is available. T8 and T14 generates a linear ramp with a
time constant of 5ms. The slow ramping of the PA avoids spurious signals in the adjacent channels. A
5V regulator supplies all stages of the transceiver except driver and PA module. These gets the
unregulated supply voltage directly.

Control circuit

A micro controller IC is used to control the whole transceiver. It polls the PTT (push to talk) line,
programs the synthesizer chip, switches receiver and transmitter path in a well defined time scheme and
checks the channel select ports. The required software is stored in the EEPROM within the chip.

Construction

The PCB (printed circuit board) artwork for the transceiver is shown in figure 5. It fits on an area of
144 x 72mm. You can find a part list at the end of this text. Those components which are marked with
n.p. in the schematics must not be placed on the board. Values of the capacitors are partly printed in
exponential expression, 102 e.g. means 1nF, 473 means 47nF. Basically it is the same as with resistors
only instead of colors numbers are used.

It is recommended to start the construction by fitting the low-profile parts (resistors, RF-transistors,
etc.), then the capacitors and AF-transistors and finally the larger parts such as crystals and filters. No
sockets must be used for the ICs except for IC1. This one however should be placed on a socket as it
makes software update much easier. The flat RF transistors have one long terminal, for the bipolar
types it is the collector for the FET it is the drain. The type numbers always look away from the PCB.
The heat sink of T8 looks to the border of the PCB. D2 (BB405) normally does not have any printed
type number on it, it can be recognized by the black body with a white ring. The resonator Q2 already
includes the two feed back capacitors. It has a bubble-shaped blue body with three terminals.

Three inductors have to be wound manually. They are marked with 3T3D in the schematics. This
means 3 turns, 3mm diameter. Silver plated copper wire of 0.4mm diameter should be used. The
terminals of L5 and L11 are stretched to the distance of the through-holes. L12 is wound as tight as

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possible . All other fixed inductors look like thick resistors, they are coded by colors. L14 (3.3uH,
orange orange gold) and L2-4 (0.33uH, orange orange silver) look very similar, so do L16 (0.1uH,
brown black silver) and L18 (1uH, brown black gold).

The only component which is soldered from the rear side of the board is the power module. It is
mounted in such a way that the heat sink looks away from the PCB. The distance between the flange
and the PCB should be 4mm. This is ensured by two 4mm-spacers. The construction step by step:

Preparing the housing:

• Insert the BNC in the wall connector without washer and tighten the nut.

• Insert the feed-through capacitor from outside the wall and solder from inside. Bend the inner

terminal so that it later fits into the hole on the PCB.

• Stick the two side walls of the housing together with the lower cover and solder the edges of the

walls from inside.

Preparing the PCB (considered that all components are fitted):

• Solder the four spacers concentric on the pads of the PCB, the 5mm parts on the middle pads, the

4mm ones below the PA module.

• Solder a 3cm piece of wire to the antenna pad for the BNC connector.

Assembly:

• Slip the PCB into the housing frame, connector pins first. Fit frame and PCB onto the lower cover.

Adjust the PCB so that the flange of the PA module flushes with the cover.

• Solder all 13 pads of the PCB to the side walls.

• Solder middle pin of BNC connector and feed-through capacitor.

• Fit the aluminum plate onto the lower cover, insert the four screws from outside into the holes and

tighten the nuts from the PCB side.

• Stick the upper cover onto the frame.

For usual packet radio operation with not more 30% transmitter duty cycle a 2.5mm aluminum plate is
absolutely sufficient as heat sink. Should the transceiver be designed for heavy duty use, e.g. at a
digipeater, a heat sink with less than 5K/W is necessary. On the PCB is space for a strong diode (D6)
in series with the power connector to prevent damage from the transceiver by applying wrong polarity.
Unfortunately the holes are too small for it’s legs. So either you drill them up or you can simply mount
the diode outside the housing as well.

Setting up the device

The transceiver has 9 adjustment points, anyway the adjustment is simple. The following test
equipment is required:

• Digital multimeter,

• Frequency counter capable to measure at least 30MHz with a sensitivity of 20mV,

• Oscilloscope,

• AF generator for sinus and square wave signals,

• A stable source for a 70cm Signal with an adjustable level between -60 and -90dBm (in case you

don’t have access to a signal generator a portable transceiver with 0.5 W RF power in 30m distance
will do).

• A receiver for the 70cm band with good demodulation capabilities (e.g. a scanner receiver with FM-

wide mode or a 9k6-capable radio).

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• A non-metalic screwdriver for tuning cores and trimmers
Start by connecting the 12V power supply voltage to the board. The current consumption should be

about 60mA. After two minutes warming up connect the frequency counter to Pin 2 of IC3 (MC3371).
This is the buffered output of the reference oscillator. The voltage at this point is below 100mVss.
Adjust the frequency with R4 exactly to 20.950 MHz. Please keep in mind that every Hz offset
produces 20 times the offset on the final frequency. This has also to be considered at the accuracy of
the counter. Due to the big coupling capacitors this adjustment reacts relatively slow.

Now enter a receive frequency of 430.000 MHz. In the next chapter it is explained how to enter
frequencies in general, for the first you can just switch to channel 0 by leaving all pins of X1 open.
Turn L1 clockwise until the voltage at test pad TP1 is 0.8V.

Enter a receive frequency of 435MHz by connecting pin 1 with pin 2 and pin 5 with pin 6 of X1 (this
settings of course works only if the PIC is in its original state and no other frequencies have been
programmed into the memory). Set the RF-generator to the same frequency and connect a digital
voltmeter to the RSSI terminal (pin 10 of X2). The DCD trimmer R54 should be in 12 o’clock position
because the RSSI voltage depends a little on the setting of R54. Without input signal the RSSI voltage
should be between 0.4 and 0.8 V. Depending on the RF signal the voltage increases. Turn the cores of
L6 and L7 and C70 recursively until the RSSI value reaches a maximum. If voltage reaches 3.5V
decrease the RF input level to continue the procedure.

The next step is to modulate the generator with a 1kHz sinewave signal of 3kHz deviation. Connect the
oscilloscope to the AF output terminal (pin 8 of X2). Turn the core of L9 so that the amplitude of the
output signal reaches a maximum and minimize the distortion with C70. An THD value of below 1%
should be reached. You can estimate the distortion very well if you have a dual trace oscilloscope
where you apply the original signal to the second channel. The receive path is now ready to use.

Before tuning the transmitter make sure that the heat sink is mounted properly and the PCB is soldered
firmly in the housing. Reduce the supply voltage to 7.5 volts and connect an AF generator to the
modulation input terminal (pin 6 of X2). The generator should be set to an squarewave output of
400mVss at a frequency of 100Hz. Plug a dummy load or a watt meter to the BNC connector and then
put the PTT terminal (pin 4 of X2) to ground. The output power should be about 1.5 W. Check the
modulation with a separate receiver tuned to the transmit frequency. An oscilloscope connected to the
output of the receiver most likely will show a heavily distorted squarewave signal at first. Turn R41
clockwise until the roof of the squarewave has a perfectly flat shape.

At last set the supply voltage back to 12V and check the output power. It should be 6W or more.

User interface

The transceiver has a 10 pin (X2) and a 14 pin (X1) connector. Table 2 shows the pinout. Pin 1 is
located in the upper right corner of the connector with view on the pins. There are female plugs
available for flat cable. X1 is used for frequency control, X2 for the link to TNC or modem.

X1: Pin Signal Pin Signal

1 D0 2 n.c.
3 D1 4 n.c.
5 D2 6 n.c.
7 D3 8 TXD
9 n.c. 10 RXD
11 PTT 12 12.5 / 25kHz
13 GND 14 +5V

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