Pozosta DF2FQ-T7F User Manual

Manual for the

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70cm-FM/FSK-Transceiver

T7F

Holger Eckardt

DF2FQ

Kirchstockacherstr. 33

D-85662 Hohenbrunn

4199HE

Technical Data

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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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X2: Pin Signal Pin Signal

1 GND 2 +5V
3 DCD 4 PTT
5 GND 6 MODULATION
7 GND 8 AF-OUTPUT
9 n.c. 10 RSSI

n.c. = do not connect

Entering frequencies

The current version 1.4 of the control software allows the use of 12.5 and 25kHz channel spacing. Due
to technical reasons the switching time between receive and transmit mode is slightly longer at 12.5
kHz operation. Pin 12 of X1 selects the spacing. By default (pin open) 12.5kHz is selected. If the pin is
on +5V 25kHz operation is active.

The transceiver covers the whole 70cm Band, for repeater operation you can choose any frequency
offset. The device has a memory for 10 pairs of channels for receive and transmit. The channel is
selected by D0 to D3 (pins 1,3,5,7 of X1) in BCD code. This can be done with a BCD switch or by
jumpers (jumper inserted=1, pin open = 0). The common terminal of the BCD switch must be
connected to ground. D0 is the least significant, D3 the most significant bit. The jumpers can be
plugged on adjacent pins, e.g. 1 and 2, 3 and 4 etc. Even that n.c. means no connection, the
microcontroller sets those pins to ground during normal operation.

In the original configuration the 10 channels are pre-programmed with 430, 431,... to 439 MHz. This is
true only if 25kHz spacing is selected.

Programming of the desired frequencies is done through the serial interface. You need a computer with
a RS232 interface (e.g. COM1 or COM2 at DOS computers) and any V24-terminal software which is
capable to send characters to it. Such a software is part of most operating systems. Depending on the
connector type please regard to table 3 for the exact configuration.

Signal T7F/X1 SUB-D 25 SUB-D 9
RXD 10 3 3
TXD 8 2 2
GND 13 7 5

The interface parameters must be set on the computer to 1200 BPS (bit per second), no parity, two
stop bits, no local echo, no protocol (e.g. for DOS: MODE COM1 12 8 1 N). Now a simple string of
characters can be entered to allocate a frequency to a particular memory location:

Cntttrrr[RETURN]

C means the upper case C on the keyboard (HEX 43), n is the memory location 0 to 9 you want to
program. rrr is the channel number for the receiver and ttt that for the transmitter. The channel number
has always 3 digits even if the first digit is zero. It can be computed from the following formula:

N=(f-430000)/R

N is the channel number, f is the desired operation frequency for RX or TX in kHz, and R is the
channel spacing (12.5 or 25kHz). The string is not editable, if you make a mistake press enter and start
again. To make it clear here two examples spacing:

Memory location 0, receive frequency 438.100 MHz, transmit frequency 430.500 Mhz, 25kHz spacing:

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The string is C0020324.
Memory location 8, receive and transmit frequency 434.125 Mhz, 12.5kHz spacing:
The string is C8330330.
All characters you enter are echoed by the T7F, this is a good way to check the physical link between

the devices. If you press E (HEX 45) you get a hex dump of the 40 bytes of memory. If you switch on
the power of the T7F the version number of the software is sent on the TXD line.

Modem signals

AF input and output is compatible to most of the existing packet radio modems. The level of the output
at 3kHz deviation is 1Vss, the modulation input needs 300...400mVss to get a deviation of 3kHz.
Some modems provide a DC level on the modulation signal. In this case you have to insert a 10µF
capacitor in the modulation line (plus pole to the modem).

The transceiver provides a fast DCD signal. Most modems generate a DCD signal internally from the
data signal. If you operate on a multi mode digipeater it can happen that the internal DCD does not
recognize the “other“ mode, so you need the external DCD from the transceiver. Adjust the sensitivity
with R53. If it is turned fully counter clockwise the function is disabled.

The delay time to key the transmitter is below 30ms so TXD 3 should be OK for packet radio
operation. However some modems takes a certain amount of time by itself for switching so the TX
delay can be considerably longer occasionally.

Voice operation and what you can do else

With little additional effort the transceiver also can be used for voice operation. A full description and
an extension PCB is available on request from the author. Of course the radio can be used for 1200
BPS packet radio as well, no modification is required. If you want to operate with 19200 BPS you
need wider IF filters. FI1 must be replaced by a 21U30A, for FI2 a CFUS450BY is required, C78
should have 330pF instead of 470pF.

Postscript

The published design may be used by everybody for private purposes. Each commercial usage, also
from parts of the design requires a permission from the author. The author rejects any liabilities for
damages which result from construction or use of the device.

Appropriate construction considered the design is compliant to all requirements of the new European
standard for amateur radio equipment ETS 300-684 as well as to the EMC standard EN 55022.
However the device is not certified by any administrative body.

For questions and further information the author is available in packet radio or by e-mail under
[e-mail].

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6, Jumper position (X1)

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Fig. 7, A picture of the assembled unit

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Partlist

C1 33p
C2 10n
C3 1µF
C4 47n
C5 10n
C6 1p5
C7 1n
C8 1n
C9 10n
C10 10µF
C11 4u7
C12 100p
C13 10n
C14 1p
C15 33p
C16 33p
C17 100p
C18 1p
C19 100µF
C20 33p
C21 .1µF
C22 10µF
C23 10µF
C24 10n
C25 47n
C26 47n
C27 47n
C28 5p6
C29 10n
C30 3p3
C31 2p2
C32 33p
C33 47n
C34 100p
C35 100p
C36 10µF
C37 10n
C38 10µF
C39 10n
C40 10n
C41 3p3
C42 100p
C43 5p6
C44 5p6
C45 2p2
C46 100p
C47 2u2
C48 1n
C50 100p
C51 100p
C52 47n
C54 100p
C55 100p
C56 10µF
C57 100p
C58 47n
C59 5p6
C60 100p
C61 1µF
C62 22p
C63 1µF
C66 47n
C67 10n
C68 10n

C69 100p
C70 12p-trim
C71 5p6
C72 10p
C73 10µ
C74 47n
C75 .1µF
C76 8p2
C77 100p
C78 470p
C79 47n
D1 BB204
D2 BB405
D3 1N4148
D4 BA479
D5 BA479
D6 1N5400 or SB320
D8 BA479
D9 BZX55-5V1
D10 BB204
FI1 21U15A
FI2 CFUS450DY
IC1 PIC16F83
IC2 MB1504
IC3 MC3371
IC4 78L05
L1 514630
L2 .33µH
L3 .33µH
L4 .33µH
L5 3W3D
L6 511765
L7 511765
L8 3.9µH
L9 455kHz,blk
L10 .33µH
L11 3W3D
L12 3W3D
L13 .33µH
L14 3.3µH
L15 .33µH
L16 .1µH
L18 1µH
PMOD M67749M
Q1 20.95MHz
Q2 CST2.50
R1 100
R2 1M
R3 1M
R4 1M-trim
R5 680
R6 3k3
R7 10k
R8 4k7
R9 4k7
R10 10k
R11 3k3
R12 100k
R13 10k
R14 100
R15 10k
R16 15k
R17 100
R18 180

R19 47k
R20 100
R21 1k
R22 6k8
R23 2k7
R24 10k
R25 10k
R26 33k
R27 10k
R28 4k7
R29 33
R30 18k
R31 220
R32 150
R33 22k
R34 82
R35 10k
R36 470
R37 2k2
R38 2k2
R39 1k
R40 100
R41 25k-trim
R42 10k
R43 100
R44 150
R45 220
R46 18k
R47 470
R48 150
R49 39
R50 470
R51 470
R52 470
R53 100k-trim
R54 10k
R55 10k
R56 270
R57 10k
R58 100
R59 100
R60 330
R61 100
R62 2k2
R63 100k
R64 1M
T1 BC547
T2 BC557
T3 BFR91
T4 BC547
T5 BF966
T6 BFR91
T7 BFR91
T8 BD140
T9 BFR91
T10 BFR91
T12 BC547
T11 BS170
T14 BC547
T15 BF255
T16 BC557
T17 BS170
X1 2X07/90
X2 2X05/90

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