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MDT Construction Manual – Issue 2

Page 11

6.5

P

OWER

S

UPPLY AND

RX/TX

SWITCHING

When the PTT is operated the TX/RX relay is energized and the transmit signal is passed to the
antenna. When the PTT is not operated the relay switches the antenna through to the receive
circuits. The relay also switches power to the TX and RX sections as required

Diode D8 provides reverse polarity protection. With the power supply connected with the right
polarity, D8 will pass current onto the board. There is a small voltage drop of about 0.3 volts but
this has minimal effect on transmitter performance. If the supply is connected in reverse D8
will not conduct and the board will be protected.

6.6

R

ECEIVE AUDIO

The low level audio signal from the mixer is applied to the audio pre-amplifier stage (U1A). This
is formed from one half of an LM833 dual low noise op-amp and has a gain of around 100 or
40dB. A reference supply for the non-inverting input is obtained from R33, R35 and C40. The
high frequency response of U1A is limited by C41. The output of U1A is DC coupled to the 2 pole
low pass filter (U1B) with a 3dB cutoff frequency of 2400Hz. This stage has only unity gain but
its main task is to set the receiver bandwidth.

The output of the low pass filter is coupled to the AF gain control VR4 via a 1uF capacitor (C44).
A LM386 (U2) is used to boost the signal from the AF gain control to drive a set of headphones.
The gain is set by C47 and R42 to about 50 times or 34dB, while resistor R43 limits the audio to
a comfortable level for headphone listening. If an external low impedance speaker is used
instead of headphones, then R43 can be replaced with a link.

A MOSFET (Q9) is turned on in TX mode and shorts the audio signal to ground. Even though the
audio amplifiers are switched off during transmit, this is still necessary because microphone
signals can make their way through to the headphones while the audio amplifiers are turning
off.

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Содержание MDT

Страница 1: ...MDT Construction Manual Issue 2 Page 1 MDT DSB TRANSCEIVER CONSTRUCTION MANUAL ...

Страница 2: ...ts 17 8 Individual Parts List 19 9 Construction 21 9 1 General 21 9 2 Building the PCB 22 10 Final Assembly 33 10 1 Microphone socket 33 10 2 Terminating plugs 34 10 3 Front Panel 35 10 4 Mounting the PCB 35 11 Testing and alignment 36 11 1 General 36 11 2 Power on 36 11 3 Receive 36 11 4 Transmit 37 11 5 Marking the VFO scale 38 12 Modifications 40 12 1 Setting the VFO range 40 12 2 Receiver alig...

Страница 3: ...11 Relay install 23 Figure 12 Non polarised capacitors 24 Figure 13 LPF capacitor marking 24 Figure 14 Trimcap install 25 Figure 15 BD139 location 25 Figure 16 BD139 lead identification 26 Figure 17 LED lead bending 27 Figure 18 LED install 27 Figure 19 Pot with tab removed 31 Figure 20 Component overlay 32 Figure 21 Mic connector wiring 33 Figure 22 Typical circuit voltages 39 Figure 23 DSB trans...

Страница 4: ...t most operators won t be aware unless you tell them Building the MDT is quick and easy The receiver doesn t require any alignment and the only setup required for the transmitter is setting the microphone gain and balancing the mixer to null out the carrier A kit of parts for the MDT including everything you need such as enclosure and front and rear panels is available from www ozQRP com MDT Speci...

Страница 5: ...he bandwidth of a SSB transmitter On a quiet band this does not cause any problems but on a crowded band it may not be as easy to find a free spot to operate without interfering with nearby stations Secondly the Direct Conversion receiver has equal response to both sidebands This means you hear signals on both upper and lower sidebands simultaneously This results in a slightly higher noise level a...

Страница 6: ...deband and 7 099MHz 7 100MHz 1KHz lower sideband The important thing to note is that only the sidebands are present at the output of the mixer as the carrier and audio signals have been suppressed by the action of the balanced mixer The diagram in Figure 1 at top right shows the DSB output signal in the time domain or how it would be seen on an oscilloscope Note the overlapping envelope shape that...

Страница 7: ...through the filter and be heard in the headphones This is the upper sideband response as the antenna signal frequency of 7 101MHz is above the 7 100MHz VFO frequency Note that there is also another antenna signal that can be heard This is the lower sideband signal at 7 099MHz This would also produce a 1KHz tone in the headphones This ability to simultaneously detect both upper and lower sidebands ...

Страница 8: ...MDT Construction Manual Issue 2 Page 8 5 MDT BLOCK DIAGRAM Figure 3 MDT Block diagram ...

Страница 9: ...nal to produce received audio The carrier signal which is much larger in amplitude than the audio signal is applied simultaneously to both sides of the mixer through trimpot VR2 The carrier signal turns on the diodes to form a low resistance and is why it is referred to as a switching mixer As the carrier is capacitively coupled it swings both positive and negative around ground potential When the...

Страница 10: ...provides a DC bias current and is enabled by shorting the EL link If a dynamic microphone is used the link is left open 6 4 TRANSMIT AMPLIFIER Transmit signal from the mixer is applied to the driver stage built around transistor Q4 A BD139 works well here when biased with about 50mA of collector current The design is well proven using both shunt and series feedback to provide low input and output ...

Страница 11: ...ain of around 100 or 40dB A reference supply for the non inverting input is obtained from R33 R35 and C40 The high frequency response of U1A is limited by C41 The output of U1A is DC coupled to the 2 pole low pass filter U1B with a 3dB cutoff frequency of 2400Hz This stage has only unity gain but its main task is to set the receiver bandwidth The output of the low pass filter is coupled to the AF ...

Страница 12: ...MDT Construction Manual Issue 2 Page 12 Figure 4 Carrier oscillator ...

Страница 13: ...MDT Construction Manual Issue 2 Page 13 Figure 5 Mixer ...

Страница 14: ...MDT Construction Manual Issue 2 Page 14 Figure 6 Microphone Amplifier ...

Страница 15: ...MDT Construction Manual Issue 2 Page 15 Figure 7 Transmit ...

Страница 16: ...MDT Construction Manual Issue 2 Page 16 Figure 8 Receive Audio ...

Страница 17: ...CC C1 C2 C7 C8 C11 C12 C16 C17 C18 C19 C20 C22 C29 C36 4 1uF 50V RB electrolytic C35 C37 C44 C45 3 10uF 25V RB electrolytic C34 C47 C48 6 100uF 25V RB electrolytic C21 C30 C39 C40 C49 C50 1 40pF trim capacitor TC1 Resistors 2 1 5Ω 1 4W 5 R19 R20 4 10Ω 1 4W 5 R13 R16 R41 R44 2 22Ω 1 4W 5 R17 R43 4 100Ω 1 4W 5 R7 R23 R28 R31 1 220Ω 1 4W 5 R1 7 470Ω 1 4W 5 R8 R11 R15 R24 R27 R32 R42 3 1K 1 4W 5 R2 R5...

Страница 18: ...386 4 audio power amp U2 1 7 2MHz ceramic resonator X1 1 5mm blue LED LED1 Coils 1 FT37 43 4T 8T 4T T1 1 FT37 43 8T 2T T2 1 FT50 43 8T L2 1 T50 2 15T L1 1 T50 2 14T L3 1 T50 2 11T L4 Hardware 1 BNC RA PCB mount connector ANT 1 3 5mm stereo PCB mount socket PHONES 1 12V DPDT DIP relay K1 1 2 1mm DC PCB mount socket PWR 2 2 pin 2 54mm pitch header MIC PTT 2 2 pin 2 54mm pitch plug with pins MIC PTT ...

Страница 19: ...al diode C18 100nF ceramic MLCC C19 100nF ceramic MLCC K1 PCB mount DIP signal relay C20 100nF ceramic MLCC C21 100uF 25V RB electrolytic L1 15T T50 2 toroid C22 100nF ceramic MLCC L2 8T FT50 43 toroid C23 220pF 100V C0G ceramic MLCC L3 14T T50 2 toroid C24 150pF 100V C0G ceramic MLCC L4 11T T50 2 toroid C25 470pF 100V C0G ceramic MLCC LED1 5mm Blue LED C26 470pF 100V C0G ceramic MLCC C27 470pF 10...

Страница 20: ...ic resonator R13 10Ω 1 4W 5 resistor R14 2 2K 1 4W 5 resistor R15 470Ω 1 4W 5 resistor R16 10Ω 1 4W 5 resistor R17 22Ω 1 4W 5 resistor R18 3 3K 1 4W 5 resistor R19 1 5Ω 1 4W 5 resistor R20 1 5Ω 1 4W 5 resistor R21 2 2K 1 4W 5 resistor R22 4 7K 1 4W 5 resistor R23 100Ω 1 4W 5 resistor R24 470Ω 1 4W 5 resistor R25 4 7K 1 4W 5 resistor R26 470K 1 4W 5 resistor R27 470Ω 1 4W 5 resistor R28 100Ω 1 4W 5...

Страница 21: ...quired You may find that sometimes solder doesn t appear to flow through to the top side This is not necessarily a problem because the plated through holes make a connection to the top side automatically Another point to consider is that plated through holes consume more solder than non plated holes and makes it more difficult to remove components So check the value and orientation of components b...

Страница 22: ...h from the top and bend out slightly underneath to hold them in place Turn the PCB over and press down slightly to press them against the surface and solder Cut off the excess pigtail with side cutters It is easier and less confusing to install a group with the same value rather than to cover a section of the PCB with mixed values You will also find it more convenient to install 5 or 6 resistors a...

Страница 23: ...entify pin 1 of the ICs and where they are installed on the PCB Figure 10 DIP ICs Step 5 Relay The relay is inserted with pin 1 towards D7 as shown in Figure 11 This end of the relay has a line printed on top of the case for identification Once inserted hold against the PCB and solder the pins Figure 11 Relay install ...

Страница 24: ...LCC These are all non polarized and can go in either way There are 14 x 100nf 0 1uF MLCC and as they are quite small install these first Figure 12 Non polarised capacitors Step 7 Low pass filter capacitors These look like 0 1uF MLCC types so don t get them mixed up Figure 13 shows how to identify the LPF capacitor markings Capacitor Marking Value C24 151 150pF C23 221 220pF C25 C26 C27 471 470pF F...

Страница 25: ...n with sidecutters before inserting The earth pin of the trimmer capacitor is highlighted in Figure 14 Figure 14 Trimcap install Step 9 trimpots Note that the carrier balance trimpot is a 500 ohm multi turn vertical mount while the Microphone gain trimpot is a 5K horizontal mount type Step 10 transistors The 2N3904 transistors are orientated to match the screen silk component overlay The BD139 tra...

Страница 26: ...d the positive lead is the longer lead The PCB component overlay has a mark to indicate the hole for the positive lead Step 13 Connectors The MDT board utilizes polarized pin headers for the microphone and PTT If preferred the wires may be soldered directly to the PCB but the connectors make for a professional looking build plus allow easy disconnection and testing if required The connectors have ...

Страница 27: ...an pass through the hole in the front panel Figure 17 shows how this done Once bent insert the LED leads into the PCB just far enough to allow them to be soldered Ensure the anode A and cathode K leads go in the correct holes as shown in Figure 18 Figure 17 LED lead bending Figure 18 LED install ...

Страница 28: ...his is winding 2 Take a 120 mm length of 0 4mm enamelled copper wire Wind 4 turns on the ferrite toroid This is winding 3 Scrape the enamel off the ends of the wires and tin with solder before installing in the PCB The winding direction is not important Transformers T2 Take a 180mm length of 0 4mm enamelled copper wire and wind the 8 turn primary on a FT37 43 ferrite toroid Take a 90mm length of 0...

Страница 29: ...for a neater alignment on the PCB Scrape the enamel off the ends of the wires and tin with solder before installing Collector Coil L2 Take a 220mm length of 0 4mm enamelled copper wire and wind on 8 turns on a FT50 43 ferrite toroid Spread the turns to cover about 80 of the circumference Note the direction of winding as this makes for a neater alignment on the PCB Scrape the enamel off the ends of...

Страница 30: ...akes for a neater alignment on the PCB Scrape the enamel off the ends of the wires and tin with solder before installing Low Pass Filter Coil L4 Take a 280mm length of 0 4mm enamelled copper wire and wind on 11 turns on a T50 2 toroid Spread the turns to cover about 80 of the circumference Note the direction of winding as this makes for a neater alignment on the PCB Scrape the enamel off the ends ...

Страница 31: ...he AF gain control is a 10K logarithmic pot Before placing into the board break off the metal tabs as shown in Figure 19 This allows the front face of the pots to sit flat against the front panel Push the pot pins through the board as far as they will go and solder in place Double check when mounted that the pot shafts are parallel to the PCB Figure 19 Pot with tab removed ...

Страница 32: ...MDT Construction Manual Issue 2 Page 32 Figure 20 Component overlay ...

Страница 33: ...lastic panels These are not required and can be set aside for some other use In this kit they are replaced with pre cut and printed front and rear panels These are actually made from PCB material with a black solder mask and white silkscreen lettering The front panel has a white circle placed around the Tune control to allow marking frequency steps with a pen more on this later 10 1 MICROPHONE SOC...

Страница 34: ... tabs Using a pair of small long nosed pliers bend the tabs around the wire to hold it in place Using as little heat as possible solder the wire to the pin The soldering iron tip is placed on the pin tabs at the end of the bare wire Be careful as too much heat and solder will melt the insulation and interfere with the contact Slide the pin into the housing ensuring the small locking piece on top o...

Страница 35: ...u may need to do this a couple of times to get the knob marker to sit equally at each end of the travel Push the LED through its cutout so that it pokes out about 2mm proud of the panel 10 4 MOUNTING THE PCB Place the rear panel onto the rear of the PCB so that the antenna connector and DC socket pass through the cutouts Place the PCB onto the enclosure base ensuring the front and rear panels slid...

Страница 36: ...in series with the power supply positive lead and set to measure current Turn on the power supply and note the supply current It should be around 50mA If it s far from this turn off immediately and look for problems The relay should be de energized and the board in RX mode To verify that there are no obvious problems do a quick probe around the board with a multimeter and check the DC receive volt...

Страница 37: ...ross the dummy load o Connect a spectrum analyser across the dummy load An attenuator may be necessary In all cases it s simply a matter of monitoring the transmitted signal for minimum output while adjusting TC1 and VR2 The carrier null is quite sharp and there is some interaction between the controls so you will need to go back and forwards to obtain maximum balance Check the TX mode DC voltages...

Страница 38: ... been recently soldered Connect a frequency counter across R8 Rotate the Tune control fully anti clockwise Read and note the frequency on the counter Rotate the Tune control fully clockwise Read and note the frequency on the counter From the two readings determine what markings you wish to make on the scale Marks at every 10KHz would be a convenient option Start at the low frequency end of the tun...

Страница 39: ...mitter 0 1 6 Q5 Q6 collector 13 8 13 6 40 Q5 Q6 base Note 3 0 0 6 Q5 Q6 emitter Note 3 0 0 02 Q7 collector 0 4 8 Q7 emitter 0 0 6 Q8 collector 0 6 3 Q8 emitter 0 0 45 U1 A pin 8 11 1 0 U1 A pin 3 5 5 0 U1 A pin 1 5 4 0 U1 B pin 7 5 4 0 U2 pin 6 13 5 0 U2 pin 5 6 6 0 Notes 1 Zener tolerance 5 2 DC measurement approximate Oscillator running 3 No RF drive Readings taken with a digital multimeter Powe...

Страница 40: ...evenly spaced over about 80 of the circumference However if the turns are compressed the inductance will increase slightly and lower the tuned circuit frequency Conversely if the turns are separated the inductance will decrease slightly and the frequency will increase To check the frequency of the modified tuned circuit do the following Inject a stable signal into the antenna socket in the middle ...

Страница 41: ...G The VFO is a free running oscillator and there will be some drift after powering on as the components warm up It is best to avoid transmitting until it stabilizes This only takes a couple of minutes and any drift after this period is quite small 13 2 1 SSB The Tune control is used to tune in stations The MDT doesn t have a fine tune control so you need to adjust the control slowly and carefully ...

Страница 42: ...onic generation and must be avoided To check your signal you can either connect the MDT to a dummy load and monitor yourself with headphones on a nearby receiver or have a friend that lives close by listen to your signal The idea is to increase the Mic gain progressively while the receiving station sweeps across your transmission looking for distortion and unwanted spurious byproducts Set the Mic ...

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