Description 23cm datatransceiver

(c) 2000, Bas de Jong PE1JPD

Introduction

This transceiver is designed for high-speed packetradio in the 23cm band using Frequency Shift Keying with baudrates from 9k6 up to 150 kbit/sec depending on the IF-bandwidth and modulationtype.

A kit or ready made unit is available from DDS Electronics, the Netherlands

Please read this manual carefully and follow the steps for construction and alignment. In this manual you will find the following chapters:

  1. Introduction
  2. Description
  3. Construction
  4. Alignment
  5. Tips and backgrounds

Specifications

  1. General  
    Vcc supply 12-14 V
    Frequency range 1241.000 – 1241.950

    1295.000 – 1295.950

    other segments possible as option
    Pullingrange within channel +/- 25 kHz
    Modulation Wide Band FM
       
       
    Receiver  
    Sensitivity -100 dBm
    Noiselevel ~3.5 dB
    RxM output 1V top-top with100 kHz dev.
    RxD output 5V top-top
    RSSI  
    Squelch status (mute) Available
       
    Transmitter  
    Power output 2 – 3 Watts
    Deviation ~10 - ~100 kHz or wider
    RX/TX switchtime ~10 ms
    Overheating during TX Without heatsink ~3 min

    With heatsink dependent on size of it

Description

The design of the transceiver is straightforward and shown in fig. 1. Central in the design is the vco which oscillates directly on the transmit-frequency around 1241 or 1295 MHz . The vco is build around a BFR91A with a stripline-filter between collector and basis of which the center stripline can be tuned with a varicap. This vco has a very low noiselevel but the tuningrange is rather limited to around 100 MHz, which however is sufficient for this purpose. The vco is followed by a mar-8 which acts as a buffer/amplifier with an output of 10-15 mW. A small part of this output is fed to the TSA5511 PLL. This IC integrates everything which is needed for the PLL: programmable devider, reference oscillator and devider, phasedetector and amplifier. The device is programmable via I2C. The output-frequency of the vco is determined as follows:

Fvco = Fref * 8 * n where Fref = Fxtal/512

With Fxtal = 3.2 MHz Fvco = n * 50 kHz so the output frequency fits in a 50 kHz raster. The referencefrequency can be pulled a bit with the trimmer in series with the 3.2 MHz crystal.

The PLL is programmed by the microcontroller AT89C2051. This is a member of the 8051-family with serial input/output, 2 parallel ports and a analog comparator. The clockfrequency of the controller is not critical at all and is determined by a crystal (12 MHz). The vco-frequency is controlled via 6 jumpers of which the most significant bit (J5) determines the bandsegment of the vco (1241 resp. 1295 MHz). The other 5 bits determine the frequency in 50 kHz steps:

543210

X00000 1241.000

X00001 1241.050

....

X10011 1241.950

As already stated, the trx is designed to be used split-frequency where the IF equals the split. This way the frequency of the vco doesn't have to be changed during rx or tx, and no time is lost (re)programming the PLL and locking up again. Therefor very fast switching times can be achieved which are needed for high-speed packet. The output from the vco/buffer is fed into a striplinecoupler at the rx-mixer build around a 3SK183 dual-gate mesfet. During rx, the antenne-input is fed via a quarter wavelength stripline to a mar-8 amplifier giving 20 dB gain, followed by a 3 line striplinefilter. This striplinefilter is tuned with sky-trimmers. The output of this filter directly couples to gate-1 of the rx-mixer fet, together with the LO-signal from the vco/buffer. The output of 54 MHz IF is filtered via 2 TOKO-coils, amplified by a BF982 DG mosfet and fed to a MC3356 IF-chain. This IC converts the input to 10.7 MHz using an oscillator build around a BFY90 and a 64.7 MHz crystal. The second IF of 10.7 MHz is filtered in a SFE10.7 IF-filter, amplified and demodulated in a quadrature detector. The demodulated output is available via a feed-through C, as well as a Receive Signal Strength Indicator (RSSI: S-meter), a squelch/mute signal and a shaped, digital output. The IF-bandwidth is determined by the type of filter and is in this case 280 kHz wide.

The tx-path is very straigthforward: via a -20 dB stripline coupler 0.15 mW of the LO-signal is coupled to a third mar-8 giving 20 dB gain. The output of around 10 mW is amplified in a Mitshubishi M67715 module giving 2-3 Watt of output. The tx-modulation from the modem is first filtered via a simple RC-lowpass and then put onto the PLL controlvoltage. The deviation is adjusted via a 100 ohm trimmer. The tx is switched on when PTT is low, which results in switching on of the 9V for the tx mar-8 and the driver-stages in the module. The 9V also supplies the PIN-diodes in the antenna-switch. During tx both diodes conduct so that the output from the module is fed to the antenna, and via the second diode the lambda/4 to the rx-input is transformed to high-Z at the antenna. Finally, the 9V switches off the power for the rx-input mar-8 and the IF-amplified.

Construction

preparation

To build this transceiver you will need the following tools and materials:

Steps

  1. Check the pcb. Make sure the two slices for the trapezium-C's around the 3SK183 mesfet are cut.
  2. Check the 3.5 and 5 mm holes for the transistors for shorts to ground because the pcb is throughmetalized. If nessecary, use a drill of say 6mm to free the hole.
  3. Put the tin box togeteher by fitting the 2 sides in the bottom lid. Be sure that a bended corner is positioned at the upper-right: at this position later on the module will be positioned.
  4. Put the pcb in box, solderingside down, not soldering anything yet. File the pcb to fit the box. Mark the positions of the antenna-stripline and the 7 feedthroughs at the sides. Take the box apart again and mark the positions for the holes 8 mm from the bottom edge of the side. Mark the right position and drill the hole using a 3.5 mm drill. Hold the side fixed on a piece of wood to prevent it from flying around when drilling.
  5. Now, put the box together again and solder the two edges of the bended sides together. Then put the walls in the bottom lid and check if it fits.
  6. Put the pcb in position, 9 mm from the bottom. Use 9 mm high distanceholders to keep the pcb on the right distance from the bottom. Check the holes in the box to fit the antenna stripline and feedthroughs. Solder the pcb to the walls on a few places and check. Assemble the complete box to check for any torgue which will make the box be unstable. If everything is ok, solder the pcb at the componentside all around to the boxwalls. Do not use an overdosis of tin, specially where the module will come to avoid problems fitting it lateron.
  7. Assemble all passive components on the pcb (R's and C's). This is not very critical but take care to use small amount of solder and cutting the wires. The components should be put tightly on the pcb. This is very important for the decoupling C's (10n) and other RF-components. The leads of most R's can be bend gently to fit the marked position, except some R's in the RF-section of which the leads have to be bend directly from the resistor-body. This certainly holds for the 56 ohm resistors at the end of striplines and the R's at the outputs of the mar-8's. The trapezium C's fit in the slices made earlier. Solder one side to the stripline at the solderside, and the other side at the componentside. Take care not to overheat the C because it easily breaks into halfs. Do not use it anymore when broken! The smd-parts have to be soldered at the solderside. Put a little tin on one pad, put the smd-part onto it holding it down with the finger or some tool and solder it. When fit at one side in the right position, solder the other side (and if nessecary the first side with some extra solder to get a good soldering).
  8. Now put the active parts in. The BFR91A in the vco, the 3SK183 mixer and the BF982 IF-amp fit in the holes in the pcb and can be soldered directly into position. The legs have to be cut to match the pads (1-2 mm). Take care that the marking at the BB405G varicap is postioned at the striplineside. Apart from the smd-parts, also the 2 BA479 PIN-diodes fit at the solderside. Bend the wires straight from the body down and cut them to 0.5-1 mm. Then solder the diode into position like an smd-part. Only the Atmel needs to be put into a socket, the other IC's can be soldered directly into the pcb.
  9. Put the module in place by pushing it as far as possible onto the pcb and mark the positions of the M2.5 screws to fixate it to the side of box. Remove the module and carefully drill the two holes (3 mm) without damaging the assembled pcb. Put the module in place, fix it with M2.5 (M3) screws and solder the leads at the solderside.Cut the remaining wires.
  10. The 7805 and 7809 regulators and BD136 powertranssitor are not screwed to the box. They can stand freely, just take care that they fit deep enough into the pcb so that they are all in the box and do not prevent the lid to be closed.
  11. Fix the 7 glass feedthroughs into position and connect them to the proper pads at the pcb.
  12. Prepare a piece of RG188 PFTE coax (3.5 mm thick) so that the inner wire is free for 5 mm and the outer braid is shiftes back. Solder the innerlead to the stripline, and the coax braid all around to the box. Take care that the innerlead does not shortens to ground!

Alignment

Preparation

First check the pcb for shorts, incorrectly placed components (IC's, transistors, regulators, fets). Of course, a programmed Atmel controller should be put in position. Remove all jumpers, this will preset the vco to a frequency of 1241.000 MHz. If it fits better, just put jumper J5 into position so that the output will be 1295.000 MHz.

VCO and PLL

If everything looks ok, power can be applied. Power should be 12-14 V, not more! Now the red LED may give a short flash but should stay OFF indicating that the vco is in lock. Check the vco by putting your (wet) finger on top of the oscilator transistor BFR91A so that the LED turnes on. During the first moments, check whether nothing gets overheated and check the output of the 5V regulator. If possible, check the vco output on a spectrum analyser or 23cm transceiver tuned to 1241 MHz.

The vco is now running on the correct frequency. Try fitting and removing jumper J5; everytime the PLL is reprogrammed and the vco swings from high to low and vv which causes the red LED to give a short flash. If it stays on, the PLL is not in lock. If this only happens either with or without J5, the PLL is ok but fails to lock at one frequency.

Transmitter

Now connect an appropriate powermeter and dummyload to the antennaconnector and ground PTT: 2-3 W should now be available. No tuning is nessecary for the poweramplifier. Only the deviation has to be set using the variable R of 100 ohm. Preset this R halfway. Final adjustment can be done using the modem later on, but in general the modulationlevel is also adjustable on the modem.

Receiver

To align the receiver a 54 MHz signalsource for the second IF is needed. If not available, you can use a 23cm signal on 1241.x or 1295.x MHz, or as a last resort, a 70cm handheld capable of transmitting on 431-433 MHz listening at the third harmonics on 1295.x. Be sure to math the frequencies on 70 and on the transceiver.

Preset the 3 skytrimmers to minimum and the IF coils so that the ferrite dips 1.5 mm in the coil.

Now, get the 64.7 MHz crystaloscillator running first. The output from this oscillator can be monitored with an ordinary scanner or 6m receiver whichever is available. Carefully tune the coil located near the 64.7 MHz crystal until the oscillator fires up. Turn the ferrite anticlockwise until the oscillator turns off again; the maximum output can be achieved just before this happens.

To tune the IF, connect an oscilloscope to the RSSI and apply a signal of 54 MHz -50 dBm to the antenna-input. Tune the IF coils to maximum, and take care to switch off the signal now and then so that output diminishes.

Now connect the oscilloscope to the RxM and tune the quadrature detectorcoil. The 54 MHz signal has to be frequency-modulated with a deviation of 100 kHz. Remove the 54 MHz signal and apply a 23cm signal –50 dBm of the appropriate frequency to the antenna-input and tune the 23cm bandpass. Optimize the IF-coils and the 23cm filter. A sensitivity of –100 dBm should easily be achievable.

tips and backgrounds

Other bandsegments

The transceiver is designed for splitfrequency use around the lower end of the 23cm band at 1241 MHz and 1295 MHz. The IF-shift is therefore 54 MHz. However, it is possible to use other shifts in the range 40-70 MHz. The 64.7 MHz crystal has to be changed accordingly:

F2xtal = IF-shift + 10.7 MHz.

In particular is it possible to use the trx as a linktransceiver in the segments 1240.x and 1299.x by changing the crystal to 69.7 MHz. Since the IF increases from 54 to 59 MHz the inductors in the first IF have to be tuned higher.

Changing the IF-bandwidth

When to be used in the interlink segment, the maximum aloowed bandwidth is 50 kHz. It is appropriate to adjust the second IF-filter accordingly. The filter used, a SFE10.7MA is 280 kHz wide but there are also filters available which are narrower with 30-50 kHz minimum.

Modulation lowpass

Standard a first order lowpass of 1 kohm/1nF is fit on the pcb. It is advisory to change this lowpass to one that fits the maximum modulation frequency.

Modulationtype and bandwidth

This transceiver uses direct FSK for modulation. It can not be used for standard, narrowband 9k6. When using WBFM however, it can be used with manchester FSK from 9k6 - ~115k2 or G3RUH-type modulation from 38k4 - ~200 kbit. Remember that the bandwidth will be

B = 2 * Fmod + 2 * Fdev

Where B is the bandwidth, Fmod is the maximum modulation frequency and Fdev is the deviation used. Fmod for G3RUH equals half the bitrate (4800 Hz for 9k6) or, for manchester, the bitrate. So, when using 38400 baud manchester, the bandwidth will be around 150 kHz.

Working simplex

In principle it is possible to work simplex with this trx. Simplex means that the rx and tx frequencies are the same. When changing from rx to tx or vice versa, the PLL now has to be reprogrammed by the microcontroller. The hardware is prepared for this (the PTT signal is available to the controller at pin 11), but the current version of the software does not support it. Remember that the rx/tx switchingtime will increase significantly because not only time is needed for the PLL to get into lock again, but also the time to reprogram the PLL has to be accounted for.