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A simple, general purpose multi-function transverter for the X-Band

A simple, general purpose multi-function transverter for the X-Band
by SV3ORA

(ex SW3ORA)

 

A little bit about the author

This article has been written by Konstantinos Giannopoulos (SW3ORA). He has graduated from the University of Portsmouth in UK and he has a BSc degree in Computer Technology and an MSc degree in Communication Networks Planning and Management. The article has been published in DUBUS magazine 2/2010.

 

General description of the project

The idea for this project was born a few years ago, during my MSc degree in telecoms at the University of Portsmouth. Fascinated by microwaves I wanted to design and construct a complete microwave telecommunications system from scratch. The difficult part was the construction of the PCBs at the X-Band frequencies, so another method had to be used. A microwave project is usually an expensive project, so it took me some years to complete the circuit.

The result was a general purpose, full duplex transverter for the X-Band, yet easy to build. This project has been made with simplicity in mind and ability of easy reproduction by anyone. Very few radio amateurs can afford expensive pcb materials and specific components and even fewer have access to expensive specified equipment required to design their own PCB circuits at high frequencies.

The transverter is based on coaxial technology which gives the ability of easily upgrading, changing or switching between components. By following the basic block diagram one can reproduce the transverter easily. The components that you can use on the circuit are not very critical and provided that you will use components with the right specs, you can have a fully working system.

Based on your requirements, specifications and budget, you can add your own taste in the circuit. For example you could use a very low noise balanced amplifier for the receiving section, you can use the VCO in a PLL configuration or you could add more oscillators and mixers for double or triple conversion. In this article, I present you the basic working circuit as a starting point for a larger project.

Despite its simple topology, the transverter offers a set of multi functions that can be very useful and can be switched on/off upon demand, from the front panel. It offers optional internal FM/FSK baseband modulation, simultaneous TX/RX operation and external antenna/amplifier option. It can be also used as simplex, half duplex or full duplex transverter.

 

The block diagram of the transverter

The main block diagram of the transverter can be seen on figure 1:

 

A simple, general purpose multi-function transverter for the X-Band

 

Figure1. The main block diagram of the transverter

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The block diagram elements starting from the signal generator towards the antenna will be first described. Just one local oscillator for both receive and transmit was used to minimize cost. Good signal generators on the X-Band are generally expensive. The operation is much like the operation of the gunnplexers, which use the same oscillator for both receive and transmit. By the addition of a circulator, the transverter can also be used in simplex, half duplex or full duplex mode. The signal from the local oscillator is split down into 2 ways using a coupler. The one portion of the signal drives the receiver mixer and the other the RF amplifier and/or the transmitter mixer.

At the receiver side, the RF signal is coming in from a short focal length dish antenna to a circulator. The circulator passes it to the receiver RF amplifier only, which is followed by a band pass filter, to reject the unwanted signals. Then the RX signal is fed into the receiver mixer and it is heterodyned with the local oscillator one, to produce the IF signal. The IF signal is then amplified and fed into a scanner radio.

At the transmitter side, the local oscillator signal that comes out of the coupler is fed into an amplifier and then into a band pass filter, to reject unwanted harmonics. Then a coupler, combined with a detector and a current meter, detects this signal and its power level, to provide an indication of the operational state and the output power of the transmitter. Finally the signal from the coupler is fed into the circulator which passes most of it to the antenna side only and not to the receiver side.

 

More options

If someone uses the transverter as it was described above, he can have a simple fully operational system. Furthermore, he could add some very useful operations by just adding three coaxial relays, to divert the signal path. The control of these relays is done easily, using external panel switches.

The first relay is inserted between the transmitter coupler and the circulator and it allows the optional use of an external transmitter amplifier. This is mandatory if you need to make some long distance contacts. By using this option, someone has to use two separate antennas, one for transmit and one for receive. It is always a good idea to use two antennas if your transmitter has a higher power, in order to better isolate the receiver from the transmitter. Circulators cannot handle high powers and they always have isolation problems (i.e. a small amount of signal flows towards directions that should not).

The second and third optional relays are inserted between the local oscillator coupler and the TX RF amplifier as shown on figure1. These relays have to be switched simultaneously to allow the diversion of the signal path. A single double relay can be used instead. In conjunction with a second mixer, the local oscillator signal can be used to up-convert a low frequency IF signal to the X-band.

Single conversion is not generally recommended because it is more difficult to filter the image signal but for the purposes of simplicity and cost, this could be acceptable here. If double or triple conversion is desired though, you can use more local oscillators and mixers to drive the IF port of the TX mixer. You could for example use a combination of UHF oscillator and mixer, which are non-microwave and easier to make, to up convert the source signal to a higher frequency to drive the microwave TX mixer. In fact, because of the RF filtering used, there is a restriction on this (especially if your filters are too sharp), which will be discussed later on. If you completely remove filtering, you may have as many conversions as you like, but this is generally not recommended.

The transverter can be used in simplex, duplex or half duplex mode by switching the appropriate blocks on or off using front panel switches. The default mode is full duplex. By switching the internal transmitter amplifier off, only the receiver operates whereas by switching the internal receiver amplifier off, only the transmitter operates. In full duplex mode, two channels are used for communication, one for receive and one for transmit. The frequency separation is defined by the IF frequency and the TX and RX tuning of the filters.

 

Modulation schemes

Another option of the transverter is the addition of an internal modulator. This can be used to modulate the transverter by the use of an external audio source. This avoids the extra cost for the transmitter mixer and external transmitter equipment if only FM or FSK modulation is needed. If this option is desired, a VCO must be used as a local oscillator. Below you can see the simple circuit of the internal modulator.

 

A simple, general purpose multi-function transverter for the X-Band

Figure 2. The VCO modulator

 

The LM317 variable supply is borrowed by the gunn modulators era and allows voltage variations to happen by the presence of an audio signal. The 50k potentiometer is used to set the modulation depth. The 1k potentiometer is used to set the voltage limits of the modulator and the 500R one, to fine tune the modulator. You can also bypass pins 2 and 3 with 1uf capacitors. If the input signal is in microphone levels, it needs to be preamplified in order to drive the modulator. When the equipment is used as a transverter or in receiving mode only, then the internal modulator must be disabled, to avoid unwanted frequency deviation. If transmitter (not transverter) and full duplex mode are enabled simultaneously, this frequency deviation is unavoidable, because a single local oscillator is used. It is pretty much like the Gunn operation in this case.

By driving a VCO with this type of modulator, FM modulation can be achieved and the modulation depth can be adjusted for your communication needs. You can also use the sound blaster programs to transmit FSK baseband in all modes (FSK, CW, SSTV etc). The equipment does not offer the option to directly SSB modulate the X-Band signal. It has to be switched into transverter mode and someone has to insert an already modulated lower frequency SSB signal into the transverter input port for upconversion.

 

Filtering

The use of filters in this simple transverter is not absolutely mandatory and for cost reasons someone could decide that he won’t use these. If you do not use filters you can easily set different TX and RX frequencies by choosing the appropriate IF frequency to receive. From the other hand, filtering will provide better isolation between the transmitter and the receiver, better selectivity in receiver side and better harmonics and spurious signals suppression in transmitter side.

The transverter uses only two filters, one for the RX and one for the TX. Easy tuning variable X-Band filters are expensive, so simple four-cavity tuned filters were used. This has the disadvantage that someone could not easily switch between different desired frequencies because he would have to retune the filters each time. This is a more complicated procedure than turning a single knob. The result is to be able to transmit and receive only within a predefined set of frequencies, defined basically by the sharpness of the filters.

When using filters, choose or tune them not to be very sharp because in transverter mode you will need this feature in order to separate as far as possible your TX from your RX signal during the upconversion. For example, if you have a TX RF bandpass filter of about 2MHz@3dB bandwidth and if your oscillator runs at 10GHz, then your IF input must be about 1MHz. Then your TX output will be 10GHz, 10GHz-1MHz and 10GHz+1MHz. This is too difficult to further filter and your local oscillator signal will be too close to your wanted output. Now imagine your TX RF bandpass filter has a bandwidth of 10MHz@3dB. Then your IF input can be at 10MHz, so the output signal from the mixer will be 10GHz, 10GHz-10MHz and 10GHz+10MHz. The local oscillator signal and the unwanted image can be easier further filtered and in case not, at least it does not interfere with the wanted signal. In conclusion, one has to decide between the use of filters or not, but have in mind that most professional equipment use filters extensively.

As far as concern the power amplifier filter, the most correct thing would be to have a filter before the power amplifier and another one after it, to completely filter out the unwanted signals produced by the mixer, when in transverter mode and also filtering the intermodulation products of the amplifier. The cost of filters in these high frequencies may be quite high, so only one filter is used in the output of the final amplifier. The problem is that this may lead to very inefficient use of the power amplifier as it is driven by both the wanted and the image signal when used in the heterodyne transmitter mode. To overcome this problem you may want to
insert the filter before the power amplifier and not after it.

 

Receiving

To minimize cost, at the receiver side no IF filter is used. All the filtering is done by the RX filters. A digital HF receiver is used as a receiver back end, as there is no cheaper and easier solution than this. If SSB needs to be received, an SSB capable receiver must be used or constructed. For FM reception, an FM demodulator can be constructed or you may use a wideband scanner with FM capability at HF. Instead of using an IF filter, you can let the receiver back end do the filtering job, by using its internal filter as a fine frequency selection. Also, in that way, the internal mixers and local oscillators of the receiver are used. This will result in dual or triple conversion at the receiving side of the whole transverter system.

Between the receiver mixer and the HF receiver, a broadband IF preamplifier is placed. The broad bandwidth feature is only needed upon RX filter removal, if wide receiving is desired. If you decide to use the transverter in a single RX frequency, then you can use a narrow bandwidth amplifier in conjunction with an IF filter, to eliminate any unwanted mixer products.

 

The power supply

The power supply unit used in the transverter is very simple, as there is no need for a demanding PSU. If the external amplifier option is chosen, an external PSU is needed. The PSU voltage and current capability is based on the componens that will be used. In my configuration, except from the LM317 modulator, three more voltages were needed, 5V, 12V and 15V. I got all these by using separate regulators (LM7805, LM7812, LM7815) for better isolation and higher current capability. The schematic of the PSU is shown in figure 3. Connect each LM78xx input at point + of the bridge, to get the desired voltage. It is always a good idea to use a power line filter (not shown) at the 220v side of the transformer to filter unwanted noise from the power lines.

 

A simple, general purpose multi-function transverter for the X-Band

Figure 3. The PSU

 

The completed transverter

The completed transverter is shown on figure 4. A standard box case was used and mounting holes were done to support the switches, tuning potentiometers, indicators and connectors.

 

 

Figure 4. The completed transverter

 

Precision multi-turn variable potentiometers (with counter heads) were used to set the VCO voltage and the FM deviation. For further precision of the frequency setting, a digital voltmeter was used, to continuously monitor the voltage in the VCO. Also, a current meter was mounted, to monitor the power level of the transmitter detected by the internal detector. The connectors used, are feed-through types, for lower losses and greater isolation between adjacent ports. Finally, a bank of four switches was used to select between different modes and options on the circuit, as described previously.

 

 

Figure 5. The back of the box

 

On figure 5, the back of the transverter can be seen. There is a WG-16/WR-90 waveguide port for the RF I/O for lower losses between the antenna and the transverter. The SMA external amplifier port is terminated with a terminator, to avoid accidentally switching into external amplifier mode when no external amplifier (or second antenna) is connected to the transverter.

The pictures below show some close ups of the different parts used in the transverter.

 

 

Figure 6. Another view of the transverter

 

Figure 7. Close up of the upconversion mixer

 

Figure 8. Close up of the downconversion mixer, the coupler and VCO connections

 

Figure 9. Close up of one of the coaxial relays used

 

Figure 10. Close up of the VCO, coupler and downconversion mixer configuration

 

Figure 11. Close up of the circulator/transition output network.

 

 

Figure 12. Close up of the Cougar 5-1000MHz broadband IF amplifier

 

Figure 13. Close up of the PSU and FSK modulator enclosure

 

Using this setup you can have a fully operational simple transverter tor the X-Band at a fraction of the cost. Most important than the cost, is the easiness of building such a system, as no expensive lab equipment is needed. Apart from 10GHz, this setup can be applied in any frequency. You can build a transceiver for any amateur microwave band as long as you change the component specifications to meet the specific band requirements.

 

Happy DXing!

de SW3ORA

 
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