Transverter Interfacing

by N2CEI

You have purchased a brand new high performance HF transceiver and are more than pleased with it when you decide that you would like to interface your 2 meter transverter to it. Why not it worked great with the old rig, it should work even better with the new one! ------------Well, ---------You start to hook it up and soon realize that its not quite as simple as the last setup you had. This paper will discuss many different methods of interfacing transverters that solely depend on the type of transceiver being used. The transceiver types will be put into categories but commercial names and model numbers will only be used if the transceiver is unique.
Some of the first transverters that early experimenters designed, and some manufactures provided, required high TX drive levels compared to today's standards. This generation of transverters used Vacuum tubes for both transmit and receive and were very forgiving about exact transmit drive levels. You would tune and adjust them just as if they were a power amplifier. Most of the time you would be using a separate transmitter and receiver so interfacing was considered fairly simple to the ham of that era. Drive levels were set by indications of plate current in the transmit converter or presetting the drive level of your transmitter for a dim glow in a No. 47 pilot lamp! It was a bit of a guess but out of this new wave of VHF operation came the circuit in Figure 1.

Figure 1.

The conceptof Figure 1. is now the basis for any solid state transverter that will tolerate more than 200 mW of drive. This circuit may be used with transmitters and transceivers that have high output power levels by increasing the wattage rating of R1 or using a large dummy load as in fig 2. If you set the adjustable attenuator (the variable capacitor) to the maximum level required by the transverter, then the transverters power output can be decreased by decreasing the transmit power of the transmitter. Then be sure to use a non radiating load or you will also be working the 10 meter band when 6 meters is open! Also use good quality coax and place the adjustable capacitor and loads in a RF tight enclosure.

Figure 2

To use Figure 2. with a transceiver, simply add a transfer relay on your transceiver before the dummy load circuit. Then use the keying circuit (that would be used to key an external amplifier) to toggle the transfer relay form the receive to transmit position. A RF sensed circuit is not recommended for full break in or SSB operation unless a large amount of "Hang Time" is designed into the circuit to prevent relay chatter (but then full break in would not work anyway!). In figure 3, the receive side of the relay would go directly to the transverters receive output port. If you are using a transverter that has both receive and transmit IF ports combined, add a transfer relay to the transverter side of the circuit, and switch both of them together. This can be a low power relay in most cases, but keep it coaxial if it is external to the transverter

Figure 3

To consolidate space , the mechanical relays in figure 3 can be replaced with a solid state switching circuit. (PIN diodes) With the use of a low power 50 ohm load or a network of resistors you will have the standard switching circuit used in microwave transverters that use low power portable 2 meter transceivers as IFs . Figure 4. Is an example of this circuit and can be interfaced directly to the transverter or put into a separate enclosure if the radiated RF from the load resistor is a problem.

Figure 4.

Further enhancements to figure 4. may be added to provide DC switching for the RX and TX sections of a transverter and a extra RX gain stage if needed. Figure 5. Shows a circuit that can also provide external DC for other gain stages such as Preamplifiers and TR switches. Adjustable attenuators for trimming the required amounts of gain on both TX and RX may also be added. This is standard on most microwave transverter kits today. It is found built in to transverter PC boards and as separate add on circuits.

Figure 5.

Adding keying line circuits are simple and a effective way to make a easy to use portable station. RF senescing should be avoided to eliminate relay chatter, and a positive keying circuit should be used. The TXON point of fig. 4 or the TTL of Figure 5. requires a positive voltage that can be found in most 2 meter transceivers. Some are external and others are found on the coax line during transmit. To use this positive keying voltage, a RF choke is connected between the common IF connector (from the transceiver) and the TXON (or TTL) point on the schematic. Be sure to use a bypass capacitor to de-couple the RF from the switching circuit.
L1 ,C2 and C3 in figure 4. are frequency sensitive and can be selected for any frequency IF. This circuit has been use with 903 and 1296 MHz. IFs but will have considerable loss and low isolation if leaded components are used. Surface mount diodes, capacitors and small, tightly wound inductors should be carefully laid out on a circuit board for best results.
If it is absolutely necessary to use RF sensing, adding a circuit with some hysteresis and delay to Figure 4. by using an op-amp and some high value capacitors for "Hang Time" will make a switching circuit that will enable any transceiver to be interfaced with any transverter. Just be careful of the power dissipation of the 50 ohm load.( See Figure 6.) This circuit will still fit into a small enclosure with a heatsink attached or could be mounted inside a transverter if the RF power levels are low enough to prevent radiation into the mixer circuits. Dissipation of 100 watts might be a little difficult inside of a transverters enclosure but it could be done with a large enough heatsink. C12, C13, and L2 are the frequency selective components.

Figure 6.

A further consideration would be to add a sequenced circuit such as in N1BWT's "Fool Resistant Switch" (Fig. 7.) This circuit is a sequencer coupled with the basic circuit found in Fig. 4 . It provides separate sequenced output voltages to all gain stages and preamplifiers. It has RF sensing built in but has options to enable or disable it. It also has just about any feed back and switching circuit required to set up a high performance microwave transverter system with mast mounted LNAs and TWTs

Transverter Interface Box (TIB)

With most of the newer transceivers having the best receivers amateur radio operators ever had available to them , makes it a natural to want to connect a transverter to it. Finding out it doesn't have a transverter port is sometimes a let down. So far the discussion has been how to interface transceivers to transverters that are basically less than 25 watts of power. The higher power examples work, but having a 100 watt dummy load connected to your transverter system not only wastes energy and produces heat, it is a bit cumbersome if you plan to operate portable. There should be a better way and there is. The use of the "ALC" circuit that is built in to most HF transceivers is one way to save a lot of energy and hardware .
The ALC circuit found in most HF transceivers has a external connection that is intended to be connected to a linear amplifier. The theory is that when the amplifier becomes overdriven, it will produce a negative voltage that is feed back into the transceiver's ALC circuitry. This voltage acts as a gain control in the transmit stages of the transceiver. This in turn, lowers the drive power automatically (the transceivers output power) and limits distortion from a overdriven amplifier. What a great idea! So, why not use it in a transverter system. Not as a feed back system to limit distortion, (that is done by a proper drive level setting of the transverter) but as a power limiting circuit in your transceiver.
There isn't a transceiver with a ALC circuit that can't be totally disabled with a negative voltage input! That's right! Negative voltage input. This can be tested on any transceiver with a variable supply(isolate the ground!) or use a 9 Volt battery and a multi-turn pot. After connecting your transceiver to a power meter, slowly increase the amount of negative voltage on the ALC input and watch the power output drop! You now have control of the power output of your transceiver. Connect a transfer relay system between your transceiver and transverter to handle the Transmit and receive functions and go get 'em!
Well, almost! There are a few things you need to be aware of when using a circuit like this. Don't use a 9 volt battery in a permanent set-up. When the battery goes dead, so does you transverter! Use a Negative voltage generator such as a ICL 7660 circuit similar to a LNA GaAs-FET power supply. ALC circuits require only a few milliamps of current It should be interfaced so that when the transverter is in line, the negative voltage is on! Also set it up so the default position of your system is your transceiver connected to a load other than your transverter!! (That would work that VSWR protection circuit!)
Still after you set that up, there are a few other items to be aware of. Some ALC circuits behave differently in different modes of operation. Switching from SSB to FM could be a nightmare! Check the output power in both modes. Some other transceivers sometimes generate a spike on initial transmit. This could cause damage to your TX mixer so check it out. This spike could be caused from not enough negative voltage. Also be sure the negative voltage is on the ALC input 100% of the time. Don't just switch it in on transmit!
When adjusting the voltage level on your ALC circuit, try to totally disable the transmitters power output. It will bottom out at a very low level output. Some around 1-2 milliwatts. Then give it a little more negative voltage if possible. Even if your transceiver says 0 to -4VDC, minus 9 VDC will work just fine on all ALC circuits. If you try to adjust the power output of your transceiver with the negative voltage, you will find a point in the curve that a small change in negative voltage will produce a large change in output power. Stay away from that point. It is too unstable! Set you transverter system up with the minimum amount of drive possible. It just make levels more manageable Figure 8. shows a fool proof circuit.

The rest of the Story!

There are still a few transceivers on the market that had to be different. For one reason or another, after establishing a precedent, somebody decides to change the method. Going from a simple Din plug connection of the past to needing to build a "Bias Tee" in the future doesn't sound like progress! You guessed it! The TS-850! It's is a less than desirable setup because if you lose the voltage on the assembled "Bias Tee" you will transmit into the RXIF of your transverter. The other draw back is that when you want to use the rig on the Low bands, you need to dis-connect everything!
So a interface is required. See Figure 9. When the circuit is shut off, the standard RF path is routed through the relays. (transmit and receive) When the circuit is power up, a regulated voltage is sent to the transverter connection on the TS-850. This voltage disables the transmitter in the TS-850. This in turn ensures that the 850 will not transmit in to the RXIF port of the transverter which is now connected by the internal RF relay in the circuit.(Yes! The only RX IN is the main antenna connector!) The best feature of this circuit is that the default is when the circuit does not have DC voltage applied.
Figure 10. shows a circuit used with a FT-757GX . It has the same draw back as the TS-850. The RXIF port from a transverter is connected to the main antenna connection. Ouch!------------ A good point is that it has a connection that can be interfaced to a transverter system with a simple switch. A jumper in the main power connector is replaced with a switch and that will put the transceiver in transverter mode or not. This switch should also be connected to the RF relay the will either connect a HF antenna or the transverter's RXIF line to the main antenna connection. Again, the default is normal HF operation with the DC power off! Only other hitch in the FT-757GX is that it has a low level drive signal for a transverter,(-6dBm or less) and the push to talk to ground key line will only sink up to 2 ma. You should use this signal to activate a relay to key all other accessories.
This is an important note. Most of the newer transceivers on the market today do not have mechanical relays as auxiliary connections for activating amplifier or transverter circuits. High current open collector circuits don't exist either. Most key circuits will only sink or source a few milliamps. Read the manual and if you get can't find the answer, call some one! Don't just try it unless you are ready to replace a SMD transistor if you can find the blown transistor!
A common set up for most Icom transceivers with transverter ports, would be to connect the transverter port of the transceiver to the TXIF of the transverter. Then connect the RXIF to one of the auxiliary antenna connectors. Well this has changed with the newer Icoms. The transverter port is used for both transmit and receive. The circuit in Figure 11. can be installed into the transverter so it becomes a simple setup. But the new trick in the IC 765 and 775 is to toggle the voltage on pin 6 of the accessory jack on and off when changing from transmit to receive. This should be done with an external set of relay contacts in the transverter . Use the "Send" line in the transceiver to key the transverter.
Some other general notes are to watch out for rigs that say they are transverter ready. A few on the market today do little more than change the frequency readout on the digital display!! Watch it! If you are in the market for a new rig and think you may want to use it with a transverter system, ask all of the questions! Sometimes a letter designator such as "D" or "MP" will make the difference between a simply or a complex interconnection. They all can be interfaced but why not catch a break every now and then.

Figure 11.


This paper has not covered everything there is to know about transverter interfacing. I am sure some one could pull out a new or a old rig that just "hasn't been done before". I am sure the major manufactures will not disappoint us in their cleverness to save a few pennies buy using a new connector or method of providing what they think a transverter should need. The days when external ports on plug-in computer boards to interface our transverters are not far away. It is a fact that major manufactures are trying to replace the need for most transverters with their newest offerings. A rig that will operate from 160M through 23 cm is also not very far away! But, the newest Satellite rigs have internal connections that are waiting for the launch of Phase IIID. If the major manufactures will provide the interfacing or converters for these transceivers remains to be seen
Being the Hams we are, we will always have a need to experiment or find our selves just not satisfied with the standard transceivers performance. With QSO's being conducted in the microwave regions becoming commonplace and future microwave satellites ready to be launched, there will always be a need to interface some sort of converter to a transceiver. We will see.
Have fun and catch you on the bands!