Electronic T/R Switching and the Ameritron QSK-5
by Greg Latta AA8V

Hot Switching

Electronic T/R Switching and the Ameritron QSK-5
 Main Page  Handshake Circuit
 How PIN Diodes and PIN Diode Switches Work  QSK Indicator Modification
 Building an Electronic Transmit/Receive Switch Using PIN Diode Switches  Hot Switching
 PIN Switch Driver - Turning A PIN Diode Switch Off and On  Circuit Board And Parts Identification
 T/R Input and Timing Circuits  Schematic Diagram and Circuit Descriptions

Safety Note: Working on or testing the QSK-5 and associated circuitry may involve operating the amplifier containing the QSK-5 with the cover off. This is extremely dangerous since very high voltages are present when the amplifier is turned on, sometimes in close proximity to the QSK-5. If at all possible, do all work with the amplifier off and unplugged. The operator assumes all risk and liability in such matters! Never operate the amplifier with the cover off unless you are experienced with working around very high voltages!

Hot Switching
 What is hot Switching?
 What problems are associated with hot switching?
 Why must hot switching must be avoided with a PIN diode switch?
 What causes hot switching?
 How do I detect hot switching?
 What do I do if I detect hot switching?

What Is Hot Switching?
Hot switching is a term that refers to turning a switch off or on while the circuit is live. In most cases, hot switching is normal. For instance, the light switches you use in your house turn the lights off and on while the circuit is live. And the circuit breakers in your house turn the branch circuit off and on while the power grid is live.

What Problems Are Associated With Hot Switching?
As everyone knows, when a switch is turned off while the current is on a spark usually occurs at the switch. This spark can burn and pit the switch unless the switch is designed for it. Most switches are designed for hot switching and it causes no problems. However, with a PIN diode switch hot switching will usually ruin the switch by burning out the diodes, as will be discussed below.

Why must hot switching must be avoided with a PIN diode switch?
When any switch, mechanical or otherwise, is turned off, the resistance of the switch changes rapidly from a very low resistance to a very high resistance. During the transition, the resistance takes on all values of resistance in between. (Imagine turning the volume control on your transceiver very quickly from one end to the other. No matter how fast you turn the knob, it must be at all points in between.) In a mechanical switch or relay the resistance changes because the contact area decreases as the switch opens. Once the contacts separate, a small arc forms and then the arc extinguishes as the contacts open further.

In a PIN diode switch there are no mechanical contacts. The resistance of the diode changes because the bias across the diode is changed from positive to negative. As in the case of the mechanical switch, during the transition the resistance takes on all values of resistance in between a very low resistance and a very high resistance. Now for the crucial point: At some point in the transition, the resistance of the PIN diode switch will equal the impedance of the load (typically 50 ohms), and at this moment half of the power being delivered by the transmitter will be absorbed by the diode switch! For a typical 100 watt transceiver this means that at some point while it is shutting off the diode switch will be asked to dissipate 50 watts! Whether the diodes can handle this depends on the diodes and on how fast the transition occurs. In the case of the QSK-5, shutting off the receive PIN switch while more than about 20 watts is flowing through it will burn out the diodes. It is safe to summarize by saying that hot switching of PIN diode switches must be avoided at all costs.

What causes hot switching?
One major cause of hot switching is a transceiver that, when switching to transmit, emits RF before, at the same time, or too quickly after it brings the T/R line low. This is, unfortunately, very common. It doesn't cause a problem with mechanical T/R relays, but can burn out an electronic T/R system. (Fortunately, the QSK-5 is fused to protect against this). Another common cause of hot switching is forgetting to connect the T/R line between the transceiver and the amplifier T/R switch. If this happens with a mechanical T/R switch/relay, the signal simply goes out unamplified. But with an electronic T/R switch, the result is hot switching and blown diodes (or fuse lamps, in the case of the QSK-5).

How do I detect hot switching?

Safety Note: Testing for hot switching may involve operating the amplifier containing the QSK-5 with the cover off. This is extremely dangerous since very high voltages are present, sometimes in close proximity to the QSK-5. The operator assumes all risk and liability in such matters! Don't do this unless you are experienced with working around very high voltages!

As you might guess, hot switching is common. The designers of the QSK-5 anticipated the occurrence of hot switching and designed the QSK-5 with a pair of fuse lamps in series with the receive PIN diode switch. These lamps light/flash if hot switching is occurring. If the problem becomes serious enough, the lamps burn out, protecting the expensive receive diodes. When the lamps start to light, their resistance increases, further protecting the receive diodes. Under no circumstances should you try to replace the lamps with normal fuses, which do not have this feature!

To test the system, hook up the amplifier and transceiver and be sure to connect the T/R line. (Connect the system to a dummy load so you don't cause interference while testing the system.) Turn the power level of the transceiver down to a very low level and begin to transmit. Allow the transceiver to drop in and out of receive as you do this. Keep an eye on the QSK-5 fuse lamps, particularly when the transceiver goes into transmit mode. Gradually increase the transceiver power while keeping an eye on the lamps. If the lamps light/flash it indicates that hot switching is occurring. Mild hot switching can be tolerated, but if the lamps start to get bright enough to burn out you have a problem that must be dealt with.

Test the system at both low and high keying speeds. Hot switching may occur at one speed but not the other.

What Do I Do If I Detect Hot Switching?
If the hot switching is mild enough, you may be able to live with it. In other words, if the fuse lamps don't light too much and don't blow, you might be able to get away with it. But that is your choice. If you have hot switching in any form, I recommend that you talk to customer support at Ameritron. You should also consult with your transceiver manufacturer's technical department. They may have some fixes that might help you out. Some transceivers may have a software update that can cure the problem.

In an extreme case, it might be that you simply can't use that particular transceiver with an electronic T/R system such as the QSK-5. Electronic switching has many advantages and disadvantages. This is one of the disadvantages: it may not be compatible with all transceivers.


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