QSK for the linear amplifier

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 I wanted full QSK in the big amplifier.




QSK, also known as full break-in, allows the transmit/receive switching system to operate fast enough that you can hear between the dits and dahs while sending CW. Once you get used to it, it is difficult to go back to conventional semi break-in operation.

Achieving full QSK requires very fast relays and, in many cases, a sequencer to ensure that all relays have settled before RF is applied.


To find out what would work, I measured a large number of relays that I had available and compared their switching times. The goal was to determine which relays were fast enough for reliable QSK service.

Below is the result. One thing worth noting is that relay speed can often be improved by driving the relay coil with a higher voltage than its nominal rating. This is a commonly accepted practice, provided that a suitable current-limiting resistor is placed in series with the coil.



P1D-1V under test


REW-14 under test. Notice the long switch time and bounce as compared 
to the P1D-1V



The vacuum relays clearly stood out as the fastest devices tested, with a complete switch-and-settle time of only 2.7 ms.

Since I use an Elecraft K3, which provides an adjustable transmit delay, there was no need for an external sequencer. The K3's default TX delay is 8 ms, giving the relays plenty of time to operate and settle before RF is applied after PTT is asserted.

For the output side, I chose a Kilovac HC-1 vacuum relay, rated at 3 A. The input side uses a Russian P1D-1V relay. To simplify experimentation and future modifications, I designed a PCB that can accommodate both conventional PCB-mounted relays and vacuum relays.

The keying circuit was borrowed from a unknown designer and has proven to work flawlessly, but I´m using a BS170 instead of a 2N7000. (also note that the zeners are the wrong way around).




Populated T/R board. The vacuum relays sit in grommets 
in their holes, to minimize noise from switching.





A very messy schematic because I decided to add 
dual footprints for both relay positions. A linear 
regulator was also added, but later removed in the amplifier.

The bias switching turned out to be more challenging than expected.

My first attempt used a conventional PCB-mounted relay. While functional, it produced noticeable switching noise. I then replaced it with another P1D-1V relay, but that relay occasionally stuck during operation.

It appears that these relays are not particularly well suited for switching relatively high DC voltages, so that approach was abandoned.

Instead, I borrowed some ideas from the KEYALL HV, a vintage CW keyer that uses a photovoltaic gate driver together with a totem-pole MOSFET arrangement. This solution worked exceptionally well and completely eliminated the mechanical switching noise.

The result is a silent and reliable bias switching system, perfectly suited for full QSK operation.



73 ,SA2CLC

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