Reduced RF susceptibility for the HamGadgets "Ultra Pico Keyer"

Over the past several years I've done a bit of POTA (Parks On The Air) operating, racking up about 1000 contacts as an activator in a number of parks - mostly as an "activator", and mostly on CW.  Typically, I have operated from a campsite using a portable antenna - usually the JPC-7 loaded dipole (discussed in this blog entry) or the JPC-12 loaded vertical (discussed here) - but I have also used an end-fed half-wave and a simple dipole on occasion.

When the power budget permits, I typically operate the transmitter at 100 watts as I wish to make it as easy as possible for those who wish to work me although I have occasionally operated QRP (5 watts or less).

Having used a number of different radios for POTA operation (Yaesu FT-100 and FT-817, Icom IC-706MK2G and even a RockMite) - none of them with a memory keyer - I decided that an "Upgrade" was in order so I got the Ham Gadgets "Ultra Pico Keyer" (Link here).  Available as a kit, this device is small, operated from a single CR2032 lithium coin cell for about US$40 (not including shipping) including a 3-D printed case. 

This is a nice, little device in that it provides a consistent interface to the user no matter which radio you might use and it has a number of message memories (up to eight) - perfect for an activity like POTA where a message (e.g. "CQ POTA") may be repeated many, many times during the course of the operation.

Getting "stuck" 

Figure 2:
The Ham Gadgets "Pico Keyer" (left) along with the
CW Morse Outdoor Paddle.
Click on the image for a larger version.

While the Ultra Pico Keyer works as advertised, I did notice a problem on the first trip out while using a portable antenna:  It would get "stuck" - that is, the radio would stay keyed up.  

Clearly, this was an RF susceptibility issue - verified by reducing transmit power and observing that it no longer happened.  In short, at 5 watts there was no issue, but at 100 watts it would fairly consistently get "stuck":  The radio would stay keyed continuously after the first Morse element - whether initiated by the paddle or from pressing a button to send a stored message - was sent.  While it was "stuck", I could still hear the sidetone - via the keyer's internal speaker - sending the message, indicating that it was not the microcontroller that had crashed.

Further testing showed that when the unit got "stuck" due to RF, simply unplugging the input from the paddle from the back of the keyer would cause it to release (get "un-stuck").  The fact that this happened using a portable antenna (it did not happen at home, or in the car with the vehicle-mounted antenna) provided further evidence of potential RF sensitivity.

Analyzing the problem

As I'm wont to do, I decided to take a look at the Pico Keyer's schematic to see if there was something about its design and construction that might make it more susceptible to RF interference - and I was surprised at what I found.  Here's the diagram found in the manual that is freely available online on the web site (link):

Figure 3:
Diagram of the Pico Keyer.  The components in question are Q1 and Q2, in the upper-right corner.
Click to get a larger image.

While there are protection capacitors on the paddle input (C1, C2) my eye was immediately drawn to the output keying (upper-right) where I was, at first, confused as to the arrangement with an N-channel MOSFET in both the keying line and the "common" (ring) of the "OUTPUT" connector (e.g. Q1 and Q2) - but then I remembered that the manual stated that this device would key both positive and negative voltages, explaining the "unusual" arrangement.

While admittedly clever, I could immediately see an RF susceptibility issue here:  The "OUTPUT" jack more or less will "float" compared to the "ground" of the keyer itself which is also connected to the "ground" lead of the cable to the paddle as well as the external paddle itself.  This configuration almost guarantees that there will be at least some RF current flowing from the radio and through the keyer for several reasons:

• If you are using this in a portable situation, the radio will surely have some RF on its chassis.  As noted in the final section of this blog entry, it's almost impossible to prevent all RF current from getting onto the feedline - even if you do use a common-mode RF choke.
• The paddle and the cable that connects it to the keyer might be considered as part of an antenna - and this situation is made worse if one is sitting at, say, a metal table and also if you, the operator, place your hand at/near the paddle/cable.
  

What this means is that there will, in many cases, be RF flowing from the radio chassis, through the keyer and then - via transistor Q2 (and Q1) find its way through the cable to the paddle.  I didn't really investigate the exact mechanism by which RF current through this path was causing the keying line to get "stuck" - but here are a couple of possibilities.

• RF may be coupling from the drain of Q2 into its gate - and subsequently into Q1's gate as well, which is tied in parallel with it with the peaks of the RF voltage turning on the FET.  Even if RF through the FET was causing it to conduct only on half of the RF cycle, this would surely be enough to key the radio - even if the transistor was not fully saturated.
  

• The RF could also be getting into the output pin of the microcontroller via the FET, causing its totem pole output to get "stuck" on.

Figure 4:
The added capacitor(s) can be seen soldered between the
"sleeve" pins of the "OUTPUT" and "PADDLE" jacks,
on the bottom of the board.  As you can see, I've made this
modification to both of my Pico Keyers!
Click on the image for a larger version.

Regardless of the cause, the fix was clear:  Add a capacitor to bypass RF current around the circuit.

When this occurred, I happened to be on a POTA activation, but I had my "electronic toolbox" in the car which included a number of useful items such as a soldering iron and a smattering of useful electronic components (a some common resistors, capacitors, etc.).

Grabbing a 1000pF capacitor, I connected one end to the "sleeve" (ground) pin of the "PADDLE" jack and the other end to the "sleeve" of the "OUTPUT" jack - effectively providing a bypass to RF energy on Q2's drain to the circuit "ground" to eliminate any RF voltage potential between the cable connecting the radio and that going to the paddle.

This completely solved the problem:  It is my opinion that this capacitor should be supplied with the kit.

* * * * *

Suppressing RF on the gear and connecting cables

Some readers of this may immediately say "You are obviously doing something wrong with your set-up if there's enough RF on your gear to cause a problem".  

The problem of RF going somewhere other than out the antenna has been known for many decades and is sometimes referred to as "Hot Mic", a situation where there is enough RF on the radio - and the microphone - that the operator can even get an RF burn from touching the gear.  When this happens RF can get into the radio itself and cause undesired operation (malfunctions, distorted audio, etc.) but accessories connected to the radio - most notably sound interfaces, computers and even keyers - can be adversely affected.

While in the case above there was apparently some RF present on the gear to cause a problem, there isn't anywhere near enough to cause issues with the radio itself, and the radio+microphone (when running SSB) seemed immune.  Some types of antennas - typically ground-plane verticals, random-wires and end-fed half-wave antennas can, by their nature, put RF on the feedline - and thus the radios - unless extra steps are taken to minimize this problem, namely:

• Common-mode choke on the feedline.  Typically placed near the antenna, this usually consists of coaxial cable wound on a ferrite toroid - typically 6-12 turns on an FT240 or FT140 core with either Mix 31 or Mix 43 as the material - the latter being generally more useful for portable operations where the higher bands (40 meters and up) are most likely to be used.
• Use of a balanced antenna.  A balanced antenna like a dipole is generally more likely to induce less RF current on its feedline than a purely end-fed antenna (a vertical is included) as it contains its own counterpoise - but having a perfectly-balanced antenna is not really possible and the feedline itself will participate in conducting/radiating RF along with the antenna to some degree.
• Counterpoise/ground plane at the radio.  If you are operating in a metal vehicle it's less likely that RFI will be a problem as one is likely to be surrounded (e.g. shielded) - plus the fact that the shield of the coaxial cable feeding the antenna can be electrically bonded to its chassis.  Barring being in a Faraday cage like a vehicle, having a counterpoise connected at the radio (particularly if it's 1/4 wave long at the operating frequency - and if there is more than one of them) this can siphon off some of the RF that might be present owing to its lower impedance.
• Place the antenna far away from the radio.  As noted, this isn't always practical - or even desirable.  In my opinion, equipment used with a radio transceiver should already have a modicum of resistance to stray RF energy so that even small/moderate amounts of RF on the gear will not cause any problems.
  

If you are operating portable, there's one thing that you probably aren't going to get away from:  The antenna itself.  Almost by definition, portable operating implies being near the antenna owing to the need to have a feedline of manageable length and also due to practicalities of not wanting to lug a long feedline along or taking up more real estate than necessary.  What this means is that it's likely that you and your radio will be immersed in a rather strong RF field - and this also means that anything made out of anything that is conductive (the radio, power cables, microphones, interconnect cables to your paddle and keyer - and even you) are likely to intercept RF energy this will get into everything.

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This page stolen from ka7oei.blogspot.com

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