{"id":44,"date":"2026-07-27T23:52:42","date_gmt":"2026-07-27T23:52:42","guid":{"rendered":"https:\/\/flyrc.info\/?p=44"},"modified":"2026-07-27T23:54:32","modified_gmt":"2026-07-27T23:54:32","slug":"servo-signal-display-pwm-frequency-display","status":"publish","type":"post","link":"https:\/\/flyrc.info\/?p=44","title":{"rendered":"Servo Signal Display – PWM Frequency Display"},"content":{"rendered":"\n

I’m constantly working on RC aircraft, both my own and for many of my fellow club members. Because of this, I have accumulated a number of testing devices and delved a bit deeper into the workings of servos, receivers, etc… than most probably ever do. So, when a discussion about this little Servo Signal Display module came up on a discussion board it immediately caught my interest. Even better, a fellow club member was part of that discussion, and he jumped on it immediately and got one on order from Ali Express. Here is what it looks like:<\/p>\n\n\n\n

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A couple weeks later he brought it to the field and offered to let me borrow it. He had tried to order a second one as I had mentioned to him that I wanted to get one, but at the time he ordered he snagged the last one in stock. So let take a moment to explain what it does. Fair warning, I have some training in Electricity and Computers and a lot of experience messing with amateur radios, computers and RC components but I don’t work directly in the field and am not an RF or digital control engineer so some of this may be “as I understand it” and should not be taken as gospel. It should however be close enough for most of us and I hope understandable to all.<\/p>\n\n\n\n

The receiver in our RC airplanes send control signals to the servos via something known as PWM (Pulse Width Modulation). We aren’t discussing the radio link between the transmitter and receiver now, just the communication that happens down that signal wire from the receiver to the servo. Generally, most sources agree on the following basic description which seems to account for what I’ve seen occurring over the time I have worked with this gear. Once every 20ms (20 milliseconds or 50 times a second) a pulse that is between 1000 and 2000us in width (1000us is 1ms so 1 to 2 ms) is sent out from the receiver. As you can see, the measurement of width is using a unit of time so in reality the “width” of this pulse is its duration. This is decoded by the servo as a position ranging from one end of travel to the other with 1500us meaning center position. This device measures that signal and displays the width\/duration of the pulses. Note that as long as the control stick\/switch on the transmitter assigned to this channel is not moved, the receiver should be sending the same width pulse each time on the channel in question… 50 times a second as previously mentioned.<\/p>\n\n\n\n

Note this description assumes we are using the system where a receiver has a port for each channel\/servo. The newer serial systems like SBus, SRXL etc… are a different animal with their own methods for getting this information to the servos.<\/p>\n\n\n\n

The Servo Signal Display is simply measuring the width of the pulses and displaying them on the LED display. You can operate it by plugging it into the appropriate channel on your receiver. You can plug your servo into the unit as well if you wish. The 2 servo connections on it appear to be common… so it doesn’t matter which one is used for what as long as you get the polarity correct between all devices… plus to plus and minus to minus so to speak. Look on the SSD where the proper polarity is marked.<\/p>\n\n\n\n

As soon as I saw it, I found this of interest because it allows me to verify a good bit of what was only very theoretical for me up to this point. I had never had the opportunity to peer “under the hood” at this before. There have been a few times where I would love to have been able to look at this. Is the servo missing center because its being told something different or because of a servo problem? Is the signal even getting to the servo? What happens when I go to servo setup and change to 125 travel or 150?? That isn’t 125% or 150% as the servo doesn’t increase its throw that much so what do those numbers really result in? How does Expo affect these values? It was time to at least start uncovering the mystery!<\/p>\n\n\n\n

My first test setup looked like this:<\/p>\n\n\n\n

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For these first tests, I am using my servo tester(s). I have a couple of these handy and I use them frequently to test servos, give them a little break-in and frequently to do initial linkage setup and testing on new aircraft. They are very inexpensive units, and it occurs to me that I should question the validity of what they do as well. They are usually sold for 3 for $10 or so at swap meets… not exactly certified scientific instrumentation!! Of course, this SSD unit is similarly very inexpensive so I wouldn’t expect laboratory grade results from it either, but we’ll see!<\/p>\n\n\n\n

I hooked up my battery to the tester and plugged the SSD into the servo output side and everything looked about as expected. My tester was set for center and the SSD was showing 1487… OK, we’d like to see 1500 but being off 13us is less than 1% so not bad. I then dropped to manual mode and twisted the dial to once end and…. suprise… the SSD showed 867! That seems a bit odd as we expect something between 1000 and 2000. Twisting the knob all the way to the other extreme showed 2053. Hmmm… that’s 133us to low at one end and 53us to high at the other. 53\/2000 is 2.65% over range and 133\/1000 is overshooting by 13.3% at the other… that seems significant. So note to self, don’t expect to get as much travel “in the real world” as you might get with this servo tester I’m using. Center point showed 1518 on this unit. Not too bad. Just to confirm, I pulled out a second servo tester from the same batch\/manufacturer and surprise… different results! This time I got 916 and 2154 respectively. An overshoot of 7-8% in each direction.<\/p>\n\n\n\n

Ok, so the little cheap servo testers varied from each other as well as getting to numbers that were well outside of the standard ranges I expected. Now for a bit more of a real-world experience. I dug out an AR600 and my NX8 radio and created a stock default new airplane setup on the NX8 and then did a bind to the AR600. <\/p>\n\n\n\n

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Attaching the SSD to each channel on the radio in turn and with sticks centered showed the center point to be 1502us… that’s less than .2% of error which gives me confidence that the SSD is at least close to accurate. Then came some slightly more interesting readings. Each stick had its two endpoints very close to 1102 and 1905. So, in a “default” setup for Spektrum, the endpoints are 100us short of the expected maximums. So 200us of the range of 1000us (20%) is “unreachable” by stick movement.<\/p>\n\n\n\n

I then confirmed that the range is linear by monitoring the throttle stick output on the monitor screen and comparing to the displayed pulse duration on the SSD. It was linear across the scale as expected. I played a bit with expo configuration as well and it worked as I predicted and as depicted on the radio display. No suprises there.<\/p>\n\n\n\n

I then went into the travel menu and started cranking. This defaults to a value of 100. Most folks call this 100% travel, but the number is not a percentage of anything that I can understand. It appears to be just a relative number with no associated units. I noticed that if I went in and turned this up to 125 in each direction, I then saw the end point values on the SSD reach the usually quoted max and min of 1000 and 2000us pulse width. Cranking them further to 150 (as far as the radio allows) got me to values beyond the expected range, reaching ~900 and 2100! Honestly, I’m sure I’ve done that in the real world and am not aware of it causing issues, but I begin to wonder. If the servos are expecting values from 1000 to 2000 and you send them 900 or 2100, can you complain if unexpected things occur?? I’m sure there’s some level of protection built into the servos to handle some over\/under range signal width but how far that goes may well depend on the manufacturer of the servo, so I’d be cautious and test carefully in the real world to avoid issues.<\/p>\n\n\n\n

I then decided to push a bit more and went back in to play with the Dual Rate configuration. This screen allows you to go to 125 on the throws, so I was curious how that mixed in with the travel setting. I was able to get to the 1000\/2000 standard expected max throws with the travels set back to 100 (default) by using the DR and moving it up to 125. I could then move the travel setting to 125 and get to the 900\/2100 range I saw before by using the DR setting at 125 as well. Moving higher didn’t apparently change anything. I did check, and doing this did not cause the stick motion to reach these max values quicker… there was no plateau as such so the radio is limiting us to 900\/2100 apparently. Whether you get there by using 125 endpoints AND 125 dual rate or just by moving to 150 in the endpoint menu doesn’t seem to make a difference. Just keep in mind if you set your travels to 150, moving the dual rates to above 100 accomplishes nothing. There’s something I learned today!<\/p>\n\n\n\n

My final thoughts on the SSD. The SSD seemed to do the job for which it is designed and did it quite well. The readings matched up with the radio outputs displayed to within a very small margin of error… really insignificant levels. It has another function of measuring the voltage which I only checked once and did not try to confirm its accuracy. It appeared to work but that’s all I can say. That function is accessed by a tiny switch on the side of the unit, and I would hesitate to use it often as I don’t expect that tiny switch is going to take much abuse just based on its physical size alone. Here is the switch on the side of the unit… its the little white square half hidden down in the slot on the side shown here.<\/p>\n\n\n\n

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The unit ships with no instructions, nor am I aware of any available online. I didn’t really need any as the servo connectors are labeled on the board as is the switch function, polarity for the connections and the acceptable input voltage (3-26V). The display is a nice bright red 10mm or so high 4 digit LED and the back case (I’m sure it is mostly protecting the PCB and connections to avoid damage and shorting) is a low-moderate quality 3D printed part that gets the job done. I would keep it far away from any moisture however as I’m sure it could easily get around that and the unit would be toast. It comes with a short servo extension wire to connect to whatever you are testing (which also needs to supply power). This is not keyed in any way to beware to maintain proper polarity which is labeled clearly. The second connection is common so you presumably can plug in a servo or whatever you like on that second connection if you want to observe that as well while testing. Here’s a closeup so you can see the polarity indications and operating voltage range as seen on the PCB itself. The Sig\/Volt label indicates which way to slide the switch on the side to select that function.<\/p>\n\n\n\n

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For the money, this is hard to beat if you have a need or desire for this sort of information. I found it quite interesting and somewhat useful and plan on grabbing a couple so I can keep one close to each of my servo testers… sort of an extension of my servo test kit.<\/p>\n\n\n\n

I hope this is useful information for you and will help you decide if this little device is one that you need or not!<\/p>\n\n\n\n

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<\/p>\n","protected":false},"excerpt":{"rendered":"

I’m constantly working on RC aircraft, both my own and for many of my fellow club members. Because of this, I have accumulated a number of testing devices and delved a bit deeper into the workings of servos, receivers, etc… than most probably ever do. So, when a discussion about this little Servo Signal Display module came up on a discussion board it immediately caught my interest. Even better, a fellow club member was part of that discussion, and he jumped on it immediately and got one on order from Ali Express. Here is what it looks like: A couple […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-44","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/44","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=44"}],"version-history":[{"count":4,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/44\/revisions"}],"predecessor-version":[{"id":53,"href":"https:\/\/flyrc.info\/index.php?rest_route=\/wp\/v2\/posts\/44\/revisions\/53"}],"wp:attachment":[{"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=44"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=44"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/flyrc.info\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=44"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}