Are You Calibrated?

May 12, 2015

Over time, you start to understand the unique characteristics of your radio - operating quirks, interface bugs, and other behaviors. A couple of useful parameters to be familiar with in your radio are it’s frequency stability as well as it’s frequency accuracy.

The easiest one to understand is accuracy. Are you tuned to the correct frequency? How do you know? One way you can tell that your radio may not be correctly calibrated (on HF, at least), is needing to consistently tune up or down from a station while listening on SSB, in order to get them to sound “right”.

The complement to accuracy is stability. As electronics warm up, their characteristics change, and thus, your radio’s accuracy could change over the course of a minute, 5 minutes, or even an hour or two!

Often, frequency stability is specified in parts per million (PPM). My TS-590s is specced to be stable within 5ppm. On the 20m band this adds up to as much as 100Hz “error” in your tuning. Kenwood would still consider this within spec! On 50MHz, the errors add up, and the radio could be off as much as 250Hz!

On my radio, I’ve found that I’ve consistently needed to tune up about 40hz on the 20m band, in order to get people to sound natural, once the radio has “warmed up”. Calibration and stability are also important when operating digital modes like PSK and JT65.

TS-590 warmup Startup frequency drift of my TS-590

Kenwood sells a temperature-controlled crystal oscillator (aka TCXO) as an add-on, which improves the stability to within +- 0.5ppm, a large step up from 5ppm without. Unfortunately, they also charge $120 for what is essentially a 15.6MHz crystal.

Recently, some Chinese clones have surfaced, and thanks to Patrick, TK5EP’s, work, I discovered the $20 alternative.

Checking your calibration

The easiest way to check your rig’s calibration is to use FLDIGI in “analysis” mode:

  • Tune your rig to the highest WWV frequency you can receive a signal (20/15/10/MHz) - the higher the better.
  • Set your rig to USB mode, and tune down 1khz exactly.
  • Set FLDIGI to “analysis mode”. You should see the WWV carrier around +1000 Hz.
  • Center the crosshair on the carrier, and click. You should see the offset in the lower area displaying the focused frequency.

FLDIGI should now be displaying the frequency it thinks you’re centered on, and it should be rising or falling over time. Over the course of a couple hours, I found my rig was off by 87Hz when tuned to the 20MHz WWV signal. This is 4.35 PPM, a bit high, but safely within the specs Kenwood laid out.

Frequency analysis using FLDIGI Running FLDIGI in Analysis mode - I think 0.3Hz is "good enough!"

At this point, if I hadn’t bought the TCXO, I would be manually adjusting the internal trimmer to bring the radio into slightly better calibration. I’m not recommending you do this, but it can be an option if the calibration is annoying to you. Keep in mind it’s extremely easy to mess up your rig’s calibration. The trimmer on the TS-590 is pretty tiny, and I’d have had to have the rig opened, upside-down, and turned on and hooked up to do this easily.

My $20 TCXO showed up in the mail today, three weeks ahead of schedule, and it was a very easy drop-in install. Turning on the rig, and watching FLDIGI, I was impressed pretty quickly. After a couple hours being turned on, my radio is now about 2.4Hz high on 10MHz, which works out to 0.24ppm, which is over 18 times more stable than without the TCXO. As a bonus, I’m close enough that I don’t feel like I need to mess with the calibration trimmer built into the TCXO.

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