Leo Bodnar injection board calibration for IC-9700

The Icom IC-9700 frequency stability can be improved by synchronising the internal master crystal oscillator with an external GPS disciplined one using the Leo Bodnar injection board. The 9700 internal oscillator is sort of pulled to the external frequency by placing a small coil close to the crystal. On the Leo Bodnar site the setup procedure is described. This procedure creates so many misunderstanding and questions that I decided to spend some words on it.

First of all, the external GPS based oscillator (GPSDO) cannot be calibrated. You can set the right output frequency and level and that’s it. Now when you switch on the GPSDO the internal crystal gets pulled to the right frequency (frequency jump). The required “calibration” is to make sure this frequency jump is as small as possible, because the pull action has a limited range.

The easiest way to do this, is to tune to some fixed frequency signal. Should this signal be very stable? No, the signal should just not change during the GPSDO on/off cycles, so you have enough time to adjust the reference setting in the 9700. Should this signal be very accurate? Well, if the calibration settings in the 9700 are completely off (all zero for example) this might help. But in such a case, it’s easier to use the 10Mhz input based calibration once before mounting the injection board.

Now when listening to this signal, switch the GPSDO on and you will hear a jump in frequency. Then go into the frequency calibration settings screen and adjust until the frequency jump is gone cycling the GPSDO on and off. And that’s all there is to it.

“I don’t have a beacon or other signal near me..” (really?) Well, for use with the injection board, you should have set the GPSDO to 49.152 MHz using the application from Leo Bodnar. Change the GPSDO to 435.000 Mhz and tune the 9700 there in CW mode. Then set the CW side-tone to 1000 Hz and look at the audio scope. Now adjust the 9700 calibration until the received tone is 1000 Hz, as seen on the audio scope. Since this requires switching screens on the 9700, it’s easier to use a phone app to measure the 1000 Hz tone.  Alternatively you can set the side-tone to 440 Hz, get a guitar tuning app, and tune for the A4 note. Then change the GPSDO to 49.152 MHz again and you’re done.

Just for fun, you can test/verify the pull action. Note your 9700 calibration settings and then start changing them, while listening to some fixed signal that produces a steady tone. When the change gets bigger and bigger, the tone will first remain unchanged, then start to “jitter” and then change. Now you have reached the end of the pull range. Then switch off the GPSDO und retune the 9700 back to the fixed signal. The frequency difference is your pull range in one direction. Note that a 100 Hz pull range on 2 m implies a 300 Hz pull range on 70 cm and so on. Now do this again while changing calibration in the opposite direction. This should give you about the same pull range, implying your 9700 calibration is right in the middle of the total pull range. I measured about 350 Hz total pull range on 2 m. Now your 9700 is safe from frequency drift due to temperature change, fans coming on, airco’s switching, variations in the earth magnetic field, etc. etc….