GPS-9700 Locked Frequency Source
The GPS-9700 is an external signal source that was developed by VK5EME and VK5WA, in March 2019 for the Icom IC-9700 Transceiver to stabilize frequency when using narrow band weak signal modes. The GPS-9700 can be used with or without an external 10MHz reference input to make a IC-9700 frequency stable to less than 1Hz on all weak signal modes including on the 23cm band. For high frequency accuracy especially when using Transverters for the high microwave bands, it is suggested that the GPS-9700 is locked to a high quality external GPS derived 10MHz frequency reference. The design incorporates a 0.28ppm low phase noise 49.152MHz Voltage Controlled Temperature Controlled Crystal Oscillator ( VCTCXO ) reference which is amplified, and filtered to produce a very clean RF output. The signal is then injected into the IC-9700’s internal 49.152MHz frequency reference using a small coupler PC board to mode lock the frequency.
Tests using a Signal Hound analyzer locked to the Trimble GPS reference indicate that the phase noise when using the GPS-9700 is identical to the the Radio on its own.
Due to the frequency stability of the IC-9700, the Ref Adjustment procedure will be required at times due to seasonal changes of temperature in many parts of the globe.
The use of this product does require removal of the Icom IC-9700 bottom cover and replacement of a cable and soldering of the small coupler PC board over the brass TCXO compartment. This should not affect Icom warrantee as No modifications are required to the Transceiver that could damage it. This also means that when the GPS-9700 is not used, the Transceiver will still function the same as from the factory and be locked to its internal TCXO. The use of the GPS-9700 however will not allow the automatic 10MHz calibration to work as the SMA to IPX cable is replaced and used with the coupler board, but this should not be an issue as manual calibration is still available.
1/ There can be an issue that the VCTCXO module has no output due to it not being successfully soldered to the board. I have come across this numerous times and it is difficult to flow solder under the device, so any intermittent or no output is probably due to soldering.
2/ Adjusting the 10MHz GPS drive level input to the GPS-9700 will affect the phase noise. It is best to use the lowest drive possible for lock, and then increase the level by 5dB for good locking. Some products have now been supplied with the NXP SA612AN which appears to require less GPS drive signal, and tests have shown that -10 to -20dB input is ideal for the lowest phase noise.
The pictures below are a final revision so should be carefully followed in construction. Probably the only really difficult part is the fitting of the AD9835. It is very important that the Kit construction notes are followed very carefully with regards to initial testing of the 5v regulator before increasing the voltage to the board. BUY THIS PRODUCT
The pictures below show the coupler board with components fitted to suit the IC-9700. Care should be taken when fitting the small IPX connector as it has to be mounted so that the centre pins connection on the underside of the connector is soldered to the PC board strip line track. When fitting the board to the IC-9700, care should be taken when dismantling the transceivers bottom cover and soldering the board into place. The SMA connector cable on the rear of the Transceiver is removed carefully replaced with a longer cable which is connected to the coupler board instead of the Transceivers main board. This means that the automatic 10MHz calibration will no longer work but it is not required when using the GPS-9700.
The Programming LZ2WSG software can be used to program the Arduino Nano for a single frequency for the AD9835.
- With the GPS-9700 disconnected, Tune the IC-9700 to a GPS locked beacon and set the mode to CW. Changing between CW and CWR should give the same tone frequency from the speaker if the IC-9700 is on frequency. Adjust the IC-9700 REF Adjust fine to mid setting, and adjust the course until the CW and CWR tones match.
Connect the GPS locked GPS-9700 to the coupler board and check that the tones still match for both CW and CWR. Some fine adjustment of the IC-9700 REF freq maybe required but is not normally necessary. If you now adjust the REF freq up and down a frequency wobble will be heard at each extreme which indicates that the GPS-9700 is trying to lock the IC-9700. This is a good indication that the GPS-9700 is working correctly. A simple way to adjust the REF freq is to centre the adjustment between the low and high frequency wobbles.
There are possible issues with the IC-9700 frequency stability changing between summer and winter temperatures. This can cause the audio to sound warbly as the GPS- 9700 is unable to pull the IC-9700 onto frequency. This will require the IC-9700 VCTCXO to be readjusted in step 3. of the testing section. This maybe more of an issue in extreme or very cold winter climates, and may not be an issue in temperate climates.
The module board is designed to be fitted into a Hammond 1455C extruded aluminium enclosure. The enclosure helps to stabilize the free running frequency of the VCTCXO by protecting it from environmental temperature changes. Assembly is quite straight forward but you will require another SMA washer to be fitted onto the RF OUT connector as a spacer before the front panel is fitted. The LED should also be fitted after the front panel has been fitted as it makes it much easier to bent the leads to fit the board before soldering into place.
1/ The original batch of Kits tested used a Philips SA612AN which has now changed to the NXP SA612AN. It appears that the optimum 10MHz GPS input drive may need to be reduced by up to 20dB when using the NXP chip version possibly due to higher gain in the oscillator amplifier. So the recommended input for the NXP chip version is -10 to -20dBm. The best way to determine this is to vary the 10MHz input level until there is lock and then reduce attenuation by a further 5dB for good lock. The lower the input with lock then gives the lowest phase noise at 49.152MHz.
2/ Low or No RF output at 49.152MHz can be due to the VCTCXO IC5 not being fully soldered to the board. Resolder the connections carefully ensuring that solder flows under the pads. his can be very difficult to do and pre tinning of the component pads and solder flux is normally required for reliable connections.
3/ No GPS lock can be due to a short to ground on capacitor C37 where it goes to pin 2 of the SA612, IC1. There was a mistake with etching of the boards and most have had the ground short cut away with a sharp scalpel knife.
4. There are possible issues with the IC-9700 frequency stability changing between summer and winter temperatures. This can cause the audio to sound warbly as the GPS- 9700 is unable to pull the IC-9700 onto frequency. This will require the IC-9700 VCTCXO to be readjusted in step 3. of the testing section. This maybe more of an issue in extreme or very cold winter climates, and may not be an issue in temperate climates.
5. No output from the AD9835 DDS can be due to the clock input on pin 6 being too low. Make sure that the clock is at least 1.2v p-p using a x10 probe on an oscilloscope. R19 may have to be adjusted to suit. On one board the 220R resistor was replaced with a 0R.