Friday, October 2, 2020

Cheap GPSDO kit - a must have for service bench.

Sometimes we all need to compare quality of our equipment, make some tunning of circuits, or just check/recalibrate rig frequency. 

For those working on TRX repair good stable and accurate signal frequency is a must have. Unfortunately, modern (mostly from BY) cheap frequency counters using (as per description ) internal TCXO generators, but the truth is, they are not locked perfectly and from my experience they are "no go" for proper RIG tuning/calibration. All this lead me to "buy" some GPSDO. 

Because I am not happy "buying" things I can make with my own hands, so decided to buy a kit instead. 

Fortunately polish ham Marek SQ7HJB designed some neat GPSDO device with Arduino Nano, GPS module and SIS5351 with two signal outputs, and neat OLED display. Max frequency is determined by SIS5351 and in my case ends up at about 155MHz.

Completed PCB - pic source:

 GPSDO diagram - pic source:

Overall cost of this GPSDO kit (without Hammond enclosure) is ~ 50 Euros. That's not the price to make bank loan.     

Below You can see my approach to this nice piece of equipment, if You decide to make something similar I can help You with Polish language :-)

Below You can see a kit - THT version of PCB, Arudino, encoder, WaveShare GPS with ceramic antenna, OLED display and SIS5351 generator.

Comple GPSDO kit - pic source:

After soldering all joints, voltage regulator and pins ( which takes ~1hour and it's very easy ) we can power the device and check for proper voltages, non intentional shorts etc. If all is ok, now time to solder heart and brain of this device. Be carefull as all of them are ESD sensitive! 

Asembling instructions is VERY accurate and exact, unfortunately in Polish but this should not be a problem. All vital information You can see on RADIOHOBBY project page, if You need more info, mail me. 

You should end up with PCB looking something like that : 

 If all well GPSDO is running form first shot showing some information.

You'll see LED on GPS module lit (red one on pic above), and will start blink immediately after picking up satellite signal.

After some minute or two when generator will be locked at programmed frequency "LOCK" LED(green) will lit as well and after few (2-3) GPS count passes we can switch the display to show frequency correction. Small @ on the right show that GPS is locked, and accuracy is better than 1Hz.

There are several ways to program frequencies - with encoder, or with PC. If You are familiar with Arduino programming sources are available on project page in package. To be honest I get mine already programmed kindly by Marek (tnx a lot).  

4 memory banks for most needed frequencies can be programmed with multi encoder, which is also suitable for QRG step change, time zone settting and display switching.  

Two outputs of GPSDO can be populated with low pass filters - i.e one 10MHz filter for frequency counter clock, and second for frequency You need. Or - just leave the second one unpopulated - feed directly from SIS5351 output for various frequencies as I did. Also, if needed (as in my case) - I had to change GPS antenna for ordinary external GPS car antenna block with 3m RG174 cable.

All this fit perfectly into Hammond enclosure, but this depends on your choice.

In case anyone need more info - don't hesitate to drop me @.


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