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I had some trouble closing the lid on the “Just good enough 10 MHz GPS reference” due to the size of the BNC jack. Therefore I changed it to an SMA (SubMiniature version A) female jack. A thin cable connects it to the K3’s SMA input and there is no need for any SMA-BNC adapter on that end.

At the same time I moved the GPS antenna to a more central location in the tin, in the hope that the walls of the tin would interfere less with GPS reception. That’s the theory anyway, if it matters much in practice is a different story.

Actually, I think I’m going to use SMA more often with these clear top tins and also Altoids tins. They take up much less space and are easier to install and to work with.

There aren’t any high power applications for circuitry in such tins, so I cannot so any reason why the SMA won’t work just as well or even better than the BNC.

Other related posts about the 10 MHz reference:

• Just good enough 10 MHz reference (3 Oct 2015)
• Curing amnesia in the 10 MHz GPS reference (19 Nov 2015)
• Improved GPS reception with a ground plane (11 April 2016)

I had some trouble closing the lid on the “Just good enough 10 MHz reference” due to the size of the BNC jack. Therefore I changed it to an SMA (SubMiniature version A) female jack. A thin cable connects it to the K3’s SMA input and there is no need for any SMA-BNC adapter on that end.

At the same time I moved the GPS antenna to a more central location in the tin, in the hope that the walls of the tin would interfere less with GPS reception. That’s the theory anyway, if it matters much in practice is a different story.

Actually, I think I’m going to use SMA more often with these clear top tins and also Altoids tins. They take up much less space and are easier to install and to work with.

There aren’t any high power applications for circuitry in such tins, so I cannot so any reason why the SMA won’t work just as well or even better than the BNC.

Some time ago I noticed that the Ublox Neo-7M GPS has a 10 MHz output which is locked to the GPS system’s accuracy. Most people kept saying how useless it was due to excessive jitter unless it was cleaned up with a phase locked loop of some sort.

At about the same time I installed the external reference input for my Elecraft K3. The K3EXREF enables the K3’s frequency to be locked to an external 10 MHz reference. What struck me was how its function is described:

• The frequency of the internal oscillator of about 49.38 MHz is continuously measured and averaged, obtaining a value to the nearest 1 Hz.
• The K3EXREF does not phase lock the K3’s reference oscillator and the external 10 MHz source has no impact on the K3’s phase noise performance.

This got me wondering if the Neo-7M would be just good enough as a reference and that all the averaging internally to the K3 would take care of the jitter.

I ordered one from Ebay for USD 12-13 together with an USB interface (USD 1.5) and hooked it up. (Actually the NEO-7M already has a built-in USB interface, but my board doesn’t support it). The result is shown above as assembled in a clear top tin. In my wooden house I can receive GPS indoors, so I have no need for an external antenna

The K3 accepts the input and I see the star in REF*CAL blinking. Just after turn-on of the K3 my 49.38 MHz reference frequency ends in …682 and after 10-15 minutes it has fallen and stabilized to …648, i.e. 34 Hz down in frequency. This is just 8 Hz off the reference value I determined manually was the right one when my K3 was new in 2009 (49.379.640).

All this taken together indicates to me that the K3 finds this 10 MHz acceptable for locking. The measurement to the nearest Hz, implies a measurement time of the order of 1 second and that seems to be enough to smooth out the jitter from the Neo-7M.

In order to get this to work I had to do some modifications to the GPS unit. First I had to get access to the timepulse on the chip’s pin 3. My connection is inspired by that of G4ZFQ and consists of a small wire from the left-hand side of the 1k resistor to the upper left hole. From there another grey wire goes below the chip and to the 5-pin header which is soldered to the Vcc, Rx, Tx, Gnd pins. The 5th pin is cut off and is just attached to the other pins through the plastic hardware.

The second modification was required in order to get it to run from the somewhat noisy USB 5 Volt supply. That took some decoupling between the Vcc and Gnd pins (220 uF and 0.1 uF in parallel), visible to the right in the image above, using good engineering practice to keep the wires as short as possible.

The timepulse is a 3.3 Vp-p output which cannot drive anything below 400-500 ohms impedance. Therefore I added a 74HCT04 driver that I have assembled on a little homemade SMD to DIL adapter PCB (easy to find on Ebay). It serves as a driver to feed the 10 MHz to the 50 ohm input of the K3EXREF.

The HCT04 IC has 6 inverters. One of them takes the input signal from the Timepulse output of the GPS IC and buffers it to drive the 5 other inverters in parallel. This is shown in the schematics at the end of this blog post.

The 5Vp-p output from the buffers is fed via 56 ohms to a connector that goes to the K3EXREF input. This is in accordance with the K3EXREF manual which says: “The 10 MHz source should have a signal level between +4 dBm and +16 dBm, nominal. For square wave sources, 2VDC to 3.3VDC peak is optimum. If the source is a 5V logic level, use a 50-ohm resistor in series with the input.“

In order to set up the GPS I have used the u-center program (Menu: View, Configuration View, TP5 (Timepulse 5)) from Ublox and set it up with the parameters shown to the right. It blinks at 4 Hz before the signal is acquired and then switches to 10 MHz. This can be observed on the green LED connected to the Timepulse output also as it switches from blinking to a half-lit status. 

See also

• Better with SMA
• Curing amnesia in the 10 MHz GPS reference
• Improved GPS reception with a ground plane

Some time ago I noticed that the Ublox Neo-7M GPS has a 10 MHz output which was locked to the GPS system’s accuracy. Most people kept saying how much jitter it had and how useless it was unless it was cleaned up with a phase locked loop of some sort.

At the same time I got the 10 MHz reference input for my Elecraft K3 (K3EXREF). What struck me was how its function was described:

• The frequency of the internal oscillator of about 49.38 MHz oscillator would be continuously measured and averaged, obtaining a value to the nearest 1 Hz.
• The K3EXREF does not phase lock the K3’s reference oscillator and the external 10 MHz source has no impact on the K3’s phase noise performance.

This got me wondering if the Neo-7M would be just good enough as a reference and that all the averaging internally to the K3 would take care of its jitter.

I ordered one from Ebay for USD 12-13 together with an USB interface (USD 1.5) and hooked it up. The result is shown above as assembled in a clear top tin. In my shack I can receive GPS indoors, so I have no need for an external antenna

The K3 accepts the input and I see the star in REF*CAL blinking. Just after turn-on of the K3 my reference frequency ends in … 682 and after 10-15 minutes it has fallen and stabilized to …648, i.e. 34 Hz down in frequency. This is just 8 Hz off the reference value I determined manually was the right one when my K3 was new in 2009 (49,379,640).

All this taken together indicates to me that the K3 finds this 10 MHz acceptable for locking.

In order to get this to work I had to do some modifications to the GPS unit. First I had to get access to the timepulse on the chip’s pin 3. My connection is inspired by that of G4ZFQ and consists of a small wire to the upper left hole. From there another grey wire goes below the chip and to the 5-pin header which is soldered to the Vcc, Rx, Tx, Gnd pins. The 5th pin is cut off and is just attached to the other pins through the plastic hardware.

The second modification was required in order to get it to run from the somewhat noisy USB 5 Volt supply. That took some decoupling between the Vcc and Gnd pins (220 uF and 0.1 uF in parallel), visible to the right in the image above, using good engineering practice to keep the wires as short as possible.

The timepulse is a 3.3 Vp-p output which cannot drive anything below 4-500 ohms impedance. Therefore I added a 74HCT04 driver that I have assembled on a little homemade SMD to DIL adapter PCB. It serves as a driver to feed the 10 MHz to the 50 ohm input of the K3EXREF.

The HCT04 IC has 6 inverters. One of them takes the input signal from the timepulse output of the GPS IC and buffers it to drive the 5 other inverters in parallel. This is shown in the schematics at the end of this blog post.

The 5Vp-p output from the buffers is fed via 56 ohms to a connector that goes to the K3EXREF input. This is in accordance with the K3EXREF manual which says: “The 10 MHz source should have a signal level between +4 dBm and +16 dBm, nominal. For square wave sources, 2VDC to 3.3VDC peak is optimum. If the source is a 5V logic level, use a 50-ohm resistor in series with the input.“

In order to set up the GPS I have used the u-center program (Menu: View, Configuration View, TP5 (Timepulse 5)) from Ublox and set it up with the parameters shown to the right. It blinks at 4 Hz before the signal is acquired and then switches to 10 MHz. This can be observed on the green LED connected to the Timepulse output also as it switches from blinking to a half-lit status. 

I’ve always enjoyed playing about with time. Accurate time is not really a fascination but I do like a clock to tell the time. The MSF 60khz time signal is one source and I have played about with that system with an Arduino and it works well, when there is a good signal for a whole minute. GPS time has always been a bit of a thing for me because you can set it to UTC and it’ll always show UTC and frankly there are a lot more libraries available to play with. GPS Tiny & GPS Tiny+ are two of those and this evening I ‘forked’ a sketch to use a cheapo off the shelf gps module to tell the time and date on a 16×2 LCD. Nothing spectacular but hey if I can do it then so can anyone. Here’s a short sweet video of it in action (near the window!)

sketch goes a little like this

#include <TinyGPS++.h>
#include <SoftwareSerial.h>
#include <LiquidCrystal.h>
/*
This sample sketch demonstrates the normal use of a TinyGPS++ (TinyGPSPlus) object.
It requires the use of SoftwareSerial, and assumes that you have a
4800-baud serial GPS device hooked up on pins 8(rx) and 9(tx).
*/
static const int RXPin = 8, TXPin = 9;
static const uint32_t GPSBaud = 9600;
// The TinyGPS++ object
TinyGPSPlus gps;
// The serial connection to the GPS device
SoftwareSerial ss(RXPin, TXPin);
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
void setup()
{
ss.begin(GPSBaud);
lcd.begin(16,2);
lcd.setCursor(1,0);
lcd.print("Tiny GPS+ Time");
delay(3000);
lcd.setCursor(1,2);
lcd.print("by Alex, g7kse");
delay(5000);
lcd.clear();
}
void loop()
{
// This sketch displays information every time a new sentence is correctly encoded.
while (ss.available() > 0)
if (gps.encode(ss.read()))
displayInfo();
if (millis() > 5000 && gps.charsProcessed() < 10)
{
lcd.print("No GPS detected");
for (int positionCounter = 0; positionCounter < 20; positionCounter++) {
while(true);
}
}
}
void displayInfo()
{
lcd.setCursor(4,0);
{
if (gps.time.hour() < 10) lcd.print(F("0"));
lcd.print(gps.time.hour());
lcd.print(F(":"));
if (gps.time.minute() < 10) lcd.print(F("0"));
lcd.print(gps.time.minute());
lcd.print(F(":"));
if (gps.time.second() < 10) lcd.print(F("0"));
lcd.print(gps.time.second());
}
lcd.setCursor(3,2);
{
if (gps.date.day() < 10) lcd.print(F("0"));
lcd.print(gps.date.day());
lcd.print(F("/"));
if (gps.date.month() < 10) lcd.print(F("0"));
lcd.print(gps.date.month());
lcd.print(F("/"));
lcd.print(gps.date.year());
}
}