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Powerpole voltage and current monitoring is quite nice to have. One can buy commercial meters, but due to the availability of nice and cheap modules, it is very easy to make them oneself.

To the right you’ll see my combined voltage and current meter as well as my volt-meter on top of the power supply.

Both of the modules have been bought on Ebay:

• Miniature 0-30 V DC LED 2 wire Digital voltmeter (371333527599) where the display is 22 by 10 mm. Cost slightly more than $1

• 0-100 V, 0-10 A Dual Voltmeter Ammeter (262455987311) costing less than $3. The module size is 48 x 29 x 26 mm and the letters are 7 mm tall just like the miniature voltage display.

My poor-man’s 10 MHz reference based on the Ublox Neo-7M GPS module didn’t always receive GPS satellites. Since I rely on reception indoors, conditions were sometimes too marginal to lock the oscillator output to 10 MHz. Inspired by the QRPlabs GPS module of Hans Summers (G0UPL) with its large 6 x 6 cm PCB groundplane, I therefore decided to do something similar.

The first picture shows the unit with the 8.5 x 6.5 cm single-sided PCB ground plane attached with double-sided tape. It definitely helped make indoors reception in my shack much more reliable. In addition to the improved conditions for the patch antenna, it probably helps too that the antenna now is shielded from the digital circuitry of the GPS module, the 10 MHz pulse shaper, and the USB interface. I also added a small LED to the right so that I could see from the outside whether the GPS locks properly.

The second picture shows the interior prior to adding the ground plane.

This post is a continuation of these other posts about the 10 MHz reference:

1. Just good enough 10 MHz reference (3 Oct 2015)
2. Better with SMA (15 Oct 2015)
3. Curing amnesia in the 10 MHz GPS reference (19 Nov 2015)

I got my AP510 APRS tracker a little more than a year ago. It kind of worked, but not very well in my car. But after the tracker got a new firmware dated 3 Nov 2015, it has become so much better. Now I can say that it is really useful.

AP510 with original short antenna and telescopic antenna

Apparently, the Smartbeacon function didn’t work properly in earlier versions of the firmware. With some good debugging and error reporting by KC5EVE, Mark, working with the software developer for the AP510, BG6QBV, the annoying errors now seem to be gone. This is all documented in the Yahoo AP510 group.

I have fitted mine with a 16-45 cm telescopic antenna and even when attached to one of the rear headrests in my sedan, the 1 Watt of output power tracks very well.

The map below shows a drive from Telemark, about 100 km west of Oslo, to Oslo with as good coverage as one can expect given the valleys and the availability of APRS digipeaters especially in the western part.

Note the missing tracks east of LA5PPA-1 which are due to a 3.5 km long tunnel, Strømsåstunnelen, between Drammen and Mjøndalen.

Both the Baofeng UV-5R handheld UHF/VHF radio and the Sainsonic AP510 APRS tracker come with interface cables with pirated chips. These are clones of Prolific USB/serial chips. Since Prolific has taken measures against this, only old drivers will work with them. That means that one has to stop automatic driver updates as explained on the Miklor site for the Baofeng UV-5R. The same is true for the AP510. This is a nuisance.

I got tired of this and got myself some USB/serial modules from Ebay based on the CP2102 chip instead. The cost was US $1.43 a piece so it should be affordable for anyone. I also got some clear heat shrinkable tube.

It wasn’t too hard to follow the instructions on the Miklor site. I ended up replacing the chip in the original Baofeng serial cable. I’m a hardware guy so I think it is a shame not to see the three LEDs for power, rx, and tx so I used my Dremel to make a 12×12 mm cut-out in the original case, and then I closed it by using transparent shrinkable tube. For a picture, see the top of the first image.

If it doesn’t work the first time, exchange the rx and tx connections and see if that works better. According to this site, the boards can be marked just opposite of what you might think.

The Sainsonic AP510 APRS unit has a cable that on first sight just looks like a standard USB cable, but it also contains such a chip. Here I made a completely new cable without any case.  It is important that 5 Volts also passes through as this is used for charging. The pinout can be found on the site of DJ7OO (use Google translate if needed). I enclosed the board in shrinkable tube which is transparent enough for the LEDs to shine through as seen in the bottom of the first image. The board with the fake chip is found in the middle. 

So now I have interface cables for both units that don’t require me to stop updates of drivers or any other special precautions and it is much easier to program the devices from any PC.

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)

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