More Frequency Reference

So,

Following my musings last time over the QRP Labs GPS module, I've now also built two further kits from QRP Labs. The S5351A Module and the VFO.

When you combine all these together in the way I have you can create a GPS disciplined frequency reference with two outputs.

One of the outputs can be fixed in the firmware and I plan to put mine on 10MHz, I've made a low pass filter for this output to clean out the harmonics. The other output can be user set to any frequency up to about 290MHz or so.

Here's the LPF for the 10MHz output:

And here's the 10MHz output before the LPF:

and after:

All good, egh?

GPS Disciplined - Really?

Well,

After a great deal of fiddling, learning, scratching my head then fiddling some more, I have finished my GPS disciplined Frequency Standard:

This is a project by VE2ZAZ:

http://ve2zaz.net/GPS_Std/GPS_Std.htm

So, what's this all about then?

Basically we have an oven controlled crystal oscillator (OCXO) which can be fine tuned by applying a voltage to an adjustment pin. Typically this adjustment is only +/- a few Hz around the target frequency anyhow. The oscillator itself is the large metal box you can see in the image above - it has a sticker on it saying "Used". This box is actually the oven - and yes - it heats up to maintain the crystal inside at a steady temperature.

So, we have a PIC processor on the green PCB you can see and this basically monitors the output of the OCXO and compares that to a 1pps signal from the GPS receiver. Having a known 1pps signal means we can determine if the voltage on the OCXO needs to be increased, decreased or left as-is.

That's the bottom line of the whole thing.

Now, the firmware (read software) does some stuff with averages and a state machine to determine if the output is within certain accuracy parameters, but basically it's keeping the OCXO on track.

How accurate can this be?

Well, I have mine configured to make an adjustment every 20 Hex or 32 samples. Each sample is fixed at 16 seconds (expecting 16 million counts from the OCXO).

So my averaging period is 16 * 32 = 512 seconds (or about 8.5 minutes).

Therefore the best possible accuracy I can expect is 1 in (10MHz * 512s) = 2.0E-10 (ish)

So, it's more than adequate enough for my purposes. I need to leave this running for a while and monitor the output - there's a serial connection to a "dumb terminal" spitting out data as the unit trundles away to itself.

The output waveform looks like this, this is the 10MHz signal:

There's also another output switchable between 5 MHz and 1 MHz.

Initially I had problems with the 1pps signal not seeming to be at 1pps! The symptom was that the count recorded by the unit after each 16 second sampling period would suddenly be wildly out and every 1 in 4 would seem to be accurate. Will some great assistance via email, eventually this was found to be due to noise on the PSU lines to the main PCB. I've ended up adding some extra smoothing to the +8V line that feeds the main board and the OCXO driving op-amps. This plus some extra capacitance where the lines physically connect to the board seems to have solved the problem. I also ended up adding a simple RC low pass filter to the 1pps line where that enters the microprocessor, but I don't think that had any effect at all.

I've used the output from this to calibrate the counter I made recently:

This is Geddy Cat, he is by far the best cat in the world:

Good, egh?

Frequency Reference - How Accurate?

Well,

You may remember I mentioned a GPS disciplined Frequency Reference project that I found back here:

http://g0mgx.blogspot.co.uk/2013/11/the-art-of-calibration-once-more.html

I ordered and received the PCB for the project, here's the start of construction:

The main PCB connects to a OCXO which it alters the control voltage to adjust, and also connects to a GPS receiver.

Now, I started off with a GPS receiver that was advertised as being a timing and 1pps (pulse per second) output device. This is exactly what I need as the 1pps signal from the GPS timing is used to gate the counting of the signal from the OCXO and hence alter the frequency to be exact.

Anyhow, the GPS I bought, despite what it was advertised as being, didn't contain the 1pps firmware!

So I have another unit now, this one certainly does contain the 1pps gubbins, but I don't have the correct connector for the antenna feed! However, here's what the 1pps signal looks like:

As this is only about 3V +ve I suspect I will need to push this through a transistor switch, but whatever happens it looks good to me!

I've got 2 10MHz OCXOs here, and you can see them below on test:

The physically larger unit to the rear of the photo above has a variable adjustment between 0 and 8V, the one near the front is 0 - 2.54V. Also the one at the front seems to pull a huge amount of current while it is warming up its oven - like 2A.  That is going to present some difficulties; the rear unit only pulls about 500mA. So I'll probably go with the 8V adjust unit initially and see how it performs.

Cat's not impressed:

Fun though. egh?

The art of Calibration (Once More)

Well,

After playing some with frequency counters in software, I thought back to the frequency standard that I have from back here:

http://g0mgx.blogspot.co.uk/2011/07/ocxo-oh-you-mean-ocxo.html

I thought it worth a quick check that it's still (as near as damn it) on frequency. It's an oven controlled oscillator which means that the actual gubbins that creates the signal is inside a small metal box that heats up to a fixed temperature; thus eliminating temperature related drift.

There is a voltage adjust pin on the OCXO giving you +/- a few Hz. How well I can actually calibrate it remains to be seen, but for the purpose of this exercise being fairly close is good enough. When I made the original box I included a multi turn trimmer to alter the voltage on this pin and calibrated it against the radio I had here at the time - an FT950.

I have to assume that the Kenwood radio I got here:

http://g0mgx.blogspot.co.uk/2013/08/introducing-new.html

has been accurately aligned, it also contains a 1ppm (part per million) local oscillator - so it should be a good reference for me.

So, tuning the radio to my 10MHz reference signal with a grounded antenna connection I see this:

The spectrum analyser sees this:

Both pictures above are at the same Spectrum Analyser configuration, however the information on the screen differs. You can see the centre frequency in the top image and the bandwidth in the lower one. Each horizontal square is ten Hz, so it looks like I am very close according to this instrument.

I am going to make some kind of GPS disciplined frequency counter at some point, it might also be good to make a GPS disciplined voltage adjustment for the OCXO like this one:

http://ve2zaz.net/GPS_Std/GPS_Std.htm

I'm going to order the PCB for that project and see how I get on, I've also found what I think will be a suitable GPS receiver with a 1pps (pulse per second) output on eBay - that's winging it's way from China.

However, there's always the problem that a man with two watches never knows the correct time....

Interesting, huh?