Computing stuff tied to the physical world

TK – Frequency Meter

In Hardware on May 24, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

Another post about frequencies – this time I’ve assembled a DFD4A from Almost All Digital Electronics:

DSC 3124

It’s a low-cost frequency counter which goes all the way up to 3 GHz. Here it’s measuring a 10 MHz signal from my Frequency Generator, while synchronized to the Rubidium frequency standard.

As you can see, it’s spot on – the last digit flips between 0 and 1 every so often, that’s all.

As with the Capacitance Meter I assembled recently, this kit comes with detailed build instructions. Except that this time I didn’t really want to build it, so I got the pre-built version instead, including the connectors and (fully) plastic enclosure. The front plate already has all the right cutouts, and a printed piece of paper (!) glued to the front. Works ok, but I suspect that it’ll get dirty over time.

The unit came with all the parts, I just had to solder a few components and wires in place after inserting all the switches and BNC connectors.

One thing missing was the 9V battery clip – but not to worry, I have a couple of those lying around anyway.

The reason to get this particular unit was its high frequency range of well over the 868 MHz and 2.4 GHz frequencies I may want to measure here at JeeLabs. The main difference with a professional unit is probably the fact that it doesn’t have many input signal options:

  • HF measures from 0 to 30 MHz, with 5 Vpp max into a high impedance input
  • UHF measures from 10 to 3000 MHz over a 50 Ω input (max 15 dBm)

No way to directly measure the 868 MHz output from an RFM12B, I suspect – i.e. it probably won’t be sensitive enough to measure 0 dBm.

The slow measurement mode continuously collects data for one second, so you get 1 Hz resolution on the HF range and 100 Hz resolution on the UHF range (since that’s essentially just a ÷ 100 prescaler).

The fast measurement mode runs 10 times per second, i.e. a gate time of 0.1s – so this gives 10 Hz resolution on HF and 1000 Hz (1 KHz) resolution on UHF.

It’s a bit odd that the display shows more significant digits than are being measured in all but FAST + HF mode, but no big deal – the current mode is clearly visible from the switch settings.

Knowing that the counter is very accurate (for now – it’ll no doubt gradually drift slightly), it’s time to find out how accurate the TG2511 AWG’s frequency is when not synchronized to the Rubidium standard:

DSC 3125

That’s only 0.4 ppm off and well within spec – excellent!

  1. Why are there two dots in “10.000.000 MHz”? Are they “European commas”, which makes it 10 THz (i.e wrong prefix), or is it a strange use of decimal points?

    • That display does not appear to have decimal points, it looks like the programmer used a digit spot for the decimal point. It would not be too confusing to to leave out a dot segment to creat a comma.

  2. Heh, indeed. You’ll have to ask the programmer, I suspect that he didn’t even notice…

    It’s a pre-programmed PIC, unfortunately – I don’t have the right setup and tools to tinker with it.

  3. maybe it’s just easier to read numbers with many digits when they are grouped into threes? At first I didn’t even notice the incorrect usage of the dots.

  4. I’m curious how this works: “MODE2 HF SLOW: max input frequency 40MHz, resolution 1Hz., gate time 1sec”. To achieve that, it must be able to do direct counting at 40 MHz, but the max. clock rate of the PIC16C71 is only 20 MHz, and its hardware counter runs at 1/4 that (5 MHz). I see there is a 4046 PLL but how do you use that here?

  5. @jbeale, it seems only the phase comparator section of the 4046 PLL is in use – perhaps just as a signal conditioner ahead of the PIC input?

  6. Hm. When run in asynchronous mode, timer 1 has a min cycle requirement of 60 ns, i.e some 16 MHz (independent of the MPU clock). Maybe he’s simply running it out of spec in that mode?

    My frequency generator only goes up to 25 MHz, and it works fine up to that rate.

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