Posts Tagged SDR

QRPme Pocket Pal II

A QRPme Pocket Pal II could be a suitable project for a Squidwrench “advanced soldering” class:

QRPme Pocket Pal II - front

QRPme Pocket Pal II – front

Yes, it comes with a tin case:

QRPme Pocket Pal II - tin case

QRPme Pocket Pal II – tin case

You must fit your own insulating sheet under the PCB; polypropylene snipped from a retail package works fine.

It’s intended as a “mint tin sized tester for all kinds of hamfest goodies”, but it seems like a nice source of small currents, voltages, and signals suitable for stimulating all manner of circuitry one might encounter in later sessions of a beginning electronics class.

Before using it, of course, one must solder a handful of small through-hole parts into the PCB, a skill none of us were born with.

For completeness, the back side, hot from the soldering iron:

QRPme Pocket Pal II - rear

QRPme Pocket Pal II – rear

The kits (always buy two of anything like this) arrived minus a few parts, which I suspect was due to an avalanche of orders brought on by a favorable QST review. Fortunately, I (still) have a sufficient Heap o’ Parts to finish it off without resupply, although a hank of 9 V battery snaps will arrive in short order.



FM DDS: SPI Mock 3

Running some serial I/O in the background adds jitter to the timer interrupt pacing the ADC samples and as-yet-unwired DDS updates. For reference, an overview of the process showing the procession from the IRQ on the left to the SPI outputs near the middle and another IRQ on the far right:

DDS Mock - 0 VAC - SPI

DDS Mock – 0 VAC – SPI

Now, speed up the sweep and delay the trace by 25 μs to put the triggering pulse off-screen to the left and the second pulse at the center division:

ADC Sample IRQ jitter

ADC Sample IRQ jitter

The orange smear in the middle should be a tidy pulse, but it isn’t.

The  25 μs timer interrupt now has the highest priority on the front burner:

IntervalTimer AudioSampler;

... snippage ...

  if (!AudioSampler.begin(AudioSamplerIRQ, SamplePeriod)) {
    Serial.printf("Timer start failed\n");
    while (true) {

Although nothing can interrupt it, other code / handlers may disable interrupts around their own critical sections and delay the tick. If the triggering tick (the off-screen one starting the trace) is delayed, then the on-screen pulse will appear “too soon”, to the left of center. If the triggering tick is on time, but the on-screen pulse is delayed, it’ll appear “too late” on the right.

The blur is (roughly) symmetric around the center graticule line, so the handwaving seems about right.

In round numbers, the jitter moves the interrupt ±325 ns on either side of its nominal position, with most of the pulses within ±100 ns. I doubt the jitter distribution is Gaussian, but vigorous handwaving says the RMS jitter might amount to 75 ns.

At the 4 kHz audio band limit, a 75 ns sampling error a phase error of 0.1°, so the maximum amplitude jitter would be sin(0.1°) = 0.002 = -55 dB, which might suffice for amateur-radio audio.

I think, anyhow.


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FM DDS: Floating Point Timing

Inserting a few simple floating point operations between the SPI transfers provides a quick-n-dirty look at the timings:

Math timing - double ops

Math timing – double ops

The corresponding code runs in the ADC end-of-conversion handler:

void adc0_isr(void) {


  AnalogSample = adc->readSingle();                     // fetch just-finished sample

  SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
  digitalWriteFast(DDS_FQUD_PIN, LOW);

  SPI.transfer(DDSBuffer.Phase);                // interleave with FM calculations
  TestFreq += DDSStepFreq;
  TestFreq -= DDSStepFreq;
  TestFreq *= DDSStepFreq;
  TestFreq /= DDSStepFreq;
  SPI.endTransaction();                         // do not raise FQ_UD until next timer tick!


The FlipPin() function twiddling the output bit takes a surprising amount of time, as shown by the first two gaps in the blocks of SPI clocks (D4). Some cursor fiddling on a zoomed scale says 300 ns = 50-ish cycles for each call. In round numbers, actual code doing useful work will take longer than that.

Double precision floating add / subtract / multiply seem to take about 600 ns. That’s entirely survivable if you don’t get carried away.

Double precision division, on the other paw, eats up 3 μs = 3000 ns, so it’s not something you want to casually plunk into an interrupt handler required to finish before the next audio sample arrives in 20 μs.

Overall, the CPU utilization seems way too high for comfort, mostly due to the SPI transfers, even without any computation. I must study the SPI-by-DMA examples to see if it’s a win.


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Motorola K1003A Channel Element: Oscillation!

A handful of Motorola K1003A Receive Channel Elements arrived from eBay:

Motorola Channel Elements - overview

Motorola Channel Elements – overview

Having three 13466.666 kHz candidates, two with gold labels (2 ppm tempco) I disemboweled a silver-label (5 ppm) victim:

Motorola Channel Element - silver label

Motorola Channel Element – silver label

They’re well-studied, with readily available schematics:



For lack of anything smarter, I put a 1 kΩ resistor from RF Out to Ground to get some DC current going, then used a 470 nF cap and 47 Ω resistor as an AC load:

K1003 Channel Element - bias lashup

K1003 Channel Element – bias lashup

Which oscillated around a mid-scale DC bias, but looked ugly:

K1003 Channel Element - 13.4 MHz output - 1k bias

K1003 Channel Element – 13.4 MHz output – 1k bias

Perusing some receiver schematics suggested a heavier DC load, so I swapped in a 470 Ω resistor:

K1003 Channel Element - 470 ohm bias - 13.4 MHz output

K1003 Channel Element – 470 ohm bias – 13.4 MHz output

It’s now running around 3 V bias with fewer harmonics; the scope’s frequency display in the upper right corner seems happier, too.

The receiver will run that through a filter to wipe off the harmonics, then multiply the frequency by three to get the mixer LO.

There are many, many different Channel Elements out there, in receive and transmit flavors, but at least I have some idea what’s going on inside.


Frequency Modulated DDS: SPI Mock 1

The general idea is to frequency modulate the sine wave coming from a DDS, thereby generating a signal suitable for upconverting in amateur repeaters now tied to unobtainable crystals. The crystals run from 4-ish to 20-ish MHz, with frequency multiplication from 3 to 36 producing RF outputs from 30-ish MHz through 900-ish MHz; more details as I work through the choices.

The demo code runs on a bare Teensy 3.6 as a dipstick test for the overall timing and functionality:

FM DDS - Teensy 3.6 SPI demo

FM DDS – Teensy 3.6 SPI demo

The fugliest thing you’ve seen in a while, eh?

An overview of the results:

Analog 4 kHz @ 40 kHz - SPI demo overview

Analog 4 kHz @ 40 kHz – SPI demo overview

The pulses in D1 (orange digital) mark timer ticks at a 40 kHz pace, grossly oversampling the 4 kHz audio bandwidth in the hope of trivializing the antialiasing filters. The timer tick raises the DDS latch pin (D6, top trace) to change the DDS frequency, fires off another ADC conversion, and (for now) copies the previous ADC value to the DAC output:

void timer_callback(void) {
  digitalWriteFast(DDS_FQUD_PIN,HIGH);                // latch previously shifted bits
  adc->startSingleRead(AUDIO_PIN, ADC_0);             // start ADC conversion
  analogWriteDAC0(AnalogSample);                      // show previous audio sample

The purple analog trace is the input sine wave at 4 kHz. The yellow analog stairstep comes from the DAC, with no hint of a reconstruction filter knocking off the sharp edges.

The X1 cursor (bold vertical dots) marks the start of the ADC read. I hope triggering it from the timer tick eliminates most of the jitter.

The Y1 cursor (upper dotted line, intersecting X1 just left of the purple curve) shows the ADC sample apparently happens just slightly after the conversion. The analog scales may be slightly off, so I wouldn’t leap to any conclusions.

The pulses in D2 mark the ADC end-of-conversion interrupts:

void adc0_isr(void) {
  AnalogSample = adc->readSingle();                     // fetch just-finished sample
  SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
  digitalWriteFast(DDS_FQUD_PIN, LOW);
  SPI.transfer(DDSBuffer.Phase);                // interleave with FM calculations
  SPI.endTransaction();                         // do not raise FQ_UD until next timer tick!

The real FM code will multiply the ADC reading by the amplitude-to-frequency-deviation factor, add it to the nominal “crystal” frequency, convert the sum to the DDS delta-phase register value, then send it to the DDS through the SPI port. For now, I just send five constant bytes to get an idea of the minimum timing with the SPI clock ticking along at 8 MHz.

The tidy blurs in D4 show the SPI clock, with the corresponding data in D5.

D6 (top trace) shows the DDS FQ_UD (pronounced “frequency update”) signal dropping just before the SPI data transfer begins. Basically, FQ_UD is the DDS Latch Clock: low during the delta-phase value transfer, with the low-to-high transition latching all 40 control + data bits into the DDS to trigger the new frequency.

A closer look at the sample and transfer:

Analog 4 kHz @ 40 kHz - SPI demo detail

Analog 4 kHz @ 40 kHz – SPI demo detail

For reference, the digital players from bottom to top:

  • D0 – unused here, shows pulses marking main loop
  • D1 – 40 kHz timer ticks = ADC start conversion
  • D2 – ADC end of conversion,”FM calculation”, send DDS data
  • D3 – unused here, shows error conditions
  • D4 – SPI clock = rising edge active
  • D5 – SPI MOSI data to DDS = MSB first
  • D6 – SPI CS = FQ_UD = DDS latch

Remember, the yellow analog stairstepped trace is just a comfort signal showing the ADC actually samples the intended input.

The ARM CPU has floating-point hardware, but I suspect fixed-point arithmetic will once again win out over double-precision multiplies & divides.

Dropping the sampling to 20 kHz would likely work just as well and double the time available for calculations. At least now I can measure what’s going on.

All in all, it looks feasible.

And, yes, the scope is a shiny new Siglent SDS2304X with the MSO logic-analyzer option. It has some grievous UX warts & omissions suggesting an architectural botch job, but it’s mostly Good Enough for what I need. More later.

The TeensyDuino source code as a GitHub Gist:


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Teensy 3.6 Double Precision Floats

Having spent a bit of effort wringing enough precision from an Arduino to make the 60 kHz quartz resonator tester, this came as a relief:

DDS frequency:  180000000.0000000 Hz
      epsilon:          0.0000001 Hz
         step:          0.0419095 Hz

Center frequency:  146520000.0000000 Hz
  146520000.0000001 Hz
  146520000.0000002 Hz
  146520000.0000003 Hz
  146520000.0000004 Hz
  146520000.0000004 Hz
  146520000.0000005 Hz
  146520000.0000006 Hz
  146520000.0000007 Hz
  146520000.0000008 Hz
  146520000.0000009 Hz
  146520000.0000010 Hz

... snippage ...

  146520000.0000099 Hz
  146520000.0000100 Hz
  146520000.0419195 Hz
  146520000.0838290 Hz
  146520000.1257386 Hz
  146520000.1676481 Hz
  146520000.2095576 Hz
  146520000.2514671 Hz
  146520000.2933766 Hz
  146520000.3352861 Hz
  146520000.3771957 Hz
  146520000.4191052 Hz
  146520000.4610147 Hz
  146520000.5029242 Hz
  146520000.5448337 Hz
  146520000.5867432 Hz
  146520000.6286528 Hz
  146520000.6705623 Hz
  146520000.7124718 Hz
  146520000.7543813 Hz
  146520000.7962908 Hz
  146520000.8382003 Hz
  146520000.8801098 Hz
  146520000.9220194 Hz
  146520000.9639289 Hz
  146520001.0058384 Hz
  146520001.0477479 Hz
  146520001.0896574 Hz
  146520001.1315669 Hz
  146520001.1734765 Hz

Which comes from a PJRC Teensy 3.6 running this code:

double DDSFreq, EpsilonFreq, DDSStepFreq;
double CenterFreq, TestFreq;

... in setup() ...

  DDSFreq = 180.0e6;
  EpsilonFreq = 1.0e-7;
  DDSStepFreq = DDSFreq / (1LL << 32);
  Serial.printf("DDS frequency: %18.7f Hz\n",DDSFreq);
  Serial.printf("      epsilon: %18.7f Hz\n",EpsilonFreq);
  Serial.printf("         step: %18.7f Hz\n\n",DDSStepFreq);

  CenterFreq = 146520000.0;
  TestFreq = CenterFreq;
  Serial.printf("Center frequency: %18.7f Hz\n",CenterFreq);

... in loop() ...

  if (TestFreq < (CenterFreq + 100*EpsilonFreq))
    TestFreq += EpsilonFreq;
    TestFreq += DDSStepFreq;

  Serial.printf(" %18.7f Hz\n",TestFreq);

The IEEE-754 spec says a double floating-point variable carries about 15.9 decimal digits, which agrees with the 9 integer + 7 fraction digits. The highlight lowlight (gray bar) in the first figure shows the slight stumble where adding 1e-7 changes the sum, but not quite enough to affect the displayed fraction.

In round numbers, an increment of 1e-5 would work just fine:

  146520000.0000100 Hz
  146520000.0000200 Hz
  146520000.0000300 Hz
  146520000.0000401 Hz
  146520000.0000501 Hz
  146520000.0000601 Hz
  146520000.0000701 Hz
  146520000.0000801 Hz
  146520000.0000901 Hz
  146520000.0001001 Hz
  146520000.0001101 Hz
  146520000.0001202 Hz
  146520000.0001302 Hz
  146520000.0001402 Hz
  146520000.0001502 Hz
  146520000.0001602 Hz
  146520000.0001702 Hz
  146520000.0001802 Hz
  146520000.0001903 Hz
  146520000.0002003 Hz
  146520000.0002103 Hz
  146520000.0002203 Hz
  146520000.0002303 Hz

You’d use the “smallest of all” epsilon in a multiplied increment, perhaps to tick a value based on a knob or some such. Fine-tuning a VHF frequency with millihertz steps probably doesn’t make much practical sense.

The DDS frequency increment works out to 41.9095 mHz, slightly larger than with the Arduino, because it’s fot a cheap DDS eBay module with an AD9851 running a 180 MHz (6 × 30 MHz ) clock.



Raspberry Pi Swap File Size

As part of some protracted flailing around while trying to get GNU Radio running on a Raspberry Pi 3, I discovered Raspbian defaults to a 100 MB swap file, rather than a swap partition, and everything I thought I knew about swap management seems inoperative. The key hint came from some notes on gr-gsm installation.

Tweak the /etc/dphys-swapfile config file to set CONF_SWAPFACTOR=2 for a 2 GB swap file = twice the size of the Pi’s 1 GB memory.

Start it up:

sudo dphys-swapfile swapoff
sudo dphys-swapfile setup
sudo dphys-swapfile swapon

And verify it worked:

cat /proc/meminfo 
MemTotal:         949580 kB
MemFree:          194560 kB
MemAvailable:     594460 kB
Buffers:           85684 kB
Cached:           377276 kB
SwapCached:            0 kB
Active:           600332 kB
Inactive:         104668 kB
Active(anon):     250408 kB
Inactive(anon):    20688 kB
Active(file):     349924 kB
Inactive(file):    83980 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       1918972 kB
SwapFree:        1918972 kB
Dirty:                40 kB
Writeback:             0 kB
AnonPages:        242072 kB
Mapped:           136072 kB
Shmem:             29060 kB
Slab:              33992 kB
SReclaimable:      22104 kB
SUnreclaim:        11888 kB
KernelStack:        1728 kB
PageTables:         3488 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     2393760 kB
Committed_AS:     947048 kB
VmallocTotal:    1114112 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
CmaTotal:           8192 kB
CmaFree:            6796 kB

Then it became possible to continue flailing …

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