Archive for category Software

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:




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.



Propane Tank QD Fitting Adapter, PETG Edition

Smoking bacon during the winter months brought the third tank into play, requiring the POL-to-QD adapter I’d had in the drawer for just such an occasion. Not much to my surprise, the old PLA fitting adapter snapped along the layers near the outside end of the triangular snout:



So I ran off the two orange ones in PETG with six perimeter layers and 50% infill density:

Propane QD Adapter Tool - Slic3r

Propane QD Adapter Tool – Slic3r

Those should last roughly forever …

The OpenSCAD source code as a GitHub Gist:


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Streaming Radio Player: CE Timing Tweak

Adding delays around the SPI control signal changes reduced the OLED glitch rate from maybe a few a week  to once a week, but didn’t completely solve the problem.

However, (nearly) all the remaining glitches seem to occur while writing a single row of pixels, which trashes the rest of the display and resolves on the next track update. That suggests slowing the timing during the initial hardware setup did change the results.

Another look at the Luma code showed I missed the Chip Enable (a.k.a. Chip Select in the SH1106 doc) change in

def _write_bytes(self, data):
    gpio = self._gpio
    if self._CE:
        gpio.output(self._CE, gpio.LOW)  # Active low

    for byte in data:
        for _ in range(8):
            gpio.output(self._SDA, byte & 0x80)
            gpio.output(self._SCLK, gpio.HIGH)
            byte <<= 1
            gpio.output(self._SCLK, gpio.LOW)

    if self._CE:
        gpio.output(self._CE, gpio.HIGH)

What remains unclear (to me, anyway) is how the code in Luma's bitbang class interacts with the hardware-based SPI code in Python’s underlying spidev library. I think what I just changed shouldn’t make any difference, because the code should be using the hardware driver, but the failure rate is now low enough I can’t be sure for another few weeks (and maybe not even then).

All this boils down to the Pi’s SPI hardware interface, which changes the CS output with setup / hold times measured in a few “core clock cycles”, which is way too fast for the SH1106. It seems there’s no control over CS timing, other than by changing the kernel’s bcm2708 driver code, which ain’t happening.

The Python library includes a no_cs option, with the caveat it will “disable use of the chip select (although the driver may still own the CS pin)”.

Running vcgencmd measure_clock core (usage and some commands) returns frequency(1)=250000000, which says a “core clock cycle” amounts to a whopping 4 ns.

Forcibly insisting on using Luma’s bitbang routine may be the only way to make this work, but I don’t yet know how to do that.

Obviously, I should code up a testcase to hammer the OLED and peer at the results on the oscilloscope: one careful observation outweighs a thousand opinions.


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MPCNC: Linear Bearing Pen Holder

The simplest way to push a pen (or similar thing) downward with constant force may be to hold it in a linear bearing with a weight on it, so I gimmicked up a proof-of-concept. The general idea is to mount the pen so its axis coincides with the DW660 spindle, so as to have the nib trace the same path:

DW660 Pen Holder - unweighted

DW660 Pen Holder – unweighted

The puck mimics the shape of the DW660 snout closely enough to satisfy the MPCNC’s tool holder:

DW660 Pen Holder - Slic3r

DW660 Pen Holder – Slic3r

The pen holder suffers from thin walls constrained by the 10 mm (-ish) pen OD and the 12 mm linear bearing ID, to the extent the slight infill variations produced by the tapered pen outline change the OD. A flock of 16 mm bearings, en route around the planet even as I type, should provide more meat.

In any event, 3D printing isn’t noted for its perfect surface finish, so I applied an epoxy layer and rotated the holder as it cured:

DW660 Pen Holder - epoxy coating

DW660 Pen Holder – epoxy coating

After letting it cure overnight, I ran a lathe tool along the length to knock down the high spots and set the OD to 11.9+ mm. Although the result turns out to be a surprisingly nice fit in the bearing, there’s no way epoxy can sustain the surface load required for the usual precision steel-on-steel fit.

A plastic pen in a plastic holder weighs 8.3 g, which isn’t quite enough to put any force on the paper. Copper weighs 9 g/cm³ = 9 mg/mm³ and 10 AWG wire is 2.54 mm OD = 5 mm², so it’s 45 mg/mm: to get 20 g, chop off 450 mm of wire.

I chopped off a bit more than that, straightened it, annealed it, and wound it around a random contestant from the Bucket o’ Sticks with an OD just over the pen OD:

DW660 Pen Holder - copper weight forming

DW660 Pen Holder – copper weight forming

The helix is 13.5 mm down the middle of the turns and 14 turns long (trimmed of the tail going into the chuck and fudging the tail sticking out as a partial turn), so it’s 593 mm long and should weigh 26.7 g. It actually weighs 27.6 g: close enough.

Which is enough to overcome stiction due to the holder’s surface roughness, but the mediocre epoxy-on-balls fit allows the pen point to wander a bit too much for good results.

The prospect of poking precise holes into 16 mm drill rod seems daunting, but, based on what I see here, it will produce much better results: rapid prototyping FTW!

The OpenSCAD source code as a GitHub Gist:

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Sena PS410 Serial Server: Capturing HP 54602 Screen Images

The objective is to capture screen shots from my HP 54602 oscilloscope, now connected to Serial Port 1 of the Sena PS410 serial server.

Although the scope command language offers a :PRINT? command, it produces output in HP PCL, from which there seems no practical way to get a raster image format like PNG. So this remains an output-only transfer triggered by poking the PRINT SCREEN softkey, after first setting the serial parameters:

HP 54602 scope serial parameters

HP 54602 scope serial parameters

In text:

  • Connect to: HP Plotter
  • Factors: ON
  • Resolution: HIGH
  • Baud rate: 19200 b/s
  • Handshake: XON

I have no idea what’s inside the serial cable at this late date (see the HP 8591 post for more musings), but the (paper!) manual says:

For three-wire operation, an XON/XOFF software handshake must be used to handle handshaking between the devices.
For extended hardwire operation, handshaking may be handled either with XON/XOFF or by manipulating the CTS and RTS lines of the oscilloscope.
For both three-wire and extended hardwire operation, the DCD and DSR inputs to the oscilloscope must remain high for proper operation.
With extended hardwire operation, a high on the CTS input allows the oscilloscope to send data and a low on this line disables the oscilloscope data transmission.
Likewise, a high on the RTS line allows the controller to send data and a low on this line signals a request for the controller to disable data transmission.
Since three-wire operation has no control over the CTS input, internal pull-up resistors in the oscilloscope ensure that this line remains high for proper three-wire operation.

Apparently, the DCD, DSR, and CTS inputs have internal pullups.

Hardware handshakings uses these signals:

  • Pin 4 RTS (Request To Send) is an output from the oscilloscope which can be used to control incoming data flow.
  • Pin 5 CTS (Clear To Send) is an input to the oscilloscope which controls data flow from the oscilloscope.
  • Pin 6 DSR (Data Set Ready) is an input to the oscilloscope which controls data flow from the oscilloscope within two bytes.
  • Pin 8 DCD (Data Carrier Detect) is an input to the oscilloscope which controls data flow from the oscilloscope within two bytes.
  • Pin 20 DTR (Data Terminal Ready) is an output from the oscilloscope which is enabled as long as the oscilloscope is turned on.

The scope wiggles them thusly:

The TD (Transmit Data) line from the oscilloscope must connect to the RD (Receive Data) line on the controller. Likewise, the RD line from the oscilloscope must connect to the TD line on the controller.
The RTS (Request To Send) line is an output from the oscilloscope which can be used to control incoming data flow. A high on the RTS line allows the controller to send data, and a low on this line signals a request for the controller to disable data transmission.
The CTS (Clear To Send), DSR (Data Set Ready), and DCD (Data Carrier Detect) lines are inputs to the oscilloscope which control data flow from the oscilloscope (Pin 2). Internal pull-up resistors in the oscilloscope assure the DCD and DSR lines remain high when they are not connected.
If DCD or DSR are connected to the controller, the controller must keep these lines and the CTS line high to enable the oscilloscope to send data to the controller. A low on any one of these lines will disable the oscilloscope data transmission.
Dropping the CTS line low during data transmission will stop oscilloscope data transmission immediately.
Dropping either the DSR or DCD line low during data transmission will stop oscilloscope data transmission, but as many as two additional bytes may be transmitted from the oscilloscope.

The “as many as two additional bytes may be transmitted” suggests the same problem as with the HP 8591 spectrum analyzer at 19200 b/s, wherein it seems to overrun the PS410 input despite the flickering CTS control line.

I set up hardware handshaking in the PS410 and discovered the CTS line flickers as it does with the 8591, but the transfer complete without overruns. Perhaps the 8591 sends more than however many characters the PS410 can handle after calling for a pause?

The Kermit setup points to Serial Port 1 on the PS410:

set host 7001 /raw-socket
set modem none

And then the rest of the script Just Works:

Calibrator waveform

Calibrator waveform

The Kermit script as a GitHub Gist:



Sena PS410 Serial Server: Capturing HP 8591 Screen Images

The objective is to capture screen shots from the HP 8591 spectrum analyzer, now connected to Serial Port 2 of the Sena PS410 serial server.

My analyzer is an old one with a 3322A serial number, so its Opt 023 came with a genuine DB-25 female connector, not the DE-9 male connector described in the HP doc for the later Op 043 hardware. With that in mind, the HP doc says the spectrum analyzer supports only hardware handshaking:

  • Baud rate 300 to 57,000 baud.
  • 8 bits per character.
  • 1 stop bit.
  • No parity.
  • Software handshake – none.
  • Xon/Xoff and ENQ/ACK not supported by the spectrum analyzer.

The manual enumerates the handshaking lines:

  • Request to send (RTS) – Output signal indicates that the spectrum analyzer is ready to communicate. This line is true at power-up and stays true while power is on.
  • Clear to send (CTS) – Input signal indicates that the external controller is ready to receive data.
  • Data terminal ready (DTR) – Output signal from the spectrum analyzer. When the input buffer is full, this line goes false.
  • Data set ready (DSR) – Is not available.
  • Data carrier detect (DCD) – Input to the spectrum analyzer. If DCD is true, the spectrum analyzer will receive data from the controller. If false, no data will be input. The data will be ignored.

Furthermore, it is written:

The spectrum analyzer checks its CTS input before transmitting data to the computer. If the CTS line is false, the spectrum analyzer will not transmit data. The spectrum analyzer transmits data when the CTS line is true.

The spectrum analyzer sets the DTR line (PC CTS) false when its input buffer is full.

They offer several wiring diagrams, none of which correspond to the hardware on my bench, but swapping the “Personal Computer” and “Analyzer” headings on this diagram seems close to reality:

HP 8591 - RS232 DB25 to DE9 wiring diagram

HP 8591 – RS232 DB25 to DE9 wiring diagram


On the other end of the cable, the PS410 does “hardware flow control using RTS/CTS”. They also offer a diagram:

Sena PS410 - RS232 wiring diagram

Sena PS410 – RS232 wiring diagram

So I rewired the cable thusly:

HP 8591 vs Sena PS410 - RS232 cable diagram

HP 8591 vs Sena PS410 – RS232 cable diagram

Pin 1 on the 8591 interface connects to both frame ground and signal ground and, back when I first made this cable, many years ago, I had wired it to the shield of the cable and thence to the DE9 shell. Alas, the PS410 took offense; for reasons I don’t understand, a shell-to-ground connection ignites a ferrite bead on the PS410’s PCB.

With the rewired cable in hand, the PS410 serial port setup looks like this:

Port 2 - 8591 serial config

Port 2 – 8591 serial config

The PS410 apparently wiggles its RTS output after every byte it receives, because the CTS input at the 8591 turns into a blur during screen captures. This seems unaffected by the Inter character time-out setting and doesn’t (seem to) produce any problems, so it’s like that and that’s the way it is.

Using 9600 b/s isn’t as slow as you might think. The HP manual  notes:

Some of the programs in this manual use 1200 baud for proper operation. If your system uses the RS-232 handshake lines, you can use 9600 baud for all of the programs.

I tried 19200 b/s and got mysterious errors that resemble overruns, which suggests the 8591 ignores the PS410’s flickering RTS output. The screen dumps require only a few seconds, so it’s not a big deal, although timing issues have a way of resurfacing at the most inopportune, uh, times.

Kermit knows how to handle network sockets and suchlike, so aiming it at the spectrum analyzer is a one-liner:

set host 7002 /raw-socket
set modem none

The /raw-socket disables Kermit’s default Telnet interface, preventing it from squirting IAC + BRK characters when closing the session; I think that’s what happens, but I don’t use Telnet enough to know better. As you might expect, the 8591 deals poorly with characters outside its lexicon.

It’s not obvious set modem none does anything in this context, but it seems reasonable.

Then the rest of the script Just Works:

FM 104.7 MHz peak hold

FM 104.7 MHz peak hold

Which is the peak-hold spectrum of a local FM station, as received through an amateur radio HT rubber duck antenna.

The Kermit source code as a GitHub Gist: