PA0RDT Mini-Whip antenna

Antenna in a bottle installed on the roof today. Stay tuned for a future web-SDR...

An article, some theory, some notes on grounding, some comparisons,

There's also a modernized circuit by PA0NHC

Frequency Distribution Amplifier 2017.01

1:8 frequency distribution amplifier based on LMH6702 and LMH6609 op-amps.

In particular the power-supply section using a common-mode choke, a Murata BNX025 filter, and low-noise regulators LT1963 and LT3015 seems to work quite well. I also used ferrites (2 kOhm @ 100 MHz) as well as an RC-filter on all supply pins. Perhaps overkill? Performance with the intended AC/DC brick is still to be verified.

Measurements around 10 MHz show a 1 dB compression at over 14 dBm and an IP3 of around 27 to 30 dBm. The gain extends beyond 100 MHz with some gain-peaking.

Some measurements of residual phase-noise with a 3120A phase-meter, at 10 MHz. My earlier distribution amplifier required shielding with aluminium foil as well as powering from a lead-acid battery to achieve a reasonably quiet phase-noise spectrum. These measurements were done with lab power-supplies for +/-12 V to the board and without any shielding.

Finally some measurements of gain vs. frequency with a Rigol spectrum analyzer.


Pulse Distribution Amplifier 2017.01

A new pulse distribution amplifier for 1PPS distribution.

The input is fed to a LT1711 comparator triggering at 1.0 V (set by reference ADR423). This edge is buffered by 74AC14 before 1:8 fan-out to output-stages with three 74AC04 inverters in parallel driving the outputs.

Preliminary measurements show around 200ps channel-to-channel propagation skew - to be improved on by further trace-length matching or tuning. More measurements to follow.

Distribution Amplifier 2017.01

A new distribution amplifier design featuring a 1PPS pulse distribution amplifier (PDA) and a 5/10 MHz frequency distribution amplifier (FDA).

1U 150mm deep rack-enclosure from Schaeffer. Prototype PCBs without soldermask or silkscreen from Prinel. Both the FDA and PDA boards have 1:8 fan-out with 9 BNC (optionally SMA) connectors spaced 16mm apart. The boards fit comfortably side-by side on a 19" rack panel. Some funky BNC-cables with unusually large connectors may not fit side-by-side 16mm apart - a price to pay for the compact design. The plan is to use an +/-12 AC/DC brick power-supply (not shown) which fits in the back of the enclosure.

Detailed posts on the PDA and FDA boards to follow.

Siglent SDG2042X liberation

Holy megacycles Batman! This thing is made for hacking. You telnet into the thing, delete one line in an XML config file, and then reboot. Whiskey-Tango-Foxtrot!?

Here's how the screen looks before and after, and a view of the 120 MHz signal on the scope. There's a bit of amplitude ripple as you sweep up from around 50 MHz.

USRP N210 quickstart

For various experiments I got an USRP N210. Here's how to get going with gnuradio.

By default it shows up at, so configure your PC eth-interface on 192.168.10.XXX. Now

$ uhd_find_devices

Should find the device:

-- UHD Device 0
Device Address:
type: usrp2

However uhd_usrp_probe suggest upgrading the firmware and FPGA image, so we do that by:

$ sudo /usr/lib/uhd/utils/

$ sudo /usr/bin/uhd_image_loader --args="type=usrp2,addr="

and after power-cycling the device we now get (in addition to a long description of the RX and TX interfaces or installed daughter-boards):

$ uhd_usrp_probe
FW Version: 12.4
FPGA Version: 11.1

To show that it works in gnuradio a simple USRP-source can be connected to a QT GUI Sink (or QT GUI Frequency Sink) that visualizes RX I/Q samples:
The source needs to be configured with "addr=" and it uses the samp_rate, freq, and gain variables.

The USRP now samples at 100 MS/s, digitally downconverts a bandwidth samp_rate around a center frequency freq, and streams the samples over Ethernet to the gnuradio source node. When setting freq=10e6 and samp_rate=200k (the minimum possible!?) and applying a 10.020 MHz sine-wave to the RF1-input we get this:

With a signal that seems close to maximum amplitude the dB-scale shows a peak at -16 dB, and without any signal applied the noise-floor close to the center frequency dips just below -140 dB rising to -135 dB at 20 kHz offset.

Awesome stuff! The 200 kS/s samples from one I/Q channel produce about 800 kB/s of Ethernet traffic.

Stay tuned for more SDR updates in the near future!

PICDIV frequency divider

I put together a PICDIV frequency divider for use with a Rubidium clock.

I used an LTC6957-3 to convert the 10 MHz sine-wave from the clock to a CMOS logic signal (square wave). The LTC6957-3 has two outputs, one is routed to a BNC connector output, the other is used as the clock for a PIC12F675. The PIC runs pd09.asm which outputs a 20 us long pulse every second - i.e. it divides the 10 MHz input frequency by 1e7. The PIC is programmed through a 5-pin 100 mil ICSP header.

Here are some test-signals with a SRS PRS-10 as the source, and recorded on a Rigol scope.

The outputs behave as expected, but the 1PPS from the PIC is only 700 mVpp into 50R - a bit low. When terminated to 1 MOhm the rise-time is much worse so this is best avoided. Perhaps a buffer or level-translator would be a good addition.

Finally phase-noise measurements on the 10 MHz CMOS output, performed with a 3120A phase-noise probe.

I tried shielding the circuit with aluminium foil and powering it from a +12 VDC lead-acid battery - however the three measurement runs look roughly similar. Perhaps the LM317 regulator is not a great choice here, and both the LTC sine-to-square chip and the PIC should have more bypass caps and decoupling (inductors, ferrites?). In any case the phase-noise is 10-20x better than the measurement noise from a typical counter (SR620 or 53230A), so any issues only show up with high-end phase-noise probes.