WB4SON

I want to replace my dead HP 3335A with a new Siglent SDG 2042X.  After all, the latter is 40 years newer, 20 pounds lighter, and maybe 1/8 the volume.  It is also silent as a church mouse (the HP’s fans produced an obnoxious roar while keeping its ECL logic from melting).  The HP cost about $14,000 in 1980, and the Siglent sells for under $500 new today.  We have come a long way in the last 40 years!

The executive summary is that it is more than capable of replacing the 3335A, from a stability point of view.  However, it lacks HP’s wonderful built-in step attenuator that allowed one to set an output signal level like -84.1 dBm and expect it to be spot on.  The Siglent has no dBm feature, only volts or millivolts. (Correction, it can be set to dBm from -50 to +23 dBm).  I set mine to an output level of 632.4 mVpp, which is the equivalent of 0 dBm, then adjust it using an external attenuator box like a JFW Industries 50BR-001.

The Siglent allows sub-hertz frequency entry.  Using the keypad you can enter 1.23456789 MHz, you will get an output signal that is 1.234 567 890 MHz and will see that final digit bouncing around by no more than 1 count up or down.  If you use the knob, you can expect to change the output in steps of 1 hertz (or 10/100/1000/10000, etc).  Watch out if you enter a frequency using the keypad that has a sub-hertz resolution, then use the knob to adjust it later.  You will no longer see those sub-hertz digits on the display, but they are still there.  Best to enter a frequency to the nearest hertz using the keypad before using the knob. (Edit – I verified that you can enter a frequency, using the keyboard, with 1 millihertz resolution).

The spec is a tad misleading as it claims to be a 40 MHz signal source.  This is true for sine wave only.  All other functions have significantly lower upper limits (25 MHz for square wave, 1 MHz for a ramp).

I wasn’t very clear when I was posting about my pre-FMT calibration.  But I did describe that the K3s was not a particularly effective rig to use for a FMT.  This is because of how the digital synthesizer was implemented.  Even with the optional high-stability oscillator, which my K3s has installed, the issue remains.

The behavior is very evident in the Spectrum Lab screen capture below.  The K3s was fed a Rubidium based 10 MHz signal that is stable in the microhertz range.  The K3s is in CW mode with a 500 Hz Pitch selected , so we would expect it to produce a 500.000 Hz tone.

On the bottom of the illustration, in the waterfall area, the horizontal dashed lines are 5 minute interval time stamps, so we are seeing the variation in tone over a 15 minutes interval.  During that time, the DDS/PLL is performing a stair-step adjustment that is causing the tone to vary from about 501.2 Hz to 501.8 Hz.  One might describe this as 501.5 Hz, plus or minus 0.3 Hertz.

By the way, the older K3 would do exactly the same thing.  This is not a defect, as the K3s is working exactly as designed.  It was done to maximize dynamic range and minimize unwanted noise from the synthesizer.  The designer saw no reason to have its short term stability be better than +/- 0.3 Hertz.  For ham use, that is more than acceptable.  It is highly stable a longer term.  It will remain at 501.5 Hz +/- 0.3 Hertz for many days.

By the way, this is also pretty much what you can expect for Doppler on a 20-meter band signal anyway.  So the rig is just as stable as the ionosphere is in the short term.

Back in the day, a superhet receiver, employed several mixer stages, requiring various oscillators, and a sometimes sketchy VFO (Anybody remember the Swan 753 — lovingly referred to as the Seven Drifty Three?).  That made for a real challenge when trying to accurately measure frequency.  All those oscillators drift at different rates (sometimes different directions).  No matter what the stability of your reference signal, within minutes things were moving all over the place.  Hertz-level measurements were about the best that could be obtained.

More modern gear started shifting to a single reference oscillator and deriving all other frequencies from that.  At least then everything drifted at a consistent rate.  Some of those 1980’s era rigs, like those made by RACAL or Watkins-Johnson, used very sophisticated digital synthesis and phase locked loops to produce a receiver capable of resolving a frequency on the display to a hertz, and yet were stable on a millihertz or microhertz scale.  Those receivers often allowed for an external 1/5/10 MHz oscillator; perhaps a Rubidium or Cesium standard to replace the internal reference oscillator (often a VERY expensive double oven based crystal oscillator).

Once ham gear started moving into the DSP era, was a simple task to have everything working off of one oscillator, and in fact many rigs do that (to save money, actually).  But several use one oscillator for reference generation, yet clock their DSP chips off independent oscillators which can cause some issues when converting the analog RF into the digital domain, then back in the audio domain for listening.  Even those that tried to keep everything tied together might have different design objectives for their digital synthesis scheme.

As a result, some rigs are not very stable on a millihertz or microhertz scale, yet are more than adequate for a human ear or decoding FT8.  An example of this is the otherwise outstanding Elecraft K3 line.   One can see the impact of the PLL causing a reference signal to appear to drift around by about +/- 100 milliherts in a staircase pattern that repeats every 5 minutes or so, but sometimes much wilder excursions of +/-500 milliherts happens too.  Nevertheless, K3s are used all the time in FMT with green band results (< 100 millihertz error).  I used my K3 as an AM detector by injecting a known carrier near the unknown signal to produce a beat tone.  Any PLL stepping has no impact as long as the passband is wider than the drift.

So a FMT purist would seek out a radio that would be stable into the microhertz range over long time periods.  Such a receiver is the RACAL RA6790/GM (1980 era superhet).  I’ve had mine turned on for a few days, locked to a Rubidium source, and monitoring a second GPS referenced Rubidium source.  Using Spectrum Labs, after calibrating the sound card, it has been providing a rock-solid reading of 300.00000 +/- 20 microhertz for the past day.  I deem my receiver more than adequate (by at least 3 orders of magnitude), so all errors will be due to operator error or inability to divine Doppler this Thursday.

By the way, prior to calibration, the sound card I was using (Behringer UCA222) was producing a tone reading of 300.02400 Hertz, off by 24 millihertz, or 80 ppm.  After calibration with +/- 20 microhertz wobble, it is more like 70 ppb.