Swiss Army Knife vs Can Opener?

In my last entry I described a feature and option-laden filter/preamp board, comparing it to a Swiss Army Knife.  Great, but sometimes you just want a can opener…

After some serious discussion about how such a design would be used, and the typical requirements, I’ve decided that a major simplification is in order.  (And, of course, a new feature)

Filter-Preamp-simple v1

The consensus was that there was no need for bypassable filters, so the stages are now:

  • 18 dB shelf filter (-9 dB @ 6 MHz)
  • 17 dB preamp
  • 30 MHz elliptic low-pass filter (or 60 MHz as a different assembly)

This board is powered by an external +12V DC supply, and draws about 20 mA.  There is an active-filter stage that should clean up most supply noise.  The new feature is simple low-loss reverse-polarity protection for the power input.

Here is the (simulated) gain of this unit:

simpleAnd the schematic:

Simple Schematic

Actually, the two filters can be bypassed at assembly time as a load-option.  Perhaps this will let this board also be used as an amplifier-only design.

I have sent this design out for PCB fabrication and ordered the components — I can’t wait to see how it all comes together!

 

A Combined Multi-Section Filter and Preamp

Many SDR receivers, especially those designed for full HF-spectrum use, have needed various filters and preamps between the antenna and the SDR.  High-power AM and FM broadcast stations can overload the receiver front-end, so these are filtered out.  The characteristics of atmospheric background noise are such that a “shelf” (or “shelving”) filter which attenuates the lower-frequency range can also be useful in optimizing the SDR dynamic range.

Finally, the SDR sensitivity / input noise-floor at  the higher end of the spectrum can be marginal.  Add the inherent losses of all these filters and a low-noise, high dynamic-range preamp becomes very useful.

So I am designing the “Swiss Army Knife” of SDR front-end modules:Filter-Preamp

No, there aren’t four SMA connectors, only one pair will be installed, giving an “in-line” version, and a “right-angle” version that can fit on a small die-cast enclosure.  There are four sections to this board:

  • AM Broadcast Band filter
  • 15dB Shelf filter
  • 13db gain preamplifier
  • FM Broadcast Band filter

Each of the filters can be bypassed using pin headers and jumpers.  The amplifier components can be omitted during assembly and a jumper used to bypass this section.

The FM band filter allows for 6-meter reception, but many SDR receivers are clocked at 66 MHz or so, and require a lower corner frequency on the low-pass filter.  This filter can optionally be built with a corner frequency at 30 MHz.

Amplifier:

Amplifier

Amplifier simulated with 2N5109

The amplifier is a simple wide-band zero-inductor design (found in the November 1984 edition of Ham Radio magazine, page 100: https://www.worldradiohistory.com/Archive-DX/Ham%20Radio/80s/Ham-Radio-198411.pdf).  It’s become difficult to find the classic 2N5109 transistor, so I am using the more modern BFU90Q, which has similar performance.  But I am simulating with the 2N5109 because that’s the model I could find.

The amplifier draws about 20 mA from a +12V power source, and my board includes a simple but effective filter that eliminates most noise and ripple on the DC source.

Here are the three filter sections:

filters 1

From left to right the first filter is the AM broadcast filter,.  This starts attenuating below 3 MHz, and is at least -50dB down at the top of the AM band.

Next is the 15 dB shelving filter.  In a previous filter I had two 10 dB shelf filters in series, with each of them bypassable.  This limited-size board requires a compromise, so there is just a single filter section.  The inclusion of the AM BCB filter should mitigate the effects of the simpler shelf filter configuration.

The right-hand filter is a elliptic low-pass design, that cuts off at 60 MHz.  The  attenuation in the FM broadcast band, and above, is approximately 80 dB.  This is also an excellent anti-aliasing filter for the SDR.

With the shelf filter bypassed the filter response from 3 to 60 MHz is essentially flat:

filters 3

With all filters and amplifier enabled, the overall gain at 60 MHz is about +10 dB.  The input (blue trace) is -60 dBm, and the output (green trace) is -50dBm :

filters+amp 1

And here is the schematic for the whole thing:

SchematicThe simulated performance shown above should be reasonably close to the actual thing.  At these frequencies the inductors are the largest source of simulation errors, but I am using Coilcraft inductors and their provided simulation models.  These have proven to be remarkably accurate in previous designs.

But this is a tight little board, and layout parasitics may be an issue.  You will see that the (unshielded solenoid) inductors have been oriented to reduce coupling, and (which you can’t see) the ground planes have been relieved underneath the most sensitive tuned circuits.  I do hope that the amplifier won’t turn into an oscillator, but the design and layout do give me some optimism.  I plan to have a working prototype in about a month.

Like the Swiss Army Knife, this design may not be the best tool for every job, but it should be useful in SDR receiver systems.

Acknowledgement: I would like to thank Clint Turner / KA7OEI for his invaluable advice on filters, amplifiers, and optimizing SDR performance.  His excellent blog: https://ka7oei.blogspot.com/2020/08/revisiting-limited-attenuation-high.html

 

HamSCI, AGU, WSPRSONDE-8

Good friend Rob Robinett (developer of wsprdaemon), along with leaders of the HamSCI Citizen Science Project, are at the American Geophysical Union (AGU) annual meeting in San Francisco, showing how WSPR is being used to study the behavior of the ionosphere.  One of the posters being exhibited features the WSPRSONDE-8   multi-channel transmitter currently in development at Turn Island Systems:

Poster

See more WSPRSONDE-8 information at TurnIslandSystems.com

Rant: WSPR and FST4W Timing — Why???

In this post I complain about the symbol-rate timing of the various FST4W rates.  I am adding WSPR and other FST4W rates to the Beacon Blaster ( which currently generates FST4W-120, see https://turnislandsystems.com/ for details) and am getting very annoyed with the timing variations.

So let’s start with WSPR.  The symbol rate is approximately 1.4648 baud (4FSK symbols per second), or exactly 12,000 Hz / 8192.  The FSK shift is the reciprocal of this, or about 0.6827 Hz.

OK, I have no complaints about this, the numbers are easy to work with and “8192″ is a nice power of two.  This makes it easy to use a timer-interrupt to run a digital filter at (say) 64x the symbol rate.  But really, any values for that 12,000 / 8192 fraction would be workable if WSPR were the only concern.

Now, look at the FST4W rates (from the wsjtx source code):

 if(mode=="FST4" or mode=="FST4W") { //FST4, FST4W
   if(trPeriod==15) txt=1.0 + 160*720/12000.0;
   if(trPeriod==30) txt=1.0 + 160*1680/12000.0;
   if(trPeriod==60) txt=1.0 + 160*3888/12000.0;
   if(trPeriod==120) txt=1.0 + 160*8200/12000.0;
   if(trPeriod==300) txt=1.0 + 160*21504/12000.0;
   if(trPeriod==900) txt=1.0 + 160*66560/12000.0;
   if(trPeriod==1800) txt=1.0 + 160*134400/12000.0;
 }

I assume “txt” means “transmit time”.  “160″ is the number of symbols in a message.  So the symbol rates are:

FST4-15:   12,000 / 720
FST4-30:   12,000 / 1,680
FST4-60:   12,000 / 3,888
FST4W-120: 12,000 / 8,200
FST4W-300: 12,000 / 21,504
FST4W-900: 12,000 / 66,530
FST4W-1800: 12,000 / 134,400

For the four FST4W rates, the divisors are 8200, 21504, 66540, and 134400.  The only common factor is 2.  And at that common rate of 12,000 / 2 there is no hope of programming the clock-generator chips, especially since the different symbol times drift past  each other.

Since the symbol-rate accuracy is critical to the kind of propagation measurements we are using FST4W for, I am stuck running different timer interrupt rates for each mode and speed, and only running one mode at a time.  Why didn’t they use something like this instead?

CXC4-15:   12,000 / 512
CXC4-30:   12,000 / 1,024
CXC4-60:   12,000 / 4,096
CXC4W-120: 12,000 / 8,192
CXC4W-300: 12,000 / 20.480
CXC4W-900: 12,000 / 61.440
CXC4W-1800: 12,000 / 122,880

Sure, in most modes the gaps between transmissions become a bit larger, but if this is an issue then pick some other numbers.  But at least have them related by a larger common factor!

*END OF RANT*

 

Latest Batch of Boards

I’ve just received another batch of QDX External Reference Interface boards.  More info at Turn Island Systems

Also, I just sent out boards for a four-channel RF Combiner / Splitter, which takes four 1W (or lower) signals (80, 40, 20, and 10 meters), and combined them into a single output for feeding a multiband antenna.  These provide at least 20dB port-port isolation, and less than 2dB loss.  If all works as planned, these will be used for Ionospheric research.

MC-CXC-Notch

RF Combiner Notch v0.1 top

And finally, here’s a 1W (approximately) power amplifier, using 74ACT04 digital integrated circuits to drive a wideband transformer.  This takes a 3.3V digital input between 3 and 30 MHz, and produces a 1W square wave output.  External filtering will be required, and the combiner shown above should provide adequate filtering.

OneWATT v0.1 top

There are “better” ways to make a 1W amplifier, but this is quick and easy, and requires no tuned circuits.  I can’t wait to test these designs!

A Little Amplifier

QWATT 0.2 top

In the process of turning my three-output Si5351-based “ClockBox” into a three-band simultaneous FST4W beacon, I decided I needed a small power amplifier to boost the (approximately) 10dBm square-wave outputs up to a 1/4-watt clean sinewave.  Rather than using FETs and transformers, or a Class-E design, I decided to try using the an 74ACT04 hex inverter as the power stage.  Each inverter output can deliver in excess of 25 mA  with about a 4.2V swing (+5V supply) so six of them in parallel looked promising.  And I happened to have a full tube of old 74ACT04 parts in DIP form (remember those?)

So I built one, tuned for the 15 meter ham band:

breadboard 1(Ignore those power-level numbers, I had a bad coax jumper)

I was worried about connecting the outputs directly, afraid that skew would result in excess output-stage current, but some tests showed low unloaded dissipation  up past 30 MHz.  Adding small resistors at each output might be a good tradeoff, but for now I decided not to waste the power.

I used a simple L/C impedance-matching network to transform the 50 Ohm load (antenna) down to about 3 Ohms which matched the ganged inverter output impedance.  Instead of a simple series inductor I used a series L/C, calculated to provide the correct reactance at the design frequency, but giving a high impedance at higher or lower frequencies, and a bit of extra filtering.  This also makes the design slightly more bulletproof — you can run any frequency into it without causing excess dissipation.

The output power of the test board was actually closer to 1/2 W, and the harmonics were slightly better than 40 dB below the carrier.

PNG2

 

The amplifier efficiency is about 60% and the temperature rise on the buffer was well within comfortable limits.  This is good enough that I am having some circuit boards made.  The only frequency-sensitive components are two capacitors, and one toroid inductor.  I would have liked to use small surface-mount inductors, and the board does provide for that option, but the toroid has much lower loss.  The board uses two surface-mount 74ACT04 parts, and a small buffer to drive the 12 inverter inputs.

QDX External Reference Interface Boards

I am having some QDX External Reference Boards being built, both the clock-multiplier and the direct interface versions.  This should make it reasonably easy for anyone who wants to stabilize the QDX to connect an external 5, 10, or 25 MHz reference clock.  I am also having some replacement QDX back-panels made, so we won’t have to drill a new hole in the existing panel.

I will have these back and tested within a week or two.  If they work and fit as planned, they will cost under $20 for the adaptor / back panel set.  Some assembly required!

If you are interested, contact me at Paul at [my callsign] dot com.

QDX Ref 2.0Multiplier Board

 


QDX REF Protection 2.0 topDirect Interface Board

PanelTest 1a

PanelTest 5

PanelTest 3

 

(All 3D images done with KiCad)