KA1QYP Half Pint QRP Transceiver Passes FCC

In our continuing quest to evaluate the output purity of various QRP kits, Willy W1LY constructed a Bill KA1QYP Half Pint Kit.

The assembled kit produced a solid 600 mW output and was one of the cleanest radios we’ve seen.  The worst case was the 3rd harmonic which was -59 dBc.

The Half Pint uses a Cauer Filter, similar to a 5-pole LPF but two of the sections are tuned to suppress the 2nd and 3rd harmonic frequencies.

The Elsie Predicted response is shown below.  Note the pronounced dips at the harmonic frequencies.  Loss at 7.040 MHz is about 0.9 dB, and the 3 dB knee is about 8.1 MHz:

The measured performance is shown below.  The 2nd harmonic is down about 63 dBc, and the 3rd harmonic is down about 59 dBc.

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Pixie with 5-pole Axial Filter Passes

Willy, W1LY, decided to try using the standard unshielded 1 uH axial inductor that the Pixie uses for it’s current 3-pole filter (C5=47o pf, L2=1 uH, C6=470 pF) as the starting point for a 5-pole filter that could easily fit on the existing board.  Two inductors are soldered in place of L2, standing up vertically and soldered together at the top.  A cap is then soldered from that bridge down to the grounded side of C6 (Willy paralleled two caps to get the required value).  See the area indicated below.


Component values are:

  • C5 changed to 820 pF
  • C6 changed to 820 pF
  • New Cap (bridge of L2A/L2B to ground at left side of C6) 1150 pF (680+470)
  • L2A (1 uH)
  • L2B (1 uH)

Measuring the output of the modified Pixie indicates a passing value!  The 2nd harmonic, worst of the lot, was -43 dBc.  Power output was 0.3 watts using a P2n2222 for Q2.

We suspect that original 5-pole Elsie values of 1.2/1.2 uH and 910/1300/910 pF would produce even better 2nd harmonic suppression and work just fine at the lower end of 40 meters.

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Elsie Plot of QRP-Labs 7-pole 40-m LPF

This is the schematic of the 7-pole LPF used in the QRP Labs 40-meter LPF:

The plot from Elsie 2.77 for the above filter looks like this:

The actual SA response curve looks like this:

The curves are remarkably similar.  Knee around 8.8 MHz, about 36 dB down at 14.08 MHz.  Loss at 7.04 MHz is a bit worse on Elsie, but that is based on Q values of 40 for the coils and 200 for the caps — both are probably a bit better.

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A “Better” QRP Transceiver?

NCRC had several reasons for building the Pixie as a club project, and getting it on the air is probably at the bottom of the list.  Fixing flaws in the Pixie design provides learning opportunities.  But by the time you fix the Pixie, would you be better off building something else?  There is no clear answer to that question, but there is a “something else” that might fit the bill.

Enter the “DC40B” Direct Conversion 40-meter QRP transceiver.  Notably different from the Pixie in that it produces between 2 and 3 watts of clean transmit power right off the bat, and has a substantially better receiver.  It also has a real sidetone and a keyer chip built-in.  All that comes at a price.  It sells for about $40, and is probably 3x the complexity of the original Pixie (about 2.5x the component count, plus 5 toroids to be wound).

Willy W1LY built the DC40B and we measured the transmit spectra today.  The fundamental was at 2.75 watts.  The 3rd harmonic, was -65 dBc, more than meeting the FCC requirement.

The user needs to provide external jacks for the headphones & keyer, plus antenna and power connectors.  It is suggested that a panel mounted cap could be used to provide some degree of RIT — otherwise a PCB mounted trimmer is used to set the desired CW offset on receive.

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Pixie Perfection

As has been noted in several posts, the stock Pixie 3-pole filter is hopelessly inadequate to suppress harmonics.  A typical Pixie will have a 2nd Harmonic that is only about -21 dB down from the carrier.  An external 5-pole filter did not improve things well enough, and ultimately a 7-pole external filter was required.  In both those cases, the original 3-pole filter remained in place.

However, what if the original Pixie filter was replaced with a 5-pole design?  Would that be enough?  A friend of mine, Willy W1LY, set out to discover just that.  He pulled the original axial leaded inductor (L2)  and replaced it with a pair of T37-2 toroids.  He then paralleled additional caps on the input and output side (C5/C6), and finally added a new cap between the two toroids. He was able to fit all of that in the space of the original filter plus a bit of open area next to the BNC connector.  In addition, Q2, the S8050 output transistor, was replaced with a P2N2222A.

Elsie 1 dB ripple 5-pole filter. Note, it shows 2 caps between the toroids — only one cap is required

  • C1/C3 input caps were set to 910 pF (standard value — Willy used 940)
  • C2 cap was set to 1300 pF (standard value — Willy used 1270)
  • L1/L2 were set to 1.2 uH (20 turns #26 on T37-6 core)

The results were pretty much ideal.  The 2nd harmonic was suppressed by 63 dB (other harmonics were even lower).  The fundamental power was increased to 25.2 dBm, or 330 mW.  (Spectrum Analyzer is fed through a 40 dB external attenuator)

Given the current 5-pole LPF filter values are producing a 2nd harmonic rejection that is about 20 dB more than required, there is some room to improve the ripple response of the filter, which would probably boost the output power to 450 mW or higher.  But 330 mW is more power than any stock Pixie has ever produced (usually they are around 200-250 mW out at 13.8 volts input).  Another experiment would be to change the toroids out for 1.2 uH axial inductors.

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What’s wrong with the serial port on the Raspberry Pi 3?

The Raspberry Pi 3 added all sorts of features to the Pi, including a much faster speed adjustable CPU, built-in WiFi, and Bluetooth interfaces.

Some of the hardware interfaces on the Pi continue to have limited functionality.  Such is the situation with the RPi 3’s serial port.  The primary serial port, which has a fully adjustable and stable baud rate generator, was dedicated to the Bluetooth interface.  That meant that the “User” serial I/O port was assigned to the secondary serial port.  This is a major issue as that serial port lacks an independent baud rate generator, but uses the system clock instead.  The problem with that is the system clock is dynamically changing all the time based on CPU load (to provide bursts of higher speed clock rates without causing the system to overheat or draw too much power on a long-term basis).

Because of the variable system clock rate, the user serial I/O port is almost useless.  For example, you can’t hook it up to a GPS Hat and expect it to work since the baud rate changes unpredictably.

There are two workarounds:

  1. Run at the slowest clock rate all the time, crippling the RPi3
  2. Disable the Bluetooth interface and assign it to the user serial port

More details here:  https://forums.adafruit.com/viewtopic.php?f=50&t=94677

And here:  https://openenergymonitor.org/forum-archive/node/12311.html

And here:  http://raspberrypi.stackexchange.com/questions/45570/how-do-i-make-serial-work-on-the-raspberry-pi3



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P2N2222A for the Pixie

On Semiconductor has a variant of the old 2N2222A in a plastic case called the P2N2222A and characterizes it as a small signal RF amplifier.  I picked up a few samples and ran three of them through the Pixie.  In all cases the RF output had increased to about 575 mW on the meter, with the SA reading 525 mW.  The second harmonic was well below FCC limits at -57 dBc.  That’s about 1.7 watts total input power on transmit, or about 1.15 watts dissipated in the transistor and filters.  Gotta keep that key-down period short!


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7-Pole Filter + Pixie

A friend of mine, Paul K1YBE, gave me a QRP Labs 40-meter Low Pass Filter Kit.  The kit came with three T37-6 cores, some #28 wire, 4 caps, a tiny PCB, and a couple of headers. Normally used with one of their WSPR Transmitter Kits.

The filter was easy to build, but I found the inductors needed to have a turn removed to yield their desired value.  This equated to 20 turns of #26 wire for L1 and L3 (1.37 uH Q17), and 23 turns for L2 (1.81 uH Q15).  The caps had a measured tolerance of +/- 3.5%. The 270 pF caps had a Q of 30, and the 680 pF caps had a Q of 260 (all Q values at 100 KHz).

Assembly was straight forward, and I used my Quad-Hands to help keep things steady while soldering – especially the flying leads I was going to use to mount the filter in the box.

I mounted the assembled PCB into a Pamona 2391 BNC box.  This is a great solution because the box will connect directly to the Pixie output using a Male BNC, and provides a filtered output on the female BNC.

The filter response was swept with my spectrum analyzer.  Being a 7-pole design, the loss was much less at 7.040 MHz, only about 0.3 dB down.  At 14 MHz the response was down about 36 dB, and 39 dB at 21 MHz.  In truth, had I not removed a turn for the two inductors, the 3 dB point (8.76 MHz) would have shifted a bit lower, probably providing a tad more rejection at 14 MHz.  Given the tiny box it was easy to hook the filter to the SA.

The real test was hooking the filter to the output of the Pixie.

Looking at the output on the SA was pretty gratifying:

The 2nd harmonic is down 59 dB and the 3rd is down 69 dB from the carrier.  The SA shows  26 dBm output (400 mW).  The power meter shows 450 mW  (these tests were run using a 2N2219 for Q2).

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Better Q == Better Response

Yesterday I replaced the inductors in my original 40-meter LPS with larger cores that wound up having about twice the Q.  This morning I replaced the ceramic caps with silver mica caps.  The old caps had a Q around 80.  The new caps have a Q > 1000.  As one might expect this had an impact on the insertion loss of the filter, lowering it by almost a full db from -1.97 down to -1.03 dB.   The tighter cap tolerances also resulted in a cutoff frequency of 7.26 MHz, probably a bit too close to the operating frequency.  Rejection at the 2nd harmonic was -39.2, and at the 3rd harmonic was -35.0 dB.

Actual transmit performance using my original S8050 transistor (which had 2nd harmonic suppression of -10 dB) had a 2nd harmonic suppression of -51.9 dB.

Ceramic Caps Silver Mica Caps
Meter Power 270 mW 240 mW
7.040 MHz -16.7 dBm -19.4 dBm
14.08 MHz -61.6 dBm -71.3 dBm
21.12 MHz <-80.0 dBm < -80 dBm

Note that the measured meter output power was 240 mW, which is down about 30 mW from the ceramic cap solution.  The SA indicated the fundamental was at +20.6 dBm or 115 mW).  Given the lower insertion loss, I suspect this is because the filter with the SM caps and T50-2 toroids is providing a sub-optimal impedance match to the output transistor.

Putting the 2N2219 transistor in the circuit boosted the output power up to almost 280 mW by the meter, with a 120 mW fundamental by the SA., and a second harmonic suppression of -50.6 dB.

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Does Size Matter?

There’s a joke in there somewhere…

Yes, size does matter when it comes to Toroid diameter (which also has an impact on winding geometry), maximum wire size, etc.  All things which impact the Q of the circuit.

I went back to my original S8050 Transistor, and swapped out my T37-2 inductors for T50-2 inductors wound with 15 turns of #20 wire (13″ of wire, measuring 1.2 uH).  This allowed the winding to be single layer and evenly spaced (the smaller T37 cores had to go to two layers and were really jammed in there).

I had seen Q readings at 100KHz of 20 for the T37-6, 25 for the T37-2.  The larger T50-2 inductors had a Q of 45.  The higher the Q, the lower the insertion loss, so larger cores with heavier gauge wire are better.  That’s the same conclusion that is reached in the Micrometals Q Curve App Note (I’m using Micrometals cores).  The filter response curve was very similar but at 7.040 Mhz the loss was now -1.97 dB, about a 0.1 dB improvement.

With the filter using the T50-2 cores the spurious response output looks like this (Meter reading just about 270 mW, 2nd harmonic down -45 dB, fundamental at 220 mW).

T37-2 17 turns #20 T50-2 15 turns #20
Meter Power 250 mW 270 mW
7.040 MHz -16.6 dBm -16.7 dBm
14.08 MHz -63.0 dBm -61.6 dBm
21.12 MHz <-80.0 dBm < -80 dBm

Proving again that basic physics never gets old, here is a great reference from Boonton Radio Corporation, circa 1955, that describes a Q-Meter and how you can evaluate components.  (Thanks to John, WA1ABI, for this great article!)

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