G3UUR Crystal Oscillator

Somewhere in the near future, I’m hoping to experiment with building my first superheterodyne receiver. While I may be putting the cart before the horse in working on new receivers before putting up a proper antenna, until the weather kindly agrees to stay nice for more than one day at a time, it’s the soldering station for me.

An essential part of a superhet receiver, and the part I’m excited to experiment with, is the IF filter. The matching (or at least characterization) of the motional parameters of a few crystals is critical for filter design, so to that end, I put together a simple test oscillator a la G3UUR to try matching some crystals.

The G3UUR is a simple Colpitts oscillator with a buffer, which you are intended to attach to a frequency counter. A small bit of machine header allows for the insertion of individual crystals for testing. A SPST switch allows for one lead of the crystal under test to either be shorted to ground, or grounded through a small capacitor. By measuring how much the frequency of the crystal is pulled by added extra capacitance to the oscillator, we can calculate some basic parameters of the filter, namely the equivalent series capacitance and inductance.

G3UUR Crystal Schematic-03.png
All credit to G3UUR and W7ZOI for the circuit above.
The circuit went together fairly simply using parts out of my junk box. This was my first time trying out “island pad” construction, making isolated islands on the copper clad with a drill or scribe. W2AEW has a nice example of planning island pad layout starting around 1:45 in this video, and Jack K8ZOA has a really nice write-up of the construction process on his Clifton Laboratories page.. Here you can see my sketching process as I roughed out the transition from schematic to island pad layout.

Since I don’t have a nice diamond core drill bit as suggested by others, I made my own sloppy island pads using a engraving bit and my cheapo Harbor Freight knock-off Dremel. Though it’s not a particularly high powered tool, it eats through the copper on copper-clad with ease. Would definitely use this method again, it was tremendously easy. It also allows for the creation of oddly shaped islands, sad for making loops to attach power or ground leads to.

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This little engraving bit made quick work of cutting the island pads. Here I’m planning out the spacing based on the TO-92 package.
With the board carved up, it was easy enough to solder the components in place. I turned my iron up to 850 degrees to help tack the components onto the copper board quickly, and the soldering process only took about 30 minutes.

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The completed crystal checker. I used one side of an 8-pin dip machine socket for the XTAL socket – the other legs give it a little more mechanical stability.
As a frequency counter, I’m using a cheapie “cymometer” I got from ebay. While not terribly robust, it’s easy to use and seems to give fairly good accuracy.

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The G3UUR method is very simple: place the crystal in the oscillator with its lead grounded, and read off the frequency on a frequency counter. We’ll call this the base frequency, f. Then open the switch, so that the crystal is only grounded through the capacitor Cs, and read off the frequency again. (It’s worth measuring the capacitance here, rather than trusting the labelled capacitance.) The difference between the two frequencies we’ll call ΔF. The capacitance of the crystal itself (measured across the leads with a meter). we’ll call C0. The motional parameters of the crystal are then:

G3UUR Motional Parameters-02

 

A quick experiment on a miscellaneous 9.838 mhz XTAL I had lying around gave results of 11 fF and 23.8mH, which are within the realm of possibility for a crystal of this size. More encouragingly, a June 2006 paper from Jack Smith K8ZOA suggests that the simple G3UUR method seems to give results within about 5% of more complicated and exact methods, like using a calibrated Vector Network Analyzer or performing phase comparisons of frequencies across the crystal.

Hopefully this is the first step toward building a “good enough” three or four crystal filter for a superhet receiver.

Hear you on the air!

73

Wind, Antennas, and a First Portable Session

This week I had two radio firsts: trying to put together a halfway decent antenna, and a first portable operating session.

I started by putting together a center insulator a dipole antenna. There are many possible designs online, but I ended up making one with materials I’m familiar with. It’s made of a 1″ PVC cross, four PVC caps, three 1/4″ eye-bolts, some nylon lock-nuts, and panel-mount BNC connector. The whole went together in about half an hour. The PVC connections were surprisingly tight as a press-fit, but they and the eye-bolts are secured with lock-tite and sealed with hot-snot just in case. The BNC connector is soldered internally to a couple pieces of hookup wire, which you can see poking out the sides of the cross. The antenna wires themselves are simply tied to the eye-bolts and wire-nutted to the hookup wire.

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The dipole center-support. This picture was taken after the fact, hence the pre-cut Nylon cord wrapped around it. You can see the two copper hookup wires poking out either end.

The dipole center, a 200′ spool of nylon wire, some “stakes” for securing the antenna to the ground (really, 8″ pegboard hooks, on sale at Menards!), some zip-ties, and a couple of rice-filled socks as counterweight all fit handily into this small plastic case I had lying around:

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I live only about a mile from Lake Michigan and the beautiful lakefront park space that runs along the East side of Chicago. I figured with the sunny-but-chilly weather we’ve been having this week, the lakeshore on a weekday afternoon would be pleasant to work in and not too packed with people.

I couldn’t find a suitable table or bench close enough to trees to set up under, but then I spotted the convenient height of the sea-wall between the outer lakefront trail and the lake itself. I couldn’t have been any closer to the lake without getting my socks wet!

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The operating position. There’s a little ledge just on the other side of this wall that was perfect for standing on.

I strung up a portable, homemade 40m dipole (wires cut in advance) as high as I could could into one of the nearby trees – I only got the center maybe 20 feet in the air, but that’s still better than the 8′ the random wire in my home office is. The ends are tied off to stakes near the ground. This made the setup more of an inverted-V than a true-dipole.

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One of the antenna-wires staked off to the ground. You can see the black coax sloping from the tree to the operating position.

The antenna is fed from ~60′ of RG-58, running into a ZM-2 Tuner for matching, and then via a short RG-58 jumped into the Virgin Receiver. I also brought along the ardiuno-controlled, Si5351-VFO I’ve been working on to allow for the ability to change frequencies, as detailed in my last post. If I’m going to spend the time setting up practically in a lake, I thought it would be nice to actually scan the whole band. Since the VFO rig is still on a breadboard, I was quite spread-out over my little stony operating station.

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Visible here: the coil of coax coming from the antenna; the Virgin Receiver, open to allow the VFO to interconnect; the breadboard with the Si5351 VFO on it; two separate 12V SLA batteries; some nice comfy headphones; and the large red cable-coiler I used to store the dipole wires (a bit overkill).

So now, the moment of truth – I hooked up the batteries, connected the receiver and VFO, did a quick by-ear tune on the ZM-2. I’ve never done something like this before – would it actually be better than my office random-wire? Was it all just a jaunt in the park? In short, would it work?

LIKE HECK IT WORKED!

Especially up in the SSB portion of the band, there were some stations that might as well have been on the other end of a phone call. (This, by the way, confirms that the Virgin can receive SSB and AM as well as CW.) In no particular order, I heard:

  • N8KKR, KA9ZXN and others on the Hams for Christ net on 7.263 (330p Central, Monday-Sat). Not my personal bag, but they seemed a very pleasant bunch.
  • W9DCQ and W4LWW having a nice QSO, between the two of them and some other operators I couldn’t quite copy.
  • KG9O and KK4FZI having a friendly chat.
  • Some other snippets of stations.

The listed stations alone include QTHs in Evergreen Park IL, Columbus OH, Middleton WI, Franklin TN, Grassy Creek NC, and Marion IN. None of them terribly far away in radio terms (the Tennessee locations is farthest at ~500 miles), but a great confirmation that the antenna was clear and working. With a height above the ground of less than 1/4 wavelength, the ideal dipole should have a high takeoff angle and be fairly omnidirectional, and that seems plausible based on results.

In sadder news, I seem to have broken some part of the WSPR functionality of my VFO in consolidating that functionality from two separate programs into one. 1W into the antenna and several repetitions netted be exactly zero receptions, either into this antenna or my office random wire. So it’s back into the code I go. While I’m at it, it would really be worth packaging up the little VFO into its own enclosure – having all those wires flapping about is a bit worrisome for transport!

All in all, a tremendously successful, if very chilly, day by the lake. Now that I know what I’m doing, I suspect it would be able to have the station set up and listening in about 10 minutes, and about the same for tear down. With the days getting warmer all the time, I’m sure I’ll be out there again soon.

Hear you on the air!

73

Giving the Virgin Some Mobility

As wonderful as it was to ‘complete’ the Virgin Receiver and have a set of ears to start listening on, more and more I find myself wishing for the ability to change frequency and explore the 40m band a bit. To that end, I’ve been experimenting with hooking my Si5351-Based VFO directly to the Virgin as an easy way of giving it the ability to QSY.

(The VFO, as it happens, has a few more features nowadays, including being able to act as  WSPR beacon and having an auto-CQ mode. The code, as always, is on Github.)

Here’s the receiver as built:

schemeit-project(1)

Jason NT7S dropped a hint in one of his older blog posts that the Ne602 likes to see about 300 mV p-p when being driven externally, and that a 10dB pad could be used to bring the Si5351 down to this level. To that end, I put together a quick Pi-attenuator consisting of one 120-Ohm and two 150-ohm resistors.

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A tiny 10dB pi-attenuator.

The attenuator plugs directly into the receiver where the crystal X1 usually sits, in a little 1×3 piece of female header. The output of the 10dB pad plugs into the side of the header that’s connected directly to pin 6 on the NE602; the ground on the attenuator plugs into the other side of the header, and is therefore connector to ground on the receiver.

The longer red and white wires you can see attached to the pad connect to ground and the CLK0 output of the Si5351. Here’s the current setup, spread out on the bench.

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It’s already quite successful! I can scan up and down the 40m band and pick out CW signals pretty well. SSB signals are faint and pretty un-hearable. Unfortunately, for some reason, my receiver has also turned into an AM radio:

I think some kind of high-pass filter is in order here. There’s always been a bit of AM bleedthrough with this receiver, but given that the AM stations are attenuated by the RF Pot on the receiver, it seems like most of this signal is coming in through the antenna and not, say, bleeding into the audio amp.

Hear you (all around the band) on the air!

73

 

Easy QRP Low Pass Filters

With an eye toward getting my rudimentary Si5351-based transmitter on the air, I’ve started putting together a number of low pass filters to knock down some of the nasty harmonics that come with using a clock chip as a frequency generator.

The filters are all based on the designs given by W3NQN in the GQRP technical pages from 2015. (This is another point at which I’m taking design inspiration from the QRP-Labs QRSS beacon.) The paper lists both the target values for each of the toroids and capacitors for a 7-pole low pass filter for each band, as well as the required number of turns for each toroid and an appropriate toroid to use. For this project, since my target frequencies range from about 7 MHz to 14 MHz, I’m using all T37-6 toroids from KitsAndParts.com. The basic design of each filter is the same:

lpf sketch
All these filters are symetrical, for 50-ohms on either side.
Here’s the values that W3NQN gives for low pass filters for the 20m, 30m, and 40m bands:

Band C1, C7 C3, C5 L2, L6 L4
20m 180pF 390pF .773uH .904uH
30m 270pF 560pF 1.090uH 1.257uH
40m 270pF 680pF 1.380uH 1.698uH

Unfortunately, I don’t have exactly the specified capacitors in my kit, so here’s my approximation to these values. (Note that the number of turns is different at 40m than W3NQN recommends, to more closely match the specified inductance). All inductors are wound on a T37-6:

Band C1, C7 C3, C5 L2, L6 (turns) L4 (turns)
20m 1000+220 (series) = 180pF 1000 + 690 (series) = 408 pF 16T = .77uH 17T = 0.87uH
30m 220+47 = 267pF 220+220+100 = 540pF 19T = 1.08uH 20T = 1.20uH
40m 220+47= 267pF 470+220=690pF 21T = 1.32 24T = 1.73uH

Some quick simulations in LT Spice confirms that these little adjustments don’t have an enormous effect on the filter’s behavior. On 40M, for example, it has the effect of moving the cutoff frequency down maybe 100 KHz:

40m LPF Filter as Built
The behavior of the 40m LPF as constructed with the parameters above. Note the nice knee just under 10MHz and that the attenuation is around 40dB at 14Mhz, the second harmonic for this band.
So far, I’ve only constructed the 40m filter. It’s built on a little 2.3cm x 10cm piece of coppper clad. (I’ve got a bulk stock of 7cmx10cm pieces, which cut nicely into threes to make little projects like these).

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The 40m filter as built. I love these little screw-on BNC connectors. There’s probably some RF leakage because of them, but they’re so easy to apply and use. And it makes changing male and female BNC connectors as necessary a breeze.
The filter seems to be working great – a little qualitative examination with an oscillloscope shows that the filters are smoothing out the waveform at 7 Mhz quite nicely. Above that, the attention seems to match the slope predicted in the simulation above.

With this filter on my little homebrew transmitter, I finally made my first (official) WSPR contact… but that’s a story for another post.

Hear you on the air!

73