While digging around the avrfreaks.net forums, I found this very interesting tidbit in a message from a gentleman who was working on using an AVR microcontroller for SAME decoding. One poster mentioned using the Goertzel Algorithm as a way to perform tone detection on a sampled signal. The algorithm is supposed to be simple enough to implement in a microcontroller. Apparently, this method has been used for years in DTMF decoding, but this is the first time that I've ever heard of it. This could potentially be very useful for performing functions such as simple RTTY decoding or RF remote control in a cheap microcontroller.
No luck in making it to FD this year. Had some more important things come up at the last minute that took priority, so I couldn't take the time out for FD. After seeing some of the comments about how welcome the slower CW ops were, I'm not too sure it wasn't for the best that I didn't even try. I guess this little fish will be staying out of the pool until he can swim with the big 35 WPM sharks.
So it looks like I will be heading up to Stub Stewart State Park to spend some Field Day time with OTVARC and some of the hams from Tektronix. Seeing that the current forecast is for temperatures near 100° in the valley (it may not be much cooler up in the mountains) and that my allergies are killing me right now, I don't predict that I will be spending a lot of time out there. I'll see if I can work up the nerve to man the key for a little while, although I have no idea how that will work out. My code speed is only about 13 WPM comfortably, and I'm sure that most CW ops working FD will be a lot quicker than that. I will probably have to troll the upper parts of the CW bands for some slow speed QSOs. Wish me luck, I'm pretty rusty!
I've had a large collection of BF998 dual gate MOSFETs sitting around in the junkbox for quite a while now (acquired from KE6F), but I've never really known what to do with them. I feel like I own a fairly decent library of QRP/homebrewing literature, but there seems to be very little detailed information on the theory of operation of the dual gate MOSFET. Most of sparse circuit information on the device seems to assume that you already know the basic theory behind it.
I decided that the best way to learn more about the workings of the dual gate MOSFET was to start building circuits and taking measurements. This post is intended to be the first in a series where I will examine different aspects of dual gate MOSFET circuits.
I started by consulting the BF998 Datasheet, as well as additional documentation provided by the manufacturer, NXP. I decided to base my first circuit off of the recommended biasing in the NXP documentation. Below is the schematic for the first setup that I used to examine the bias characteristics of the BF998. A constant voltage (VAGC) was applied to G2 for this test. The datasheet recommended a nominal AGC voltage of 9 V, so I tried to get a voltage on G2 somewhere near that value. I applied a variable bias voltage (VBIAS) to G1 and measured the voltage drops across R6 and R7 in order to calculate the currents in each resistor. The difference in the currents in R6 and R7 indicated the drain current (ID).
|0 V||2.19 V||9.94 V||0 uA|
|2 V||2.43 V||9.70 V||890 uA|
|4 V||3.97 V||8.15 V||6.60 mA|
|5 V||4.78 V||7.35 V||9.58 mA|
|6 V||5.60 V||6.53 V||12.62 mA|
|8 V||7.20 V||4.92 V||18.54 mA|
This data gives us a starting point to see the effect of VG1 on ID. One feature to note about this biasing scheme is the voltage divider bias applied to the source. The literature states that this is used to bring the source to a higher potential than would be possible using a simple low-value source resistor. The rationale for this is to allow the AGC voltage on G2 to be able to swing lower than the source voltage, which will give a much wider AGC range than a biasing scheme with a single source resistor. (This negative voltage in relation to the source is necessary because the BF998 is a depletion-mode FET, similar to your common N-channel JFET. If you look at the datasheet, you see that the pinchoff voltage is negative). I'll touch on this a bit more shortly.
What is interesting is how this biasing scheme differs from the one encountered in most of the QRP literature. Next, I tried to duplicate the biasing scheme scheme used in the W7ZOI 50 MHz preamp. However, I was unsuccessful in getting any significant amount of drain current with G1 grounded (not nearly the amount that W7ZOI was reporting, even when using the same source resistor values that he reported). I could get much more drain current to flow when I applied bias voltage to G1 as I did in the previous experiment. This puzzled me, so I decided to set it aside for now.
A bit more searching on the net lead me to a circuit in another radio; the Electroluminescent Receiver. Dual gate MOSFETs are used liberally in this radio. It turns out the dual gate MOSFET circuit in this receiver derives its lineage from a QRP classic; the Progressive Communication Receiver (ARRL members only). The unique bit about the amplifiers in these receivers is the use of a diode or LED in the source leg to provide biasing. The forward voltage of the diode(s) or LED(s) raises up the source voltage to allow that AGC swing that was mentioned earlier. This is an alternate (and I believe a simpler) way to do the same thing as the voltage divider biasing used previously.
Armed with this information, I changed the biasing to try a LED in the source leg. A green LED was used, since they have a forward voltage of approximately 2 V. I tried two different values of R6, and as before I applied various bias voltages to G1 to see the effects on drain current.
|0 V||0 V||0 uA|
|2 V||512 mV||1.55 mA|
|3 V||1.25 V||3.79 mA|
|4 V||2.03 V||6.15 mA|
|5 V||2.82 V||8.55 mA|
|6 V||3.63 V||11.00 mA|
|7 V||4.42 V||13.39 mA|
|8 V||5.21 V||15.79 mA|
|9 V||5.70 V||17.27 mA|
|10 V||5.83 V||17.67 mA|
These results seem pretty good, although it looks like I was running into the limit of increasing the drain current somewhere around 9 to 10 volts. I suspect that's because the voltage on G1 was approaching the voltage on G2. However, I may be wrong about that, so feel free to correct me.
Next, I decreaed the source resistor to 100 Ω, a value commonly seen in other dual gate MOSFET amplifiers.
|0 V||0 V||0 uA|
|2 V||355 mV||3.55 mA|
|3 V||912 mV||9.12 mA|
|4 V||1.52 V||15.2 mA|
|5 V||2.15 V||21.5 mA|
|6 V||2.79 V||27.9 mA|
Since the maximum drain current for the device is listed as 30 mA, I stopped at 6 V of bias on G1. A bit of tweaking of the bias voltage showed me that I could get a drain current of 10 mA with a G1 voltage of 3.16 V. Since the manufacturer documentation lists a nominal drain current of 10 mA, I decided to stick with this value for now. When it comes time to test IMD, I may want to change this a bit, but for now it's a good starting place.
The plan is now to use the LED biasing scheme when I start investigating the RF characteristics of the device. I suspect a bit more tweaking will be done, but I feel that I have a better grasp of the basic biasing of the circuit. I may look at taking the G1 bias off of the top of D1, or I might use a voltage divider...that's still to be decided by future experiments.
Wayne McFee, NB6M has created a custom PCB for his variation of the MARC LC Meter, along with providing a kit of parts. I just received my kit yesterday and was very eager to build the thing, since it's one vital piece of test gear that I don't yet have.
Stuffing the PCB according to Wayne's instructions was quick and easy. There were a few minor hitches to watch out for, but nothing big as long as you take the time to read the documentation. It only took me about 45 minutes to get the on-board components installed.
One nice thing about the kit is that the PCB fits the exact dimensions of the white-on-blue LCD display that is provided. The mounting holes line up quite nicely, which allows you to use standoffs to secure the display to the main PCB. A few external switches and connectors are wired to the PCB to finish the electrical construction of the meter.
The smoke test passed successfully and the meter read a capacitor and an inductor pretty close to the nominal values, satifying me that everything was working correctly. I know that the leads from the DPDT switch to the DUT terminals should be shorter to minimise stray inductance, but the accuracy was reasonably good with the setup that I have above. I'll probably end up switching out those binding posts anyway, since I'm not too fond of them. I've got a supply of 5-way binding posts from Jameco that I like to use, so I'll substitute those with shorter leads.
All that is left to do is mount the meter in an enclosure, which I still have to procure. I'll probably make a trip down to the local Radio Shack later on to get a project box. (I can't believe that I'm actually not ashamed to shop at this RS). Kudos to Wayne for an excellent and very useful kit. I know that this one will be getting a good workout on my bench.
Terry, WA0ITP asks on qrp-l.org:
With a low Solar Flux and a double digit A index, it's time
to hunker down and melt solder, snort rosin, and burn
I've got a half dozen unbuilt kits, an oscillator, single
ended mixer, and an audio amp all destined to be a DC
receiver (hopefully). An 80M amp nearly installed in a CR
enclosure. Oh, and a buncha Dayton RCA connectors for a
straight key/keyer distribution box, err tin..
So What's On Your Bench?
The answer at NT7S is: way too much stuff. On the kit side, I have two from NB6M of the pQRP group: the TinEar receiver and his version of the MARC L/C Meter. I recently ordered the N3ZI Digital Dial, and have that about half assembled. I plan on pairing this up with my beta version of the Willamette when I get a chance to put it in an enclosure. I've also got a big pile of the W8DIZ RF Toolkit modules ready to patch together into a receiver. The plan is to pair this stuff with the HYCAS IF amp.
On the homebrew side, I've got a whole bunch of irons in the fire. I've really been taken with AVR programming lately, so I've been developing some different applications for these uCs. The development environment is Ubuntu Hardy Heron, using gedit and the avr-gcc tools, and the Adafruit USBtinyISP programmer. The big sooper sekrit project here is an in-line QRP SWR meter, which is rumored may make an appearance as a kit if I can learn how to use Eagle CAD. Many other AVR-related ideas are also on the back burner. Believe it or not, I've also been doing some analog RF stuff. I have a dual-gate MOSFET mixer (based on the BF998) that I've been tweaking for possible use in a simple receiver. I think the plan is to put it on hold for a while, in order to build up some more test gear (such as a return loss bridge, noise figure test rig, etc.) so I can quantify the performance better than any other circuit I have yet developed.
So as you can see, I've got way too much stuff on my hands. Even if I could retire really, really early, I would have enough projects to keep me busy for very long time.
Now that school is done for the summer, I've finally got some time to put into finishing the neglected final documentation for the Willamette transceiver project. Here's what I have on the agenda for the document:
- Finish the final section, which includes operating instructions, mods, and credits.
- Fix a few typos and technical errors/inconsistencies in the released sections.
- Merge everything into a single document
I'd like to have this done before the 2nd group of kit builders receive their kits, which means that I should get moving on it soon! Then I believe that I can finally feel that the project is "finished".