Tag Archives: oscilloscope

40 Meter Ceramic Oscillator

November 4, 2015 KK6GXG

The Beach 40 saga continues. My original VXO design, well… it stunk. I’m not sure why, but the best selectivity I could get out of it was only a couple of kilohertz, very annoying.

I have been working on a VFO design to get full band selectivity but in the meantime I came into some ceramic oscillators for 7.2 and 3.68 MHz. The best part is that I replaced the entire Super VXO section with a 140ρF variable capacitor and two ceramic resonators.

By placing the 140ρF var cap in series with the two paralleled resonators and feeding the output directly into the local oscillator buffer… Ta da! A working VXO.

This time, instead of a few kilohertz the VXO pulls a full 148 kHz, from 7.173 MHz down to 7.025 MHz. It is certainly not the full band but it is a useable range. Over a five minute period, I noticed a slow 50 Hz drift that eventually settled out to a point where it continued to float ±10 Hz.

This is not a final solution, but it will work well enough to get this QRP DSB transceiver on the air for testing.

Of course now that I have a working VXO attached to the local oscillator buffer, it’s time to hook it up to the product detector/balanced modulator… success! With the VXO, buffer, product detector chain attached to an antenna it was time for a little listen.

The audio amp still has some issues as far as power drain off when power is disconnected, but it does fine as an amp. I plugged everything together and plugged the audio out into an external amp so I could tune around without headphones.

Not much on the band, but my frequency generator with a small antenna plugged into it put out enough RF for the receiver chain of the Beach 40 with the temporary VXO to pick up its 400 Hz signal and cross reference the VXO frequency displayed on the oscilloscope with that of the frequency counter connected to the frequency generator. So one could say I have a working receiver, mostly anyway.

Next up: First and foremost I need to get my documentation in order. In particular, I need to get the schematics in conformance with the actual circuits. After I get the documentation in place I need to get to work on the audio amp.

After the RX stage is all dandy, my attention will turn to the TX/RX switching. I can’t do anything more with the TX side until I get the TX/RX switching squared away.

Anyway, that’s all for now.

Until next time,
~Jon KK6GXG

UPDATE: As promised, the conformed schematic for the VXO/Buffer section…

Amature Radio

RTFM aka Read The Datasheet

May 28, 2015 KK6GXG

A lot going on these days. Lets start with something educational… No, don’t run away… it’s not that bad.

RTFM or Read The Freaking Manual is directed in this case to datasheets for semiconductors. And this is where we begin our lesson for today.

20150527_113247 — The VXO dismounted from the LO/Buffer for testing

I have been working on the Beach 40 project for a few weeks now and have made some progress but I was having some difficulty with the VXO (variable crystal oscillator) and the LO/Buffer (local oscillator and buffer.) The oscillator section is supposed to generate an RF carrier frequency, in this case close to 7.2 MHz. I wasn’t generating the carrier and I couldn’t figure out why.

I decided to put these sections aside and move on to the next section and ruminate on the problem for a while. The next section in line was the Balanced Mixer / Product Detector but I was missing a component that I was still waiting to arrive, so on to the next stage, the Microphone Amplifier.

20150525_135559 — The Mic Amp has the mic element temporarily direct wired for testing

Laying out the parts physical locations on the circuit board I came to the transistor, one I hadn’t worked with before. So I decided to look up the data sheet and be sure of the pin-out. It wasn’t what I was expecting it to be, and then it hit me, like a Log from BLAMO! My oscillator wasn’t oscillating because I had the pinout wrong on the transistors! I finished the Mic Amp and the part arrived for the BM/PD.

20150527_154019 — The Balanced Mixer/ Product Detector. Ain’t she a beaut!

I moved on to the BM/PD and finished it then looked up the transistors on the LO/Buffer and sure enough, I had them in backwards.

With a little coaxing and cajoling, and construction of a really scaled down crystal oscillator for testing, I got them turned around and everything back in place, well sort of. I missed a jumper and had to go back and solder that back down, but the after that the oscillator was oscillating like a good little oscillator should. Oscillation!

With a little tweaking and and the removal of the Fine Tuning circuit that I still haven’t figured out, I am back on track and only three sections shy of a full transceiver. At present I have the sections built for a QRPpp transmitter a very, very, very, very low power transmitter, somewhere around the microwatt range but I should be able to modulate a voice transmission a foot or so to my DC receiver.

The Mic Amp test is actually a test of the VXO, LO/Buffer, Balanced Mixer, and Mic Amp sections and has me transmitting some AF (audio frequency) along with the carrier through the Balanced Mixer to help locate the signal in the band. Once I locate the signal on a separate receiver it’s time to tune out, or “suppress”, the carrier signal on the BM/PD section. I’ll be doing just that in the next few days. Hopefully I will remember to video the test and post the video.

I already checked out the VXO and LO/Buffer when I checked the frequency. I can check the Mic Amp just by hooking it up to a speaker, which I will. Then the BM/PD gets it’s big on-air check out.

20150527_163139 — CCW from top left: Balanced Mixer/Product Detector, Local Oscillator/Buffer with VXO mounted vertically, tuning capacitor, Mic Amp, and Audio Amp with just the tinned pads on the board

All that’s left to have a working receiver is building the Audio Amplifier which I have already laid out, I just need to solder the parts down and test. After that I need to build the RF Amp to have a transmitter. I will want to also complete the Low Pass Filter before transmitting though. I don’t want to splatter the band or anything.

So the schedule for now looks like a completed Beach 40 transceiver should be on the bench next weekend or there abouts.

That’s all for today.

73,
~Jon KK6GXG

Lab Work, Repairs

New-To-Me Signal Generator

April 18, 2015 KK6GXG

On Saturday I picked up a few things at the De Anza Electronics Flea Market. The most impressive acquisition would be the Lafayette Signal Generator for $20.

I have been needing one and I was dreading the prospect of having to build one and having a tough time without the right tools for calibrating it, so this was a great find and a steal of a value, though I didn’t know it for sure until today.

One thing I have learned about buying used electrical/electronic equipment, particularly in at a flea market, is that you have to take your time and open it up, get documentation, inspect and clean the equipment before attempting to use it, or plug it in for that matter.

Sunday was opening day. I noticed right away that I would need to replace the power cord grommet as it was in two pieces. I didn’t actually get to anything else until today.

Today I inspected all of the wiring and components, looked for hot spots and anything that would indicate an over heat. Having found nothing of the sort I moved on to cleaning, which there was surprisingly little to do. I made sure the tubes were cleaned and had no fingerprints on them. Now that I have a piece of tube equipment I will need to get a tube tester. 😉

With the cleaning done it was time for the plug-in and smoke tests. No pops and and no smoke! I let everything warm up and burn in for a good twenty minutes before starting any tests.

Testing with the oscilloscope began with the AF (audio frequency) side of the generator. After the twenty minute warm up period the “approximately 400 cycles” audio tone as specified in the manual turned out to be stable at 388 cycles (Hertz). I can work with that.

“kc” is kilocycles. The term is essentially the same as the more familiar kilohertz along with “mc” megacycles being the same as megahertz. The change over in terminology occurred slowly from the mid 70s to the mid 80s. Many hams still use kc and mc. I use them interchangeably depending what I am referencing or who I’m talking with. This piece of equipment has frequency labeled on the dial as kc and mc.

Moving on… The generator did its first diagnostics job with a portable amplifier I have had for a very long time (the one on the right). I clipped on the amp to the audio out and it turn out the volume control on the amp is trashed from banging around in my tool bags for a couple of decades so I will need to replace the potentiometer in that this weekend.

I then moved over to the RF (radio frequency) side of the generator and clipped on the oscilloscope. The RF side is divided into 5 switchable bands. I checked each band by referencing the frequency on the dial with the frequency on the O-scope. All five bands checked out very closely to the dial. A little lead or lag here-and-there, but overall pretty close for this equipment and its age which I’m guessing is about as old as me (made in the mid/late 60s).

The frequency counter I purchased last year has been giving me problems. New equipment, cheap (figuratively and cost), and no manual. With the help of the new sig gen and the oscilloscope I managed to fix a couple of minor problems and figure out the modes on the counter in the process. Looks like the electronics bench is finally coming together.

Now that I can check the receivers I build I can also check the transmitters I plan on building. I can also check portions of the radios as the construction moves forward. I plan on building more of my own test equipment but it’s nice to have a reference point or two to calibrate off of.

I also did some diagnostics on the 40 meter direct conversion receiver project and updated the project page… some very interesting results, you should take a look. 🙂

A productive day I think.

73,
~Jon KK6GXG

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