Doubling to 6m

SPRAT 140 magazine had an article by G3XBM's Sixbox 50 MHz AM transceiver. It is also documented on this blog/website. His TX is a 3rd overtone XTAL oscillator followed by a buffer and an amplifier both voltage controlled to generate AM modulation. The RX is a freerunning LC regen. The output power is about 200mW peak (modulated).


I got intrigued by his simple circuit and decided to replicate his design.


It turns out that I have one surplus XTAL that would put me on 50.8 MHz and a canned oscillator that goes to 50.35 MHz. All simulations were done using my xtalfind Perl program.

The XTAL has too short leads to be used and it is not an overtone cut, so I decided to use the 25.175 MHz canned oscillator and try to double it with the EX-OR doubler described in the previous post.


The local oscillator signal has a period of T=40ns. I need to delay it of 1/4T, so 10ns. According to the 74HC86 datasheet this amount is right in the typical propagation delay range for 5V supply.


I've assembled dead-bug the circuit and done two tests:

1. check the output at the oscilloscope (max BW 100 MHz, so quite at the limit)
2. use my finger to check for the base frequency
3. see the output spectrum on a spectrum analyzer

Test #1 is visual. The resulting waveform is more sinusoidal than it actually is because I've operated the instrument at the limit of its bandwidth. But the output period is half the input, so there's no 25 MHz output.


Test #2 confirm. Using my FT-817 receiver tuned on 25.175 MHz carrier I touched the canned oscillator output and noticed a signal increase. Then I touched the 74HC86 output and heard only an increase on 50.35 MHz.


Test #3. My spectrum analyzer starts at 45 MHz, so I have no mean to check what happens at the LO frequency. But I measured 5dBm at 50 MHz (through a 7dB attenuator pad) and about -6dBm and 100 MHz.
Curiously there was a rather strong product at 75 MHz that I could null adjusting the delay line.



So in theory I have a 12dBm signal (15mW) at 50.35 MHz. I could directly modulate the 74HC86 chip to produce AM, but the propagation delay is a function of Vcc too.


Now I can either generate AM or DSB-SC. Both need a linear amplifier chain, with DSB being more energy efficient. Since I have a digital signal I could probably drive a simple balanced mixer, or an NE602. In case I use a NE602 then it can be re-used as RX mixer too (for AM or SSB, not DSB).



If the canned oscillator could be pulled I could generate NBFM too, but I think this is not the case.

The EX-OR frequency doubler

After few years I've had this idea in mind, I could finally melt some solder and put together a frequency doubler using EX-OR digital gates.

The idea for my application is: feed a 2-IN XOR gate from a 74HC86 with properly phased signals and it generates a 2 * f_in (square) wave.

So, what are "properly phased signals"?

If the base frequency f_in, which is in form of a square wave, is delayed of T/4 and fed to the second input of XOR gate, then given the XOR truth table the gate will output a 2x frequency.

I leave the reader draw the two wave forms and XOR them graphically to see the effect.

How to delay the square wave then? Easy, use the intrinsic propagation delay of digital gates! Per datasheet a 74HC86 gate has a propagation delay of 11ns with a load of 50pF at 6V supply. According to another document of OnSemi the delay introduced by a gate varies linearly with the capacitive load, so I see a simple way to control delay between 50ns down to 5-6ns.

Why am I doing this? Besides for the scientific progress :-) I'm aiming at a simple TX/RX system for 50 MHz band.

FT817 keypad - a completed one

I have built one of my FT-817 keypads for another person, and here's how it looks like when unassembled:


I have included connectors on the serial cable and on both LEDs. All connectors are different so the user will not mix them. The overall footprint of the completed keypad is slightly larger than the matrix keypad itself, which sits on the circuit using a matched pin-header strip.

Except for the book, what is shown is worth 20€ (@2009) of materials and circa 3 hours of work.

DTT switch-over

2009-10-07: switch over from analogue over-the-air TV to digital (DTT, DVB-T) in my area.

I grabbed the TV remote control and scanned all programmed channels, from 1 to 29: white noise. I ran a complete search in case the condo antenna could pick up some remote signal once masked by stronger local stations: nothing.

RIP analogue TV.

I still have to erect my antennas on the roof. With digital TV signals it will be harder to identify even the lightest TVI, or perhaps the whole system will more immune. At least people won't hear my voice or CW through their TV set loudspeakers, just loose audio or video quality.

Maybe some analogue TV will be thrown away, becoming a good source for scrounged parts to keep aside for future experiments.

Inside Gould 4072 DSO

In August this year I have received a Gould DSO model 4072, 100 MHz bandwidth, 400 MSPS. It is a totally different concept of oscilloscope to what I am used to (analog), and without documentation it needs some thorough exploration.

The unit appears to be in working conditions, except for the power button that doesn't depress and perhaps broken CRT graticule illumination (don't remember).

According to the OSD menu this DSO has onboard memories for measured waveforms, so it must have a backup battery somewhere. I opened it up removing the upper lid (careful with the printing device connecting cable!) and was surprised to see that it has plug-in boards.
I wouldn't say that these are serviceable, but at least replaceable with similar boards.



With a bit of lucky intuition I found the battery on the "DIGITAL" board. It is 2.4V 110mAh and according to my DVM it still has some charge (~1V). No visible leak. I will replace it as soon as possible.

At the same time I am working on the power button, that seems to close only one contact towards the chassis (!). Or wait, it must be some mechanical thing instead. The button pulls/releases a metallic strip that goes all the way to PSU. I need to follow it all the way in there...

Edit 2014-02-11: I do not own anymore the Gould 4072.

QRP from Rab Island (2009)

This note will look old, since more than one month has passed since we've returned back home. But at least will report what I have done in 2009 on Rab island.

I had planned to take part to Apulia VHF QRP contest from the top of Kamenjak mountain. All I could carry was the 4 element Yagi. The day before the contest I had forgotten headlight on and the car battery went flat dead. Fortunately I could get it started for 100 HRK from the local service station.

The contest day I went up to the top, prepared the setup and called for two hours beaming Italy (organizing country) without receiving a single reply.

So I decided VHF was not the way to go and I switched to HF 20m. I moved the 9m fishing pole away from the car so it could support the 20m EFHW antenna radiator, tuner laying on the rocky ground and one counterpoise.

I did get odd looks from hikers/bikers/tourists passing by, but nobody asked a thing.



The band was crowded, signals were good, so I tried some QRP SSB contacts. I could work one station, others were having pile up (it was too hot up there for fighting). Finally I fired up the netPC and called CQ on PSK31. This mode was more productive.

The game was soon over since we were expected for lunch. PSK31 operations continued from home with 100% battery power (RTX and computer) and 20m EFHW antenna out of the balcony.



Overall I managed fewer QSOs than last year, but I did not take part in any contest. There was no pile-up on my calls even if I was from a IOTA/IOCA reference.

I missed the warmth of CW contacts but I enjoyed these operations nevertheless.

Alpe Adria VHF 2009: mission aborted

Every year I can hardly wait for Alpe Adria VHF contest, the first Sunday in August. After my best performance of year 2008, I wanted to improve the setup.

I started building a Yagi with 4m long boom: on an empty mountaintop it should still be possible to handle it. On Friday the boom was ready but not the rest (dipole, elements, ...). Moreover one battery pack is not suitable anymore for my FT817, so I had half power reserve than last year.

I decided to keep things simple and short: same antenna as 2008 (5el on 2m boom) and just one battery pack. I would not be able to beat my own A.A. record but at least to improve my operating skills.

On Sunday morning I drive up to Colle delle Finestre (2100 masl), there are some clouds in the sky but don't look too serious:


Once at Colle, I see dark clouds climbing fast from the other valley, and my target destination already covered. I change plan: I'll go to a closer peak at 2500 masl.

At the beginning of my hike I have to pass a barrier, I pull my backpack, I pull ... crash ... damn! What was that? I put the backpack down and the 2m long boom sticking out of it has broken in two. I can still build a 3 el antenna or fix it on the fly once on top. So I continue hiking.

Air gets colder. I feel something on my hand: it starts raining.


Far away I hear a couple of sounds that look like a thunder, or a motorcycle roar.

-> abort <-

I walk back to the car, take few pictures and a strong storm begins. It lasted two hours, so I am glad I decided to return home rather than wait for clouds to pass by.

See you next year, Alpe Adria!

The word of experience

Finally I put my Tek 7603 oscilloscope into service. A friend (the one from optical experiments) needed to verify a circuit he's building. Those were routine tests, not troubleshooting.

While probing for a couple of signals (ca 3 kHz, 15Vpeak) I noticed a strange random behavior: either the signal was as expected, or was passing through a high-pass filter (only spikes instead of edges).

The first problem was easily tracked down to a cold solder joint since it showed properly when touching the generating IC.

The second ghost was trickier: all joints were ok. It was a digital signal... where was the high-pass effect coming from? While checking for continuity with an ohm-meter backwards from target to source pins (with ICs removed!) I came to a homebrew connector with 2.54mm (1") pin headers ... on the corresponding pin tip I could not get a proper contact, while at the pin base it worked alright. Very suspicious. I touched the pin and asked right away: "what did you do to it? Glue?"

He admitted that the building instructions showed a way to hold the strip while soldering: put it into a roll of packing tape. !!!!!!!!!!!!!!!!