Month: March 2016

A Crystal Checker (Oscillator and Buffer)

So today among other things, I decided to pull out the crystals and start playing with simple crystal oscillators buffers and perhaps even at a later stage, expand on this to VXO and Super VXO’s and make myself a pretty simple rock bound rig with some frequency agility.

So i digress, today i knocked up a simple collpits oscillator and buffer to be used as a crystal checker. We all have crystals we cannot read the value of, so having a simple oscillator to stick them into to find out their frequency, is a handy thing to have and being simple, you can knock one up in a lazy afternoon and keep yourself busy for a while.

 

12916278_1275356985811342_7200592174495787135_o

So i laid the board out ans routed up a board. There is an error in the schematic. The value of the 470 capacitors should be pf (Pico Farads) not nf.

12916879_1275431735803867_5627072237247994163_o

The board was assembled and smoke tested. Ran into a little issue at this point, no output from the buffer. The oscillator was doing its dance and making nice sine waves, but the buffer was not playing its part and buffering. After a lot of hair pulling, asking questions of much smarter people than me and getting some good and some not so helpful answers, I ripped the DC blocking cap out, measured it and found it was short to ground. Replaced said cap, buffer fires up and strts buffering.
12928129_1275365339143840_8339005945985913345_n

Output of an 80m colour burst crystal, showing nice harmonic content as the oscillator is being driven a little too hard and going non linear.


 12512392_1275438662469841_462139532436070531_n

Output from a 10MHz crystal i bought to use as an IF filter in a Superhet, nice and clean and no harmonics present. Was a good little bit of fun, next I will have to build another and turn it into a VXO and then a Super VXO and eventually a rock bound rig.

Facebooktwitterredditpinterestlinkedin

The Art and Guesswork of 10m

Sometimes you just gotta try things, spin the dial and see what happens. You just never know really what paths are open unless you stop, listen and call cq. And even if you listen and do not hear something, it is often worth while having a call for a while just to see what eventuates.

I will do this every now and again and the results always seem to be worth it. So today I had a quick tune about 10m, i heard nothing, then i heard 1 weakish signal in Marshal Islands V93NS and i gave him a quick call and got in the log, so i found a vacant freq, spotted myself on the cluster and pushed the auto caller on the rig and sat back and waited to see if anything came of it.

And low and behold, it was not long before i worked a couple of JA stations on a band that was dead quiet. Moral of the story is, a dead band is often open to somewhere and unless someone calls CQ, others will never know you are listening. Self spotting on the cluster while forbidden in contests, is kind of acceptable when you are calling into a dead band, you might as well try and alert someone to your presence calling cq and hope they come on frequency to see if you can be heard.

Mostly, get out there and have a go. Less assumptions about what the band is, and more trying to make contacts regardless of the lack of traffic. 73.

Facebooktwitterredditpinterestlinkedin

Input and Output Impedances

Impedance is one of those concepts in radio that can be hard to fathom conceptually. If you read a description of what impedance is, it generally goes something like “The resistance in a circuit by the combined effects of resistance and reactance” With reactance being “The effect of capacitance and inductance causing the current to be out of phase with the EMF causing it”

All in all, it sounds like a bunch of gobbledygook to me. What I think is important to know, as a total beginner like me, is that impedance exists and that mismatches in impedance between circuits leads to loss and inefficiency and at other times, probably some other nasties as well. So, if we know it exists, even if we cannot conceptualise the concept, how can we measure it so we can design transformers to match the output impedance of once circuit with the input of another.

All that is needed is a variable resistor or decade resistance box, signal source and an oscilloscope and understanding a basic principle that if you vary the input or output load resistance until you have a 50% drop in the output amplitude, the resulting resistance is equal to the input or output impedance.

Input Impedance

Zin-test

Starting with the variable resistor at 0 ohms and working up, output amplitude decreases, when amplitude is 50% reduced, resistance of the variable resistor = input impedance.

 

Output Impedance

Zout-test

With output impedance you want to start with variable resistor at maximum and decrease its value till the output amplitude is 50% of the no load value. At this point, resistance again equals output impedance.

Alternate Method

Do not have a scope or power meter, this is no problem either as you can acheive the same using nothing more than a fixed resistance and a multi meter. For this method you take 2 measurements, voltage at no load and under a fixed load. With those values you can then use the following equation to determine the output impedance.

Output Impedance = Rload(Vopen/Vload -1)

Or, Output Impedance is equal to the Load Resistance multiplied by the (open voltage divided by the load voltage minus 1). Both methods will give very similar results and if applied to things like amp chains, will allow the builder to match input and output impedances across stages and reduce losses in the system.

Images are from: HERE  Also this site is a very good resource for all things introduction to electronics theory and practice.

 

Facebooktwitterredditpinterestlinkedin

Diode Ring Mixer

There comes a time when you build something to just understand it, tonight was one of those nights for me. I mostly use mini circuits mixers in my home brew as they are cheap and convenient, but tonight i build a diode ring mixer just to learn about how they really work and understand the principles involved.

drm

So i laid out the circuit in Spring Layout.

IMG_20160319_203242

Routed out the board, wound the transformers and assembled the few other bits and bobs i needed.

IMG_20160319_210345

Assembled the board and used a sharpie for my very professional looking silk screen HIHI.

IMG_20160319_212730

Next step was to feed the thing and check to see if it was actually working. LO was fed with 3v p-p at 8.4mhz from a DDS Signal Generator, RF was fed with 100mv at 1mhz and the IF was connected to a spectrum analyser to see what the output products were.

12874491_1265752343438473_1412500626_o

As you can see in the image above, we have the + and – peaks of the inputs showing in the spectrum analyser, 7.4 and 9.4 mhz. Harmonics are there but are orders of magnitude down on the desired signals. This surprised me a little as I expected greater harmonics with the 2 inputs not being 50 ohm terminated. LO was 200 ohm and RF has an unknown impedance, but i really should measure it just so i know.

All in all, this has been a very successful little project that produced good results and a very useable mixer.

Facebooktwitterredditpinterestlinkedin

New QSL Cards

_cfimg-1664236993218596108

An interesting story with this one. I had been calling cq for a while, having a qso with the odd station here and there, when i got into a ragchew with Cliff, a ZL station. We had been chatting away for 15 mins or more when John said his call during a pause, he was a little low down to me, but, 5mins later when he was called into the qso, he was a nice easy copy. I could only have a brief qso with John as i needed to use the loo, i was about to explode, but, we made an exchange and a few other pleasantries and I do hope to catch him on the bands again. Tu 73’s John.

_cfimg5018402711211806278

I have worked Michel a number of times, always a big signal, with those antenna is it any wonder 🙂

_cfimg-4535136507211518452

First time i have worked Dennis, was nice to get another Philippines station in the log.

Abendstimmung auf Farm Ameib Erongo-Gebirge

I was really pleased to have a qso with Gunter, he was calling VK/ZL and with next to no takers i was able to get him 2nd call, this was also my 2nd African country.

Facebooktwitterredditpinterestlinkedin

Esk National Park Activation

Esk NP is a small un-signposted remnant wetland park just west of Brisbane. There are no walking track, no facilities, no nothing. So i found some space just off the side of the road and setup my gear. 10m was a bust when i tried at 3pm, but would later provide the surprise of the activation.

So, 15m was next and it started off great, Hawaii, Russia, VK6, Sth Korea, VK5, Japan, Vk4 all in the log in very short order and New zealand added a little latter on, then a whole lot of calling where i could not buy a contact. I then went and played 2m ssb and cw with Rick VK4RF and made my first 2m contacts with a chaser 100km away.

Jumped on 40m next and made a bunch of contacts, vk1,2,3,4,5. 40m was its usual horrid noise and the FT897 is not the nicest receiver to have in any conditions other than perfect, I am so lucky to have an IC-7410 at home with modern DSP filtering that does wonders to noise and close by stations. Hope to upgrade the 897 soon to an IC7300 once i know what the price is like.
Anyways, i digress, from 40 i flipped over to 10m to play with rick, and while we were sitting there gas bagging about stuff, cause calling cq with no takers was getting boring, we had Jonathan VK6JON and Ted VK6NTE call in and get in the log. Being on a 10m tuned up 40m dipole made for tough going to vk6, but we made the exchange and then qsy’d to 15m and had a nice QSO at 57 or better. After this i started calling cq and well 15mins of calling later and nothing, i could not buy a contact. So off i went playing search and pounce, adding a couple more dx contacts to my tally.

A quick trip back to 40m for a few calls while the last of the daylight faded away behind the mountains and I was done. Packed up, went to a friends place who live near by and had a beer and some dinner.

CW, i did call on 10m and 15m cw and did not go very well at all, like most of this activation, i could not buy a contact, but i did make some CW contacts with John 6NU, Rick 4RF/HA, Gerard 2IO, Steve 3MEG and Ben 6FBLJ. All in all, 58 QSO or there abouts from 35 calls, not sure if bands and modes matter in making the 44, but making 44 is not a high priority for me, having fun is, and to be honest, even though i started to lose my voice and today my throat feels like gravel today, I had a bunch of fun this activation, and for me, that is everything.

Facebooktwitterredditpinterestlinkedin

Failure Is Always An Option

For anyone who watched the Mythbusters, the catchphrase “Failure is always an option” is one you will hear often. It is also one that i subscribe to in general, where, no matter how well you research and plan, no matter how great your methodology, failure is always a very real possibility.

I was reminded of this in the last couple of days when an audio stage for a DC receiver did not produce the intended results. The audio output from a mixer in a direct conversion receiver is very low and needs a lot of gain to bring it up to speaker driving levels. With that in mind, i have a preamp prior to the mixer giving 10x gain to the incoming RF, and then lots of audio gain post mixer to bring the AF up to a nice listening level.

With that in mind, i thought, OP AMP preamp would be a good idea followed by a LM-386 at 100x gain. Trouble is, I also set the OP AMP to deliver 100x gain and that is 10,000x gain too much and obviously something is going to distort. Changing the gain of the OP AMP to 10x was tried next, and while the audio was nice, it was much lower than the LM-386 in isolation, and the whole point was to have nice sounding audio that would drive a speaker, this was certainly not that. I am guessing there is some impedance miss match there, that i have not accounted for, but to be honest, i really have no idea.

Now, i could run the LM-386 by itself, the audio will be a little lower than i would like, and lets face it, they sound like arse when it comes to CW, which is the main mode this RX will be used on. So i am back at square one, needing to find an audio stage that both sounds nice and gives 150 to 200x gain and is power efficient. Wish me luck HIHI.

12514063_1251606018186439_1294500111798729066_o

The offending circuit and its implemented circuit board.

Facebooktwitterredditpinterestlinkedin