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Old 18th May 2017, 2:43 pm   #1
Andrew2
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Default My pantry transmitter.

The last few days have seen me faffing around with a pantry TX to enable me to listen to Serenade Radio on my 60's trannies around the house. I was going to go with a simple VFO/buffer/modulated PA, but VFOs are not particularly stable in the long-term and are prone to FM'ing if they pick up a sniff of the modulated output.
So I swung to the other end of the spectrum and tried a fully synthetic type with 9KHz steps. This worked nicely, but I realised it would be a waste of my last MC145151 chip as I probably wouldn't need access to more than one channel.
Finally I decided to get the dreaded xtal drawer out and sit down with a calculator to see what would land where when divided by whatever divider was to hand (in this case a HC4017 decade counter).
To my surprise quite a few crystals fit the bill, landing nicely on or near legit channels. I chose a rock marked 14.169 MHz, which I found would tweak up to 14.175 MHz and this divided by nine would produce 1575 KHz which is quiet in these parts.
The osc is a simple Colpitts and the output is taken from the emitter of the transistor straight into the clock input (p14) of the counter. The reset pin is taken to p11 to provide a f/9 signal and the o/p comes from p12 which is the 'carry out' and is a nice 50:50 square wave.
This is DC coupled to an emitter follower buffer with a pair of 1k resistors in its emitter to deck and the junction of these goes to the modulator. I'd tried several ways to get nice AM from various high-falutin' ccts, but I found they all caused compression in the 'troughs'. They sounded OK, but the 'scope said otherwise. So I decided to go simple and tried a single-ended common-emitter stage with a 1.5k in its emitter and a ferrite toroid transformer (about half a dozen turns pri and sec) in the collector. Bias is from a pot across the supply, wiper to base via a 10k. The base has a 1nf to deck to remove RF here. There's also a 300pf to deck on the collector to clean up the RF waveform a bit.
With the RF from the buffer coupled to the emitter via a 10n cap and the audio AC coupled to the base via a 1uF and a 1k, the results amazed me - beautiful AM, right up to 100%, at which point the crests of the mod were exactly twice the size of the unmodulated carrier. Adjustment of the pot varies the carrier output and you adjust the mod input to suit.
I forgot to measure the power coming from this stage, but I'd guess it was about 30 mW.
I decided a bit more urge was probably a good idea, so I went on to build the final stage - a VN67 fet PA. Very simple. The secondary of the ferrite transformer in the modulator has one end going to the gate of the fet via a grid-stopper of 22 ohms, the other end going to deck via a 1uF cap. The top end of this cap is fed from the wiper of a 4k7 pot across the supply. This sets the bias so that the fet draws about 60 to 80 mA without RF drive. There is another ferrite transformer in the drain of this fet, the secondary feeding a LPF which goes over at around 1.8 MHz. Power out is 160mW into 50 ohms at full tilt. Could probably be better if I optimised the ferrites.
The whole thing runs off an 8 volt stabilised supply, but as I get confused over exactly what types of CMOS will stand 15 volts and which won't, I just zenered the supply down to 5v for the chip.
For an aerial I wound a few turns of wire around the inside of a 3-foot dia hula hoop and brought it to resonance with a large compression trimmer and a few thousand pf in series to provide a nominal 50r match point at the junction. Works a treat, now Serenade Radio is audible all over the house at good strength, even with the wick turned well down.
Only trouble is it suffers terribly from modulation hum. I get this blasted buzz on any strong MW station, and my own transmissions are not immune. I can get rid of it by rotating the radio, but it really is a pain!
Sorry about the scruffy diagrams, I quickly drew them out for a friend who is interested. There may be a few small differences from my description.
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Last edited by Andrew2; 18th May 2017 at 3:04 pm. Reason: Boxing gloves on again...
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Old 19th May 2017, 7:14 pm   #2
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Default Re: My pantry transmitter.

Quote:
Originally Posted by Andrew2 View Post
Only trouble is it suffers terribly from modulation hum. I get this blasted buzz on any strong MW station, and my own transmissions are not immune. I can get rid of it by rotating the radio, but it really is a pain!
Hi Andrew,

I'm just building an AM internet here so I'm interested to see your version and results tailored to the top of the waveband. It sounds like a pleasing little build and with good results, must be gratifying to have done this.

I wonder if you could break up the text of your OP with the Mods' permission - it does take a bit of effort to read through all that in one block, not that you're doing anything wrong, just that it does look more offputting and I wonder that you might get more responses?

Also a photo of the actual build is always pleasing to see!

What do you think is behind the modulation hum? It doesn't seem fair that mine doesn't suffer from it when your super-careful design does!

How did you design the loop antenna and matching?
(not that I'm the expert, I'm just curious.)
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Old 20th May 2017, 12:21 am   #3
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Default Re: My pantry transmitter.

Hi Al, thanks for your interest. I wasn't going to include diagrams but they are there at the bottom of the post now, so really the description is not required.
Modulation hum is a subject that comes up quite often on here these days. It seems to be caused by the RF from strong transmissions getting on the mains wiring and mixing with local 'buzz' noise also on the mains. Our local Gold 1458 KHz is a very strong signal and it suffers terribly, although rotating the radio stops it.
I didn't so much design the loop as throw it together and 'suck it and see'! I think there are about half a dozen turns in the loop and it is tuned/matched with a 1000pf compression trimmer (one of several bought surplus many moons ago) in series with a 10nf ceramic cap. The TX output goes across the 10nf and the trimmer adjusted for max field strength. A later check with my return loss bridge showed a decent match. Tuning is pretty sharp and it's easy to end up with wonky sidebands!
I'll post some piccies if I remember....
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Old 20th May 2017, 10:04 am   #4
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Default Re: My pantry transmitter.

Hi Andy, yes, I wasn't questioning the meaning of modulation hum (I've been part of such threads and even started one recently) - more just querying why it's bugging you...

Happy listening!
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Old 20th May 2017, 10:19 am   #5
Andrew2
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Default Re: My pantry transmitter.

There seems to be a high level of general buzz-type interference carried on the mains around here, probably a rogue PC power supply. It's been going for years.

Re the pantry TX, I'm just doing a proper cct desription, stage-by-stage. Also some photos and details of the loop antenna. I made no notes at the time of building, just building it up 'off the cuff' so to speak. More later.
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Old 20th May 2017, 10:32 am   #6
paulsherwin
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Default Re: My pantry transmitter.

It would be helpful to members if you could try the design with an alternative bipolar PA stage. Not many of us will have spare VN66s in our biscuit tins of transistors.
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Old 20th May 2017, 12:38 pm   #7
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Default Re: My pantry transmitter.

Quote:
Originally Posted by paulsherwin View Post
It would be helpful to members if you could try the design with an alternative bipolar PA stage...
Paul, yes, I had the same thought.

And for example the common IRF series, like IRF510, commonly seen in such a role (although way too much power unless designed carefully) isn't a substitute as the threshold voltage is much higher... and also needs say 12V power supply.
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Old 20th May 2017, 1:14 pm   #8
Andrew2
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Default Re: My pantry transmitter.

All comments noted. Here's a more detailed description, but as it was built 'from head to board' with only minimal prototyping and measurement, the notes are still rather poor. Still, anyone with a basic understanding could knock one up I reckon.
Re the bipolar PA. Please no. Push pull ones are nice, but single-ended ones always make me (and my scope) shudder with the nasty-looking waveforms. The bias supply is a pain too. I'll let one of you brave souls do it!


Here's a better (?) description of the pantry TX.

Osc and divider.

The oscillator is a crystal-controlled Colpitts and it's about as basic as it can get. The xtal in this one is 14.169 MHz and is tweaked onto 14.175 MHz by the 20pf trimmer in series with the xtal. The transistor is a BC548, so probably anything will do. The only thing to fiddle with (apart from the xtal trimmer) is to set the base bias so that the DC level at the emitter sits at about 2.5 v. Adjust the bias to ensure reliable clocking of the HC4017 counter chip. This chip must be an HC part as the standard CMOS 4017 won't work at anything more than a couple of MHz.
There should be about 1v p-p of RF at the clock input (pin14). Dont forget to enable the chip by decking (pin 13).
(Pin 15) is the reset pin and is returned (in this case) to (pin 11) which will cause an f/9 division, outputting a nice 5v p-p square wave at 1575 KHz from the carry out (pin 12).
The diagram shows a crude potential divider to provide a 5v supply for the 4017, but since then I have reduced the 560 ohm resistor to 390 ohm and stuck a 4.7 v zener to deck to replace the 2.7k.


Buffer.

Not much to see here. Another BC548 is pressed into service, this time as an emitter follower. Input at 1575 KHz is directly applied to the base and the output is taken from a potential divider in the emitter. This divider is a hangover from the prototype, when I was driving the modulator from my sig gen. When building, I simply tried to match the signal levels I had used at the time. In my completed unit, I get 4v p-p at the buffer's emitter and about 1 v p-p at the junction of the resistors. If you are testing as you build, you may get higher readings than this as the loading effect of the next stage (the modulator) is absent.


Modulator.

The first transistor that came to hand from the mounting pile of scrap on my bench was a 2N2222, but I'm sure another BC548 would work a treat. This circuit was the result of thinking 'there must be a simple way of applying AM that is linear, without resorting to LTP's and stuff'. And this is what came out. The RF from the buffer goes to the emitter via a 10n cap, and the audio goes on the base via a 1uf cap and a 1k series resistor. There's a 1n to deck from the base to get rid of any RF.
OK, the first of the dreaded wound components appears here, in the collector. It uses a junk-box ferrite toroid about 1cm dia, and I wound about 6 turns each for primary and secondary. It may be worth experimenting with the turns ratio when you have finished the TX, but as mine worked 'well enough' I didn't bother. There's a 300pf cap to deck from the collector to remove some harmonic energy. My scope sees about 500mV p-p without modulation, but the exact amount will vary with the setting of the bias pot. A 4.7k preset was used here as I had a couple in the drawers.
At this stage you can try out the TX, getting a feel of what the 'set mod bias' pot does and how much output it can provide without squashing the audio.

Final amp.

I used a VN67 fet device here as I find them much nicer to work with than bipolars for this type of thing. I believe VN66/67 devices are now like hen's teeth, but perhaps an IRF540 or similar would do if the bias is turned up. Make sure you don't buy cheap Chinese copies - I did once and I was led a very merry dance.
There's nothing much to it. Input comes from the secondary of the modulator toroid via a 22 ohm gate stopper to the gate of the fet. The other end of the secondary goes to deck via a 1uF cap. Bias for the fet comes from another 4.7k preset to the top end of this cap. Another junk-box ferrite toroid goes in the drain, again with half-a-dozen turns pri & sec. I'm sure this could be substantially improved with a bit of fiddling with turns ratio.
Without RF drive, set the bias pot for about 60 to 80 mA drain current. A small heatsink might be advisable. Before applying drive, solder a 47 ohm 0.25 watt resistor across the secondary of the output toroid.


Testing...

On with the volts and the set up can begin. Trim your osc to frequency and check the various waveforms look something like sensible. Without modulation you should be able get about 3.6 v p-p into the 47 ohm resistor, and at 100% modulation the p-p voltage should double to over 7 v p-p. You will find that you can get a lot more than this by advancing the mod bias pot, but doing this will leave less room for the modulation. 7 v p-p into 47 ohms is 130 mW peak envelope power and I find it easily enough to cover our house when fed to my homebrew hula-hoop loop. Note, these junk box ferrites are pretty random things and if you cannot achieve the results I have, it may be worth trying different ones. The LPF is optional as the loop aerial (see later) is very sharp and should (maybe...) attenuate any harmonic content quite well. However, I'm a bit anal about such things so I included it.


Loop aerial.

Trying to radiate MW signals from a short wire aerial is a pretty depressing thing to do. It never travels far. I've had very good results with a 3 foot diameter frame aerial hung on the wall of my upstairs shack. It's just a normal plastic hula-hoop, the type with a join that can be pulled apart. I can't remember exactly how many turns I threaded through the hoop, but I've just measured the inductance of the winding and it is 23 uH. Leave a few inches of wire sticking out at each end and push the loop back together. The tuning/matching circuit can be made up on a scrap of Veroboard and left to dangle on the loop wires. My method of feeding the TX into this is shown in the diagram. Basically it is brought to resonance on the right frequency by the combined capacitance of the trimmer (a large 1000pf compression type) and the 8.5n capacitor in series. The fixed cap can be made up from several caps in parallel if necessary. The RF from the TX goes across the fixed cap. Note the tuning is pretty sharp. If you don't have a way of finding when the loop hits resonance, just leave your scope probe nearby and tune the trimmer for a sharp increase in the scope display.

So there it is. Sorry I've had to be a bit vague in parts and as usual many of the bits are un-specified junk-box components, especially the ferrites. Suck it and see is good when it comes to wound components.
Re the photos. I've started building the board into an old Icom case so I couldn't get the shots I wanted, but I think they show what's what. The smaller board next to the RF board is just a bit of audio amplification and HF filtering and there's a little circuit that flashes an LED when the modulation approaches 100 per cent.
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Last edited by Andrew2; 20th May 2017 at 1:23 pm.
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