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Old 18th May 2017, 3:43 pm   #1
Andrew2
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Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,034
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 4:04 pm. Reason: Boxing gloves on again...
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