Short Wave

[Albert H]

Here's a quick SW PLL synthesiser. This has been used by a number of SW stations, because they don't have to get crystals cut for the specific frequency that they want to use. The PLL uses a standard 4 MHz crystal, and provides a 5V squarewave output.

SW-Synth3-5.png

The VCO can be disabled by opening the wire link connecting pin 5 of the '4046 to ground - there should be a pull-up resistor (10k) from pin 5 to the 5V rail (missing on this drawing - oops).

The LEDs can be a single bi-colour type - red for out-of lock, green for locked, and the "SW 12V" terminal goes high on lock.

Setting the output frequency is done by choosing which diodes are installed around the '4040 to divide the carrier frequency down to 5 kHz (the reference that's fed to pin 14 of the '4046). The frequency can be programmed in 10kHz steps (since the lowest possible divisor on the 4040 is 2). If you need incremental tuning, the trimmer connected to the crystal will allow you to "bend" the frequency slightly up or down (by 2 - 3 kHz at most).

The ICs MUST be the 74HC or 74HCT type as these are the only types that will go fast enough.

Before anyone points out that you can buy DDS synthesisers from China for $5, they require a controller - a PIC or an AT IC - and can be a complete pain to use!

NB: If you want to use a push-pull PA, with bi-phase drive, program the PLL for twice the output frequency, and then use a 74HCT74 to divide by two and give the two phase outputs from the Q and not Q outputs.

[reverend]

Is the VCO on the 4046 clean enough for this? I had always thought it was a rather noisy thing and that an external VCO would be necessary.

[radium98]

74als74 can be replaced by 74hct74 ? out of topic question ,what is the maximum input clock .

[Albert H]

The 74HCT74 will happily clock at over 20 MHz.

The 4046 VCO is actually better than most people believe. Part of the problem is often poorly filtered supply rails, and the other part of the problem is badly designed loop filters. I've used the 74HCT4046 VCO in many projects and never had much issue with it.

If you want to connect a more sophisticated VCO, you can disable the one in the '4046 by pulling pin 5 high, and use the phase comparator output at pin 13 to feed your loop filter, and put your sampled signal from your VCO into pin 3. I've used this approach to add PLL to old VFO rigs - even adding PLL (and some frequency selection switching) to an old Heathkit DX50! That rig is in pretty regular use on the border between the Netherlands and Germany.....!

[matt_ne]

Before anyone points out that you can buy DDS synthesisers from China for $5, they require a controller - a PIC or an AT IC - and can be a complete pain to use!

I know what you mean, I did something for Medium Wave using the cheapest Chinese DDS I could find and an Atmega8. It was pretty straightforward to get running; however, you get what you pay for, the DDS does have its 'funny days' where it won't start up properly.

I've got buttons for +/- 9 kHz steps along the bottom of the board, and the third button there allows for 1kHz steps if held down, as I had a requirement to go off-piste in the extended Medium Wave band.

DDS runs at twice frequency and is fed through and edge triggered D type flip-flop for a nice square wave.

Not sure what to try next, maybe try a more expensive DDS from a reputable source, or may try your PLL design.