Whilst trying to make the Not-so-superhet (Blogs passim) slightly more super, I decided I needed a stable, defined RF source to feed directly into the antenna. I have used my Wobbulator with some success - but that, of course, is at IF and I wanted to test operation of the entire receiver.
Fortunately, those nice guys at the NorCal QRP club have a design for a simple generator, which produces s9 and s1 signals at any frequency for which you have a crystal (up to the 15m band). Although the NorCal unit is produced as a kit (currently listed as "Out of Stock") they also make the assembly manual available online for download - including a circuit diagram!
It won't surprise readers to hear that I made a pirate copy or, to make it sound less like IP theft, a "replica" - here's the breadboarded version for initial mock-up...
I then made an ugly version, which is slightly more practical...
The (very) ugly circuit board sits atop the PP3 which powers it, has a socket into which the chosen crystal may be plugged, a BNC output and a switchable attenuator, to select s9 or s1.
The NorCal documentation suggests that it produces s9 and s1 - but actually their design produces s9 (50 microVolts) and 1 microVolt (which is between s3 & s4 - NOT s1).
You can find a really nice table of the S levels at the G-QRP website.
I decided to build a 48 dB attenuator (rather than the 34 dB pad in the NorCal design), to give me s9 and s1. This pi attenuator was designed using Alan, vk2zay's excellent resistive pad calculator (Alan's website isn't responding as I write, so I'll add the link later).
I tried the unit into the nearly new FT 817 (as it has a hugely convenient front antenna socket!) and - sure enough - I get s9 and s1. It "does what it says on the tin".
I like the NorCal design - especially how the LED is used as both a "power on" indicator and a "voltage reference"! I think I'll make a PCB version, to make it more rugged and I may even mount it in an Altoids Tin.
I hope they won't mind me plagiarising their circuit - my first rig, which I still have, was a NorCal 40, so I am a customer of sorts (to tell the truth that was purchased second hand).
...-.- de m0xpd