Experimenting with and repairing radios requires signal sources. I have used three different signal sources so far:
- My MFJ259B antenna analyser is a useful tuneable signal source covering 1.8-170MHz. Its output have various spurious and harmonics. The biggest issue I have is that it drifts in frequency. But as a starting point, it has been very useful.
- A Direct Digital Synthesiser based on the Analog Devices AD9851. Mine came from a Chinese supplier via ebay for around £30. This is a 10 bit DDS clocked at 180MHz, so able to give a sinewave output from near DC up to around 70MHz. It also generates an alias frequency at (180-F)MHz, which I've been able to use to generate a signal in the 2 metre band. The DDS is stable and has very fine tuning steps and its output is essentially harmonic free; but being a 10 bit DAC the noise floor is relatively high.
- An Si570 based oscillator using the PA0KLT module (http://sdr-kits.net/PA0KLT_Description.html). This has very fine tuning steps and covers approx 4-280MHz at around 0dBm. It's a square wave output, so has very high harmonic content but essentially it's otherwise spurious free. This is a form of digital synthesiser and is to all intents and purposes as stable as a crystal, but you can set it to any frequency in that rangeMine cost about £35.
We are somewhat spoiled with things like this nowaways - stable sources, synthesised, and low cost!
Ideally I'd like a bench oscillator with the tuning range of the Si570, an ability to do frequency sweeps (for passband checking), and low harmonic level. I'm contemplating making a set of diode switched low pass filters, possibly with a gain block to give ~10dBm out, as an add-on. That would give a very useful bench signal source. I'm also contemplating a 12 bit DAC DDS (AD9854) for which there's a module available for around £50 which will be able to do automatic, programmed sweeps.