Having spent the weekend debugging several of the transceiver modules I have managed to breath some life in to the receiver section, firstly a few notes on design and construction snags that have caught me out along the way.
- When using a design somebody else has published as a base for your own, make sure you take in to account that if you deviate from it, for example an IF Amplifier stage, that you check the reactive component values such as capacitors and inductors at the IF frequency you choose, I spent several hours wondering why my IF stage was attenuating my IF frequency rather than amplifying it – the inductive reactance at 455KHz is far different than at 9Mhz, certain coupling capacitors must be check to have a low reactance at the frequency of interest.
- Blindly assuming that a 6dB pad at the output of a post-mixer amplifier is standard, no it is not – I was losing around 10dB in the mixer/filter stage so I changed both of the post-mixer amplifier attenuation pads to 1dB and the I got my gain back. Make sure all elements of a module/stage are calculated for gain and loss, the first mixer will shave off 6dB, its diplexer will cost you around 2dB and a crystal filter, depending on your configuration and choice of filter will claim around 5-6dB.
- When buying emergency components from the likes of Maplin, don’t assume the sales assistant has gave you the correct value parts – check them first before you fit them, the diplexer on the second mixer which mixes the 10.7MHz signal with a 10.245MHz local oscillator was very lossy at the frequency of interest – I only figured this out once I swept the stage with my Spectrum Analyser and noticed a heavy attenuation curve near 455Khz – not good. It turned out the store gave me 680nF capacitors instead of 6.8nF, I have a LCR meter so no more excuses!
Having ironed out a few of the wrinkles I made a very quick lash-up of the core receiver modules, place a random length of 3 core mains cable in the first mixer and strung the cable over the curtain rail in the office, not an exact science but i was too eager to start monkeying around making antennas – I was cooking on gas and wanted to hear the result of my hard work!
So, here is a brief on the architecture of the receiver so far, it will probably change before it all goes in to the fancy equipment enclosure but for now it is what it is.
- 1st and 2nd mixer with 10.7MHz roofing filter at 7.5KHz bandwidth followed by a 455KHz 2.8KHz bandwidth SSB filter.
- Hybrid cascode IF amplifier.
- AD831 Mixer for the Product Detector.
- NE5534 low noise AF amplifier with low pass filter from the Product Detector.
- TDA2030 AF Power amplifier.
Couple of videos taken for reference.