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MrBungle · 21st Jan 2018, 2:51 pm

Spent a few hours on mine today finally.

Turns out 3mm red LEDs from Tayda (1.4 pence each) make better varactors than the official parts. There’s a far more linear response. Consistency is no good though so you have to go through a few until it will sweep the range required. Two in parallel works quite well too.

Got a problem with the output power from the VCO now though at the higher end of the sweep. I think the issue I have is lead lengths which is difficult to manage so I’m going to rebuild the VCO using a different construction method, design and parts.

The plan is to chicken out and integrate it with the first mixer so I will use an SA612AN with LED based varactors as the VCO and first mixer. Also moving to a slightly more robust construction method. Doing it dead bug causes some serious problems because anchoring scope probes seems to bend things and consequentially knock tetchy things way out of alignment. Also need to improve VCO shielding. To give you an idea how bad this got, it was shifting up to 10Mhz from last time I got it out.

21st Jan 2018, 2:51 pm	#98
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Spectrum Analyser Spent a few hours on mine today finally. Turns out 3mm red LEDs from Tayda (1.4 pence each) make better varactors than the official parts. There’s a far more linear response. Consistency is no good though so you have to go through a few until it will sweep the range required. Two in parallel works quite well too. Got a problem with the output power from the VCO now though at the higher end of the sweep. I think the issue I have is lead lengths which is difficult to manage so I’m going to rebuild the VCO using a different construction method, design and parts. The plan is to chicken out and integrate it with the first mixer so I will use an SA612AN with LED based varactors as the VCO and first mixer. Also moving to a slightly more robust construction method. Doing it dead bug causes some serious problems because anchoring scope probes seems to bend things and consequentially knock tetchy things way out of alignment. Also need to improve VCO shielding. To give you an idea how bad this got, it was shifting up to 10Mhz from last time I got it out.