Saturday, 29 June 2013

CaPs LoCk

After the 12 or so hours of preparation yesterday at home and today at the village hall, the couple of hours of Toms birthday party were brilliant. On arriving home afterwards, there was a small packet awaiting my return, marked Saffron Electronics, residing within which were 10 65pF trimmers.

Eventually, I managed to slip out to the workbench for a short while. Much of my time was spent putting a small cable entry chassis mount BNC socket on the end of a bit of RG-174, and sticking F-connectors onto a length of RG-59. Those two jobs done, I added one of the new trimmers to the crystal already mounted on the 10m WSPR transceiver board. I can at least now start building the LO proper, and hopefully have it running on the 14MHz fundamental soon.

Being rather knackered, I didnt wish to work on the LO buffer amp on the 4m transverter tonight. I have a particular aversion to dual gate MOSFETs as they are rather expensive four legged fuses, and with my antistatic wristband being 'somewhere' in a box under the bench, decided best leave that task for when im a little more awake. Instead, I decided to rebuild the input coil on the 70MHz receive side. This is a 7t 4mm dia air wound coil, tapped 1t from ground. I had built the original from 18swg wire, very sturdy, but found its associated trimmer capacitor had to be fully meshed for best results. Hmmm, I much prefer it when I can peak a tuned circuit and tune out either side. Being so sturdy, I couldnt open the turns up to reduce the inductance a fraction, and hence increase the resonant frequency. So, a new coil was would with 22swg wire, still sturdy enough, but now more manipulable. I suspect this inability to adjust both coil and cap, and hence not be able to tune the pair up to a high enough frequency, was responsible for the -87dBm receive measurement. I also this evening, took the liberty of running a 12dB SINAD test on the MVT-7100 at 28.4MHz, and it came in at better than -122dBm, so it does look like the loss is in the converter stage.

The photo above shows the transverter so far. The new coil can be seen on the right above the antenna changeover relay. On the left, roughly in place but not yet soldered on, is the SBL-1 double balanced mixer. Being a hot carrier diode ring, this can work at a higher level than the NE602 Gilbert Cell mixer used in the receive chain, and will form the heart of the transmit up-converter. However, before it can be fitted, the MOSFET buffer amp needs to go just above it.

Before going any further on the transmit side, I think I will connect the nice new coax connectors and retest the receive.The bulkhead BNC sockets mean I wont have the mismatch of a pair of crock clips during the test as was the case last time. But for now, im turning in for a good kip.

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