Last night, after one of my quite regular bouts of total inability to sleep, hatred of the covers, and before the Nytol had chance to kick in, I decided to think back over the receive sensitivity problem. So, I sat and thunked. Suddenly, the folly of my argument hit me. If I have to have the capacitor fully meshed, then my resonant frequency is too high, and no amount of widening the inductor turns spacing will make it lower! What I need is more turns, or more capacitance.
Tonight then, once the boys were in bed, I slunked my way back out to the workbench. Having already put the new coil on, and having during the day found a few moments to build the LO Buffer and put the SBL-1 DBM in place, I decided to properly connect the IF and Antenna connectors, and to run the tests again with a view to seeing what needed to be done.
Well, tonight I had to put a stonking -67dBm signal on in order to even hear the received signal, let alone measure the SINAD. Very odd. Both AM and FM were the same, and couldnt hear a thing in SSB. So, in frustration i disconnected the audio out from the scanner - and bang, stonking loud received audio tone!. It seems the funny issue I had with the scanner must be some sort of ground problem due to it being on battery and all the rest of the kit being off the mains. Test set up works fine until I connect the audio lead from the scanner to the 2955, which kills the received audio.
So, doing an FM SINAD test by ear, I found a perfectly acceptable -117dBm for 12dB SINAD. With the test set in AM mode and the modulation off, the slight frequency reading error of the scanner allowed me to hear the high pitched difference tone down to -127dBm just discernable, thats a pretty decent 0.091uV!. The audio tone was perfectly obvious at -120dBm/0.27uV.
But, heres the real surprise! I adjusted the test set to zero beat the scanner, and then backed it off a little to give a strong to my ears 800Hz tone. This was at 70.401MHz on the test set, 28.400MHz on the scanner. I then peaked the input tuned circuit by squeezing the coil turns together a bit, and adjusting the trimmer. It peaked best signal with the cap just short of fully meshed. So, I started turning down the signal, and down, and down, until the test set wouldnt go any lower! At -135dBm the tone was still very readable, had it been CW it would have been reasonable if not easy copy. Thats 0.039uV! I even swapped to the BNC output on the test set, but all that happened is it got better! (which shows that my N to BNC adaptor needs cleaning). It was even obvious back in the house listening from the open kitchen door!
I have to say I am amazed. Such a simple circuit, just a tuned circuit and an NE602 IC, and its sensitivity is awesome, even when used with lets face it a pretty poor IF receiver. The video above shows the set-up and the circuit under test. The warble is the signal from the Marconi 2955 test set.
It does of course mean that I owe Roger an apology, as I suggested that perhaps there was a typo in his schematic, as the diagram says a 35pF trimmer in this circuit, but his photo shows that his transverter has a 65pF one. Mine has the 35pF. I will stand by however my ascertion that 35pF is too small. Mine is very close to fully meshed, I think with a 65pF it would be more like half meshed, which I think is preferable in this case, I dont like things to be at the limit. I probably wont change the trimmer though, but instead add a 27pF or so fixed cap in parallel. So Roger, if you happen to read this, my apology on record, but also my gratitude and admiration for an amazing circuit!
Strangely, when I tried to check the LO Buffer, I couldnt get a frequency reading on my counter, neither could I from the oscillator (which was quite clearly running properly!). This is probably an issue with the counter rather than anything else, something to look at another day now.