Inside a Mini-Circuits Mixer, and the Dissing of DSB

I'm not crazy about using integrated circuits, especially the really complex ones for which true understanding of how they work is impossible. The Mini-Circuits mixers are not, of course ICs, but for me they too have always had a whiff of the "black-box" about them. Thomas, OZ2CPU, pulls away the veil with this photo of the innards of an SBL-1.

I came across Thomas's very interesting web site when I was installing in my QRSS beacon rig the SBL-1 mixer sent to me by Jim, AL7RV. That poor SBL had died suddenly during testing. Thomas alerted me to the cause of death: In his caption for the picture of the innards, he notes, "I was unlucky to kill this one during my first transmit experiment. The IF input can NOT accept more than +20dBm audio level."

Joop and I have been discussing diode ring mixers, and I've been reading an article about them by Paul, NA5N. It looks to me like the designers of the SBL mixers weren't really thinking of balanced modulators when they created these things. Just look at the schematic above. In a normal receiver application, LO energy goes in from the L port, your RF goes in R, and your IF (or, in a DC receiver, your AF) comes out from the I port. But when we use these things as balanced modulators, we have to put the Audio INTO the I port. You can see how too much voltage on that port would quickly release the smoke from those little hot-carrier diodes.

The data sheets are oriented to the standard application (RF into R, LO into L, IF output at I). We are told to keep the LO level at +7dBm and that the 1 db compression point for the RF input is +1dBm. But at what level should you put the AF input to the I port if you are using this thing as a balanced modulator in a weird WSPR DSB rig? Similarly, the data sheets give SWR data across a wide frequency range for the L and R ports... but not for the I port. DSB is getting dissed!

K1JT Spots My DSB QRPp WSPR Signal

2009-05-03 16:42	N2CQR		-29					K1JT		7035	4371
2009-05-03 16:38	N2CQR		-24					K1JT		7035	4371

I got into WSPR because I wanted the gratification that comes from seeing a map readout indicating that my QRP signal has crossed various oceans. Well, yesterday I crossed the pond for a second time, but this time the receiving station heard the signal more than once, so the rx station callsign appeared on the display (if the signal is heard only once, you get the line showing the path, but NOT the rx station call). And what a fine callsign it was: K1JT, Joe Taylor, Nobel Prize winner and the inventor of WSPR.

In the last SolderSmoke I got Joe's name wrong (I called him John). It must have been too early in the morning here. Anyway, I was gently corrected by Wes, W7ZOI, and I promissed to make ammends. Sorry Joe! (Thanks Wes).

The Dover Construction Club

Last night there was a meeting of the Dover Construction Club (discussed by Nigel, M0NDE, in Solder Smoke #194). For the first time, the members agreed to be photographed.

Above, in the foreground, Merv checks the main board of his softrock, Ian explains to Paul how a mod to an old FT101ZD will work, and Nigel is seen using the microscope and observe to Merv he has not soldered all the IC legs down on a SMD board. He says he knows. grin!!

Ian Keyser, G3ROO, host of Dover Construction Club. At his bench in a separate part of the workshop where he has test equipment to diagnose the inevitable faults. Here he is working on the RAMU featured in a past edition of Sprat.

In a corner of the garden a brick arch leads to the workshop door, behind the brown door hides the secret world of the amateur radio constructor. Obviously, this is something of a clandestine operation.

Tony, sitting at the other end of Ian's test area, taking down some CW.

In this workshop club members have bandsaw, milling machine, lathe, and pillar drill.

Wow! That's the kind of club that we'd all like to be part of -- Nigel's comments on SS 104 generated a lot of envious e-mail . Thanks a lot guys!

WSPRing Across the Mighty Atlantic

You have to look carefully, but in the picture above you can see a little green line stretching from Rome to Maine. That's my 20 milliwatt WSPR signal crossing the Atlantic. W1CDO's receiver picked me up at 0452 UTC yesterday.

Date	Call		SNR				W	by	loc	km	mi
2009-04-29 4:52	N2CQR		-22				0.020	W1CDO	FN43ou	6541	4064

The day/night terminator was in the mid-Atlantic at that time, so I think I had a bit of a tailwind from the gray line! But still, not bad for 20 mw in Maunder Minimum II. K1JT's software and W1CDO's receive system did the heavy lifting. Antenna here is just an end-fed wire among the buildings of central Rome.

Goats Go QRP to the Field!

Another great video from Steve and the goats.