The LM373 Transceiver - What a Rig!
The LM373 is a Real Gem!
6/7/2017 ~ New Video of the LM373 Transceiver!
6/6/2017 ~ Added >10 dB Attenuator to handle super strong signals. It is a "T Type" controlled by a small relay with the Series arms being 33 Ohms and the Parallel arm being 22 Ohms. A switch on the front panel controls the In/Out. Kind of amazing --with the pad Out of the circuit you can hear the noise on the signal and with it In --the noise drops like a rock. Good affirmation about RF amplifier stages and their contribution to noise. The Pad/Relay was mounted on a small circuit board which was soldered vertically to the main board.
The panel mounted toggle switch that selects Pad In/Out is located below the volume control and above the earphone jack. This is a solution to the strong signal overload.
Every so often the planets align and you end up with a really nice rig. No nice is not the right word but the correct terminology is that you end up with a Superb Rig. I have fully documented the building of the LM373 Rig at http://www.n6qw.com/LM373.html
Frequently I joke that I have two boxes of electronic projects with a giant box containing rigs/projects that worked once, or ones that don't work or ones that will never work (the 6 Pole Dishal Filter is in this group). Then I have a smaller box of rigs/projects that work very well. The LM373 Rig is definitely in that box.
Rigs can look good --or can even be called pretty; but how do they work is the real test. Typically the unofficial criteria is how do they sound on the other end, can they work DX , how do they receive, and for those who lurk the bands with their SDR store bought radios with 72 inch screens is there any energy below 200 Hz or above 2800 Hz. This latter criteria seems to be the one that is the most controversial with those of us who homebrew their rigs.
So now I would like to share some LM373 Performance information. With IRF510 for a final it will put out in excess of 5 watts (6-8 can be easily had). With my intermediate amp it will do >110 watts and with the SB200 in line following the intermediate amp it will do 700 watts (easily). Couple that with a 2 element beam (which I will talk more about a bit later) and you can break pile ups --which I have. Stations have been worked running QRP and most of the stations worked at 110 watts and of course stations worked at high power. Interestingly about 1/3 of the DX stations were worked running the 110 watts -- the beam antenna is a great field leveler.
Now how does it sound -- I get frequent rave comments about the audio (subjective from non SDR monitors). Punchy and articulate are frequent descriptors --kind of amazing since the audio stage is a single 2N3904 that was modeled in LT Spice. There have been no reports about spurs, lack of sideband suppression or pinched sounding. All this from a homebrew rig!
On the receive side it hears really well and that has been a frequent question from the other end after hearing the transmit side. Well I have found the hearing part an interesting aspect of the rig. The LM373 has built in AGC Circuitry and the data sheets include suggestions for making this adjustable versus fixed values. That will be a further experiment.
In W5BAA's design he used a second LM373 as the receiver mixer stage and riding along with that is that you do get some gain from that stage since it is an active (versus passive) mixer. In my rig I used an SBL-1 as the mixer stage which as a passive mixer (non-gain) does have a conversion loss. Ahead of the SBL-1, I added a single manually adjustable gain stage. Mind you the manually adjustable is a trim pot on the circuit board. My original intent was to set it for about 10 dB and forget it. Well I have found that on really strong signals and with the beam down the throat of the station that the rig will overload. Crank back that trim pot ( 2 to 3 dB of gain) and everything clears right up without any other adjustments. This does not happen all of the time --just on the rock crushing signals and sometimes I do see those kinds of signals. With the beam moving off the signal heading this clears up the signal too. So I smile as I now know the beam is doing its job!
The solution to the infrequent overload can take many forms and two of those would be to make the AGC a panel control as shown in the LM373 Data Sheets or add a 10 dB attenuator (a simple relay with a 10 dB pad on one side of the contacts) as I did with the KWM-4. The attenuator would be easy to implement right on the 2N3904 Rx RF Amp Board. This has perhaps a better approach as the engagement is a simple toggle switch versus a panel mounted pot. So this whole discussion boils down to that the receiver is sensitive and can be too sensitive if upstream there is not some signal limiting for extremely strong signals. A more elaborate AGC would also 'save the day'.
Now how does it work with DX -- well my DX standard is the European / VK-ZL path. I have worked the following pre-fixes S51 (Slovenia), EA3 (Spain), E51(Cook Island), LU1 (Argentina), XE1(Mexico), ZL1(New Zealand) and 9Y4 (Caribbean). Mind you that so far I have made 50 contacts total with the rig and 15% have been DX.
Sides have been added to the case (painted black) and now I am working on a top cover. One possible top cover might be a sheet of 1/8 thick Lexan plastic so you can the "innards" another is a metal cover with a speaker built into the cover. Still noodling that piece.
There is just something about being able to work DX stations (some running Flex 6700's) and to have them say nice signal --all from a homebrew rig using 1970's technology.